596 lines
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596 lines
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<title>GNU Compiler Collection (GCC) Internals: Processor pipeline description</title>
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Insn-Attributes.html#Insn-Attributes" rel="up" title="Insn Attributes">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Processor-pipeline-description"></a>
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<div class="header">
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<p>
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Previous: <a href="Delay-Slots.html#Delay-Slots" accesskey="p" rel="prev">Delay Slots</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Specifying-processor-pipeline-description"></a>
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<h4 class="subsection">16.19.9 Specifying processor pipeline description</h4>
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<a name="index-processor-pipeline-description"></a>
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<a name="index-processor-functional-units"></a>
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<a name="index-instruction-latency-time"></a>
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<a name="index-interlock-delays"></a>
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<a name="index-data-dependence-delays"></a>
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<a name="index-reservation-delays"></a>
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<a name="index-pipeline-hazard-recognizer"></a>
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<a name="index-automaton-based-pipeline-description"></a>
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<a name="index-regular-expressions"></a>
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<a name="index-deterministic-finite-state-automaton"></a>
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<a name="index-automaton-based-scheduler"></a>
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<a name="index-RISC"></a>
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<a name="index-VLIW"></a>
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<p>To achieve better performance, most modern processors
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(super-pipelined, superscalar <acronym>RISC</acronym>, and <acronym>VLIW</acronym>
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processors) have many <em>functional units</em> on which several
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instructions can be executed simultaneously. An instruction starts
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execution if its issue conditions are satisfied. If not, the
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instruction is stalled until its conditions are satisfied. Such
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<em>interlock (pipeline) delay</em> causes interruption of the fetching
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of successor instructions (or demands nop instructions, e.g. for some
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MIPS processors).
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</p>
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<p>There are two major kinds of interlock delays in modern processors.
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The first one is a data dependence delay determining <em>instruction
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latency time</em>. The instruction execution is not started until all
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source data have been evaluated by prior instructions (there are more
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complex cases when the instruction execution starts even when the data
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are not available but will be ready in given time after the
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instruction execution start). Taking the data dependence delays into
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account is simple. The data dependence (true, output, and
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anti-dependence) delay between two instructions is given by a
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constant. In most cases this approach is adequate. The second kind
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of interlock delays is a reservation delay. The reservation delay
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means that two instructions under execution will be in need of shared
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processors resources, i.e. buses, internal registers, and/or
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functional units, which are reserved for some time. Taking this kind
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of delay into account is complex especially for modern <acronym>RISC</acronym>
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processors.
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</p>
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<p>The task of exploiting more processor parallelism is solved by an
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instruction scheduler. For a better solution to this problem, the
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instruction scheduler has to have an adequate description of the
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processor parallelism (or <em>pipeline description</em>). GCC
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machine descriptions describe processor parallelism and functional
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unit reservations for groups of instructions with the aid of
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<em>regular expressions</em>.
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</p>
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<p>The GCC instruction scheduler uses a <em>pipeline hazard recognizer</em> to
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figure out the possibility of the instruction issue by the processor
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on a given simulated processor cycle. The pipeline hazard recognizer is
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automatically generated from the processor pipeline description. The
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pipeline hazard recognizer generated from the machine description
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is based on a deterministic finite state automaton (<acronym>DFA</acronym>):
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the instruction issue is possible if there is a transition from one
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automaton state to another one. This algorithm is very fast, and
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furthermore, its speed is not dependent on processor
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complexity<a name="DOCF5" href="#FOOT5"><sup>5</sup></a>.
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</p>
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<a name="index-automaton-based-pipeline-description-1"></a>
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<p>The rest of this section describes the directives that constitute
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an automaton-based processor pipeline description. The order of
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these constructions within the machine description file is not
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important.
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</p>
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<a name="index-define_005fautomaton"></a>
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<a name="index-pipeline-hazard-recognizer-1"></a>
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<p>The following optional construction describes names of automata
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generated and used for the pipeline hazards recognition. Sometimes
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the generated finite state automaton used by the pipeline hazard
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recognizer is large. If we use more than one automaton and bind functional
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units to the automata, the total size of the automata is usually
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less than the size of the single automaton. If there is no one such
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construction, only one finite state automaton is generated.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_automaton <var>automata-names</var>)
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</pre></div>
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<p><var>automata-names</var> is a string giving names of the automata. The
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names are separated by commas. All the automata should have unique names.
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The automaton name is used in the constructions <code>define_cpu_unit</code> and
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<code>define_query_cpu_unit</code>.
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</p>
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<a name="index-define_005fcpu_005funit"></a>
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<a name="index-processor-functional-units-1"></a>
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<p>Each processor functional unit used in the description of instruction
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reservations should be described by the following construction.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_cpu_unit <var>unit-names</var> [<var>automaton-name</var>])
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</pre></div>
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<p><var>unit-names</var> is a string giving the names of the functional units
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separated by commas. Don’t use name ‘<samp>nothing</samp>’, it is reserved
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for other goals.
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</p>
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<p><var>automaton-name</var> is a string giving the name of the automaton with
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which the unit is bound. The automaton should be described in
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construction <code>define_automaton</code>. You should give
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<em>automaton-name</em>, if there is a defined automaton.
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</p>
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<p>The assignment of units to automata are constrained by the uses of the
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units in insn reservations. The most important constraint is: if a
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unit reservation is present on a particular cycle of an alternative
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for an insn reservation, then some unit from the same automaton must
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be present on the same cycle for the other alternatives of the insn
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reservation. The rest of the constraints are mentioned in the
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description of the subsequent constructions.
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</p>
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<a name="index-define_005fquery_005fcpu_005funit"></a>
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<a name="index-querying-function-unit-reservations"></a>
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<p>The following construction describes CPU functional units analogously
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to <code>define_cpu_unit</code>. The reservation of such units can be
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queried for an automaton state. The instruction scheduler never
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queries reservation of functional units for given automaton state. So
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as a rule, you don’t need this construction. This construction could
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be used for future code generation goals (e.g. to generate
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<acronym>VLIW</acronym> insn templates).
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_query_cpu_unit <var>unit-names</var> [<var>automaton-name</var>])
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</pre></div>
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<p><var>unit-names</var> is a string giving names of the functional units
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separated by commas.
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</p>
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<p><var>automaton-name</var> is a string giving the name of the automaton with
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which the unit is bound.
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</p>
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<a name="index-define_005finsn_005freservation"></a>
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<a name="index-instruction-latency-time-1"></a>
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<a name="index-regular-expressions-1"></a>
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<a name="index-data-bypass"></a>
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<p>The following construction is the major one to describe pipeline
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characteristics of an instruction.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_insn_reservation <var>insn-name</var> <var>default_latency</var>
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<var>condition</var> <var>regexp</var>)
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</pre></div>
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<p><var>default_latency</var> is a number giving latency time of the
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instruction. There is an important difference between the old
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description and the automaton based pipeline description. The latency
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time is used for all dependencies when we use the old description. In
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the automaton based pipeline description, the given latency time is only
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used for true dependencies. The cost of anti-dependencies is always
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zero and the cost of output dependencies is the difference between
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latency times of the producing and consuming insns (if the difference
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is negative, the cost is considered to be zero). You can always
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change the default costs for any description by using the target hook
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<code>TARGET_SCHED_ADJUST_COST</code> (see <a href="Scheduling.html#Scheduling">Scheduling</a>).
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</p>
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<p><var>insn-name</var> is a string giving the internal name of the insn. The
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internal names are used in constructions <code>define_bypass</code> and in
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the automaton description file generated for debugging. The internal
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name has nothing in common with the names in <code>define_insn</code>. It is a
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good practice to use insn classes described in the processor manual.
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</p>
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<p><var>condition</var> defines what RTL insns are described by this
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construction. You should remember that you will be in trouble if
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<var>condition</var> for two or more different
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<code>define_insn_reservation</code> constructions is TRUE for an insn. In
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this case what reservation will be used for the insn is not defined.
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Such cases are not checked during generation of the pipeline hazards
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recognizer because in general recognizing that two conditions may have
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the same value is quite difficult (especially if the conditions
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contain <code>symbol_ref</code>). It is also not checked during the
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pipeline hazard recognizer work because it would slow down the
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recognizer considerably.
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</p>
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<p><var>regexp</var> is a string describing the reservation of the cpu’s functional
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units by the instruction. The reservations are described by a regular
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expression according to the following syntax:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> regexp = regexp "," oneof
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| oneof
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oneof = oneof "|" allof
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| allof
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allof = allof "+" repeat
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| repeat
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repeat = element "*" number
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| element
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element = cpu_function_unit_name
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| reservation_name
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| result_name
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| "nothing"
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| "(" regexp ")"
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</pre></div>
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<ul>
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<li> ‘<samp>,</samp>’ is used for describing the start of the next cycle in
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the reservation.
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</li><li> ‘<samp>|</samp>’ is used for describing a reservation described by the first
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regular expression <strong>or</strong> a reservation described by the second
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regular expression <strong>or</strong> etc.
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</li><li> ‘<samp>+</samp>’ is used for describing a reservation described by the first
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regular expression <strong>and</strong> a reservation described by the
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second regular expression <strong>and</strong> etc.
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</li><li> ‘<samp>*</samp>’ is used for convenience and simply means a sequence in which
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the regular expression are repeated <var>number</var> times with cycle
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advancing (see ‘<samp>,</samp>’).
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</li><li> ‘<samp>cpu_function_unit_name</samp>’ denotes reservation of the named
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functional unit.
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</li><li> ‘<samp>reservation_name</samp>’ — see description of construction
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‘<samp>define_reservation</samp>’.
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</li><li> ‘<samp>nothing</samp>’ denotes no unit reservations.
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</li></ul>
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<a name="index-define_005freservation"></a>
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<p>Sometimes unit reservations for different insns contain common parts.
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In such case, you can simplify the pipeline description by describing
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the common part by the following construction
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_reservation <var>reservation-name</var> <var>regexp</var>)
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</pre></div>
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<p><var>reservation-name</var> is a string giving name of <var>regexp</var>.
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Functional unit names and reservation names are in the same name
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space. So the reservation names should be different from the
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functional unit names and can not be the reserved name ‘<samp>nothing</samp>’.
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</p>
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<a name="index-define_005fbypass"></a>
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<a name="index-instruction-latency-time-2"></a>
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<a name="index-data-bypass-1"></a>
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<p>The following construction is used to describe exceptions in the
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latency time for given instruction pair. This is so called bypasses.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_bypass <var>number</var> <var>out_insn_names</var> <var>in_insn_names</var>
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[<var>guard</var>])
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</pre></div>
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<p><var>number</var> defines when the result generated by the instructions
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given in string <var>out_insn_names</var> will be ready for the
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instructions given in string <var>in_insn_names</var>. Each of these
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strings is a comma-separated list of filename-style globs and
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they refer to the names of <code>define_insn_reservation</code>s.
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For example:
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</p><div class="smallexample">
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<pre class="smallexample">(define_bypass 1 "cpu1_load_*, cpu1_store_*" "cpu1_load_*")
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</pre></div>
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<p>defines a bypass between instructions that start with
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‘<samp>cpu1_load_</samp>’ or ‘<samp>cpu1_store_</samp>’ and those that start with
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‘<samp>cpu1_load_</samp>’.
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</p>
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<p><var>guard</var> is an optional string giving the name of a C function which
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defines an additional guard for the bypass. The function will get the
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two insns as parameters. If the function returns zero the bypass will
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be ignored for this case. The additional guard is necessary to
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recognize complicated bypasses, e.g. when the consumer is only an address
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of insn ‘<samp>store</samp>’ (not a stored value).
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</p>
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<p>If there are more one bypass with the same output and input insns, the
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chosen bypass is the first bypass with a guard in description whose
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guard function returns nonzero. If there is no such bypass, then
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bypass without the guard function is chosen.
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</p>
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<a name="index-exclusion_005fset"></a>
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<a name="index-presence_005fset"></a>
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<a name="index-final_005fpresence_005fset"></a>
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<a name="index-absence_005fset"></a>
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<a name="index-final_005fabsence_005fset"></a>
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<a name="index-VLIW-1"></a>
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<a name="index-RISC-1"></a>
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<p>The following five constructions are usually used to describe
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<acronym>VLIW</acronym> processors, or more precisely, to describe a placement
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of small instructions into <acronym>VLIW</acronym> instruction slots. They
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can be used for <acronym>RISC</acronym> processors, too.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(exclusion_set <var>unit-names</var> <var>unit-names</var>)
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(presence_set <var>unit-names</var> <var>patterns</var>)
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(final_presence_set <var>unit-names</var> <var>patterns</var>)
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(absence_set <var>unit-names</var> <var>patterns</var>)
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(final_absence_set <var>unit-names</var> <var>patterns</var>)
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</pre></div>
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<p><var>unit-names</var> is a string giving names of functional units
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separated by commas.
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</p>
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<p><var>patterns</var> is a string giving patterns of functional units
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separated by comma. Currently pattern is one unit or units
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separated by white-spaces.
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</p>
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<p>The first construction (‘<samp>exclusion_set</samp>’) means that each
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functional unit in the first string can not be reserved simultaneously
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with a unit whose name is in the second string and vice versa. For
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example, the construction is useful for describing processors
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(e.g. some SPARC processors) with a fully pipelined floating point
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functional unit which can execute simultaneously only single floating
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point insns or only double floating point insns.
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</p>
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<p>The second construction (‘<samp>presence_set</samp>’) means that each
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functional unit in the first string can not be reserved unless at
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least one of pattern of units whose names are in the second string is
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reserved. This is an asymmetric relation. For example, it is useful
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for description that <acronym>VLIW</acronym> ‘<samp>slot1</samp>’ is reserved after
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‘<samp>slot0</samp>’ reservation. We could describe it by the following
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construction
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</p>
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<div class="smallexample">
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<pre class="smallexample">(presence_set "slot1" "slot0")
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</pre></div>
|
|
|
|
<p>Or ‘<samp>slot1</samp>’ is reserved only after ‘<samp>slot0</samp>’ and unit ‘<samp>b0</samp>’
|
|
reservation. In this case we could write
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(presence_set "slot1" "slot0 b0")
|
|
</pre></div>
|
|
|
|
<p>The third construction (‘<samp>final_presence_set</samp>’) is analogous to
|
|
‘<samp>presence_set</samp>’. The difference between them is when checking is
|
|
done. When an instruction is issued in given automaton state
|
|
reflecting all current and planned unit reservations, the automaton
|
|
state is changed. The first state is a source state, the second one
|
|
is a result state. Checking for ‘<samp>presence_set</samp>’ is done on the
|
|
source state reservation, checking for ‘<samp>final_presence_set</samp>’ is
|
|
done on the result reservation. This construction is useful to
|
|
describe a reservation which is actually two subsequent reservations.
|
|
For example, if we use
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(presence_set "slot1" "slot0")
|
|
</pre></div>
|
|
|
|
<p>the following insn will be never issued (because ‘<samp>slot1</samp>’ requires
|
|
‘<samp>slot0</samp>’ which is absent in the source state).
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(define_reservation "insn_and_nop" "slot0 + slot1")
|
|
</pre></div>
|
|
|
|
<p>but it can be issued if we use analogous ‘<samp>final_presence_set</samp>’.
|
|
</p>
|
|
<p>The forth construction (‘<samp>absence_set</samp>’) means that each functional
|
|
unit in the first string can be reserved only if each pattern of units
|
|
whose names are in the second string is not reserved. This is an
|
|
asymmetric relation (actually ‘<samp>exclusion_set</samp>’ is analogous to
|
|
this one but it is symmetric). For example it might be useful in a
|
|
<acronym>VLIW</acronym> description to say that ‘<samp>slot0</samp>’ cannot be reserved
|
|
after either ‘<samp>slot1</samp>’ or ‘<samp>slot2</samp>’ have been reserved. This
|
|
can be described as:
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(absence_set "slot0" "slot1, slot2")
|
|
</pre></div>
|
|
|
|
<p>Or ‘<samp>slot2</samp>’ can not be reserved if ‘<samp>slot0</samp>’ and unit ‘<samp>b0</samp>’
|
|
are reserved or ‘<samp>slot1</samp>’ and unit ‘<samp>b1</samp>’ are reserved. In
|
|
this case we could write
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(absence_set "slot2" "slot0 b0, slot1 b1")
|
|
</pre></div>
|
|
|
|
<p>All functional units mentioned in a set should belong to the same
|
|
automaton.
|
|
</p>
|
|
<p>The last construction (‘<samp>final_absence_set</samp>’) is analogous to
|
|
‘<samp>absence_set</samp>’ but checking is done on the result (state)
|
|
reservation. See comments for ‘<samp>final_presence_set</samp>’.
|
|
</p>
|
|
<a name="index-automata_005foption"></a>
|
|
<a name="index-deterministic-finite-state-automaton-1"></a>
|
|
<a name="index-nondeterministic-finite-state-automaton"></a>
|
|
<a name="index-finite-state-automaton-minimization"></a>
|
|
<p>You can control the generator of the pipeline hazard recognizer with
|
|
the following construction.
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(automata_option <var>options</var>)
|
|
</pre></div>
|
|
|
|
<p><var>options</var> is a string giving options which affect the generated
|
|
code. Currently there are the following options:
|
|
</p>
|
|
<ul>
|
|
<li> <em>no-minimization</em> makes no minimization of the automaton. This is
|
|
only worth to do when we are debugging the description and need to
|
|
look more accurately at reservations of states.
|
|
|
|
</li><li> <em>time</em> means printing time statistics about the generation of
|
|
automata.
|
|
|
|
</li><li> <em>stats</em> means printing statistics about the generated automata
|
|
such as the number of DFA states, NDFA states and arcs.
|
|
|
|
</li><li> <em>v</em> means a generation of the file describing the result automata.
|
|
The file has suffix ‘<samp>.dfa</samp>’ and can be used for the description
|
|
verification and debugging.
|
|
|
|
</li><li> <em>w</em> means a generation of warning instead of error for
|
|
non-critical errors.
|
|
|
|
</li><li> <em>no-comb-vect</em> prevents the automaton generator from generating
|
|
two data structures and comparing them for space efficiency. Using
|
|
a comb vector to represent transitions may be better, but it can be
|
|
very expensive to construct. This option is useful if the build
|
|
process spends an unacceptably long time in genautomata.
|
|
|
|
</li><li> <em>ndfa</em> makes nondeterministic finite state automata. This affects
|
|
the treatment of operator ‘<samp>|</samp>’ in the regular expressions. The
|
|
usual treatment of the operator is to try the first alternative and,
|
|
if the reservation is not possible, the second alternative. The
|
|
nondeterministic treatment means trying all alternatives, some of them
|
|
may be rejected by reservations in the subsequent insns.
|
|
|
|
</li><li> <em>collapse-ndfa</em> modifies the behavior of the generator when
|
|
producing an automaton. An additional state transition to collapse a
|
|
nondeterministic <acronym>NDFA</acronym> state to a deterministic <acronym>DFA</acronym>
|
|
state is generated. It can be triggered by passing <code>const0_rtx</code> to
|
|
state_transition. In such an automaton, cycle advance transitions are
|
|
available only for these collapsed states. This option is useful for
|
|
ports that want to use the <code>ndfa</code> option, but also want to use
|
|
<code>define_query_cpu_unit</code> to assign units to insns issued in a cycle.
|
|
|
|
</li><li> <em>progress</em> means output of a progress bar showing how many states
|
|
were generated so far for automaton being processed. This is useful
|
|
during debugging a <acronym>DFA</acronym> description. If you see too many
|
|
generated states, you could interrupt the generator of the pipeline
|
|
hazard recognizer and try to figure out a reason for generation of the
|
|
huge automaton.
|
|
</li></ul>
|
|
|
|
<p>As an example, consider a superscalar <acronym>RISC</acronym> machine which can
|
|
issue three insns (two integer insns and one floating point insn) on
|
|
the cycle but can finish only two insns. To describe this, we define
|
|
the following functional units.
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(define_cpu_unit "i0_pipeline, i1_pipeline, f_pipeline")
|
|
(define_cpu_unit "port0, port1")
|
|
</pre></div>
|
|
|
|
<p>All simple integer insns can be executed in any integer pipeline and
|
|
their result is ready in two cycles. The simple integer insns are
|
|
issued into the first pipeline unless it is reserved, otherwise they
|
|
are issued into the second pipeline. Integer division and
|
|
multiplication insns can be executed only in the second integer
|
|
pipeline and their results are ready correspondingly in 8 and 4
|
|
cycles. The integer division is not pipelined, i.e. the subsequent
|
|
integer division insn can not be issued until the current division
|
|
insn finished. Floating point insns are fully pipelined and their
|
|
results are ready in 3 cycles. Where the result of a floating point
|
|
insn is used by an integer insn, an additional delay of one cycle is
|
|
incurred. To describe all of this we could specify
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(define_cpu_unit "div")
|
|
|
|
(define_insn_reservation "simple" 2 (eq_attr "type" "int")
|
|
"(i0_pipeline | i1_pipeline), (port0 | port1)")
|
|
|
|
(define_insn_reservation "mult" 4 (eq_attr "type" "mult")
|
|
"i1_pipeline, nothing*2, (port0 | port1)")
|
|
|
|
(define_insn_reservation "div" 8 (eq_attr "type" "div")
|
|
"i1_pipeline, div*7, div + (port0 | port1)")
|
|
|
|
(define_insn_reservation "float" 3 (eq_attr "type" "float")
|
|
"f_pipeline, nothing, (port0 | port1))
|
|
|
|
(define_bypass 4 "float" "simple,mult,div")
|
|
</pre></div>
|
|
|
|
<p>To simplify the description we could describe the following reservation
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(define_reservation "finish" "port0|port1")
|
|
</pre></div>
|
|
|
|
<p>and use it in all <code>define_insn_reservation</code> as in the following
|
|
construction
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">(define_insn_reservation "simple" 2 (eq_attr "type" "int")
|
|
"(i0_pipeline | i1_pipeline), finish")
|
|
</pre></div>
|
|
|
|
|
|
<div class="footnote">
|
|
<hr>
|
|
<h4 class="footnotes-heading">Footnotes</h4>
|
|
|
|
<h3><a name="FOOT5" href="#DOCF5">(5)</a></h3>
|
|
<p>However, the size of the automaton depends on
|
|
processor complexity. To limit this effect, machine descriptions
|
|
can split orthogonal parts of the machine description among several
|
|
automata: but then, since each of these must be stepped independently,
|
|
this does cause a small decrease in the algorithm’s performance.</p>
|
|
</div>
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Previous: <a href="Delay-Slots.html#Delay-Slots" accesskey="p" rel="prev">Delay Slots</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
|
|
</div>
|
|
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|
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