266 lines
11 KiB
HTML
266 lines
11 KiB
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<head>
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<title>Using as: Alpha-Relocs</title>
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<meta name="description" content="Using as: Alpha-Relocs">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Alpha-Syntax.html#Alpha-Syntax" rel="up" title="Alpha Syntax">
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<link href="Alpha-Floating-Point.html#Alpha-Floating-Point" rel="next" title="Alpha Floating Point">
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<link href="Alpha_002dRegs.html#Alpha_002dRegs" rel="prev" title="Alpha-Regs">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Alpha_002dRelocs"></a>
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<div class="header">
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<p>
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Previous: <a href="Alpha_002dRegs.html#Alpha_002dRegs" accesskey="p" rel="prev">Alpha-Regs</a>, Up: <a href="Alpha-Syntax.html#Alpha-Syntax" accesskey="u" rel="up">Alpha Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Relocations-1"></a>
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<h4 class="subsubsection">9.2.3.3 Relocations</h4>
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<a name="index-Alpha-relocations"></a>
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<a name="index-relocations_002c-Alpha"></a>
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<p>Some of these relocations are available for ECOFF, but mostly
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only for ELF. They are modeled after the relocation format
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introduced in Digital Unix 4.0, but there are additions.
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</p>
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<p>The format is ‘<samp>!<var>tag</var></samp>’ or ‘<samp>!<var>tag</var>!<var>number</var></samp>’
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where <var>tag</var> is the name of the relocation. In some cases
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<var>number</var> is used to relate specific instructions.
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</p>
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<p>The relocation is placed at the end of the instruction like so:
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</p>
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<div class="example">
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<pre class="example">ldah $0,a($29) !gprelhigh
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lda $0,a($0) !gprellow
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ldq $1,b($29) !literal!100
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ldl $2,0($1) !lituse_base!100
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</pre></div>
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<dl compact="compact">
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<dt><code>!literal</code></dt>
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<dt><code>!literal!<var>N</var></code></dt>
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<dd><p>Used with an <code>ldq</code> instruction to load the address of a symbol
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from the GOT.
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</p>
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<p>A sequence number <var>N</var> is optional, and if present is used to pair
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<code>lituse</code> relocations with this <code>literal</code> relocation. The
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<code>lituse</code> relocations are used by the linker to optimize the code
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based on the final location of the symbol.
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</p>
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<p>Note that these optimizations are dependent on the data flow of the
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program. Therefore, if <em>any</em> <code>lituse</code> is paired with a
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<code>literal</code> relocation, then <em>all</em> uses of the register set by
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the <code>literal</code> instruction must also be marked with <code>lituse</code>
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relocations. This is because the original <code>literal</code> instruction
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may be deleted or transformed into another instruction.
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</p>
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<p>Also note that there may be a one-to-many relationship between
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<code>literal</code> and <code>lituse</code>, but not a many-to-one. That is, if
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there are two code paths that load up the same address and feed the
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value to a single use, then the use may not use a <code>lituse</code>
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relocation.
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</p>
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</dd>
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<dt><code>!lituse_base!<var>N</var></code></dt>
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<dd><p>Used with any memory format instruction (e.g. <code>ldl</code>) to indicate
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that the literal is used for an address load. The offset field of the
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instruction must be zero. During relaxation, the code may be altered
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to use a gp-relative load.
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</p>
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</dd>
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<dt><code>!lituse_jsr!<var>N</var></code></dt>
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<dd><p>Used with a register branch format instruction (e.g. <code>jsr</code>) to
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indicate that the literal is used for a call. During relaxation, the
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code may be altered to use a direct branch (e.g. <code>bsr</code>).
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</p>
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</dd>
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<dt><code>!lituse_jsrdirect!<var>N</var></code></dt>
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<dd><p>Similar to <code>lituse_jsr</code>, but also that this call cannot be vectored
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through a PLT entry. This is useful for functions with special calling
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conventions which do not allow the normal call-clobbered registers to be
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clobbered.
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</p>
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</dd>
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<dt><code>!lituse_bytoff!<var>N</var></code></dt>
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<dd><p>Used with a byte mask instruction (e.g. <code>extbl</code>) to indicate
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that only the low 3 bits of the address are relevant. During relaxation,
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the code may be altered to use an immediate instead of a register shift.
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</p>
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</dd>
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<dt><code>!lituse_addr!<var>N</var></code></dt>
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<dd><p>Used with any other instruction to indicate that the original address
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is in fact used, and the original <code>ldq</code> instruction may not be
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altered or deleted. This is useful in conjunction with <code>lituse_jsr</code>
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to test whether a weak symbol is defined.
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</p>
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<div class="example">
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<pre class="example">ldq $27,foo($29) !literal!1
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beq $27,is_undef !lituse_addr!1
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jsr $26,($27),foo !lituse_jsr!1
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</pre></div>
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</dd>
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<dt><code>!lituse_tlsgd!<var>N</var></code></dt>
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<dd><p>Used with a register branch format instruction to indicate that the
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literal is the call to <code>__tls_get_addr</code> used to compute the
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address of the thread-local storage variable whose descriptor was
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loaded with <code>!tlsgd!<var>N</var></code>.
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</p>
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</dd>
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<dt><code>!lituse_tlsldm!<var>N</var></code></dt>
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<dd><p>Used with a register branch format instruction to indicate that the
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literal is the call to <code>__tls_get_addr</code> used to compute the
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address of the base of the thread-local storage block for the current
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module. The descriptor for the module must have been loaded with
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<code>!tlsldm!<var>N</var></code>.
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</p>
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</dd>
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<dt><code>!gpdisp!<var>N</var></code></dt>
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<dd><p>Used with <code>ldah</code> and <code>lda</code> to load the GP from the current
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address, a-la the <code>ldgp</code> macro. The source register for the
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<code>ldah</code> instruction must contain the address of the <code>ldah</code>
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instruction. There must be exactly one <code>lda</code> instruction paired
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with the <code>ldah</code> instruction, though it may appear anywhere in
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the instruction stream. The immediate operands must be zero.
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</p>
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<div class="example">
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<pre class="example">bsr $26,foo
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ldah $29,0($26) !gpdisp!1
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lda $29,0($29) !gpdisp!1
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</pre></div>
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</dd>
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<dt><code>!gprelhigh</code></dt>
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<dd><p>Used with an <code>ldah</code> instruction to add the high 16 bits of a
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32-bit displacement from the GP.
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</p>
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</dd>
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<dt><code>!gprellow</code></dt>
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<dd><p>Used with any memory format instruction to add the low 16 bits of a
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32-bit displacement from the GP.
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</p>
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</dd>
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<dt><code>!gprel</code></dt>
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<dd><p>Used with any memory format instruction to add a 16-bit displacement
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from the GP.
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</p>
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</dd>
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<dt><code>!samegp</code></dt>
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<dd><p>Used with any branch format instruction to skip the GP load at the
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target address. The referenced symbol must have the same GP as the
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source object file, and it must be declared to either not use <code>$27</code>
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or perform a standard GP load in the first two instructions via the
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<code>.prologue</code> directive.
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</p>
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</dd>
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<dt><code>!tlsgd</code></dt>
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<dt><code>!tlsgd!<var>N</var></code></dt>
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<dd><p>Used with an <code>lda</code> instruction to load the address of a TLS
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descriptor for a symbol in the GOT.
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</p>
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<p>The sequence number <var>N</var> is optional, and if present it used to
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pair the descriptor load with both the <code>literal</code> loading the
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address of the <code>__tls_get_addr</code> function and the <code>lituse_tlsgd</code>
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marking the call to that function.
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</p>
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<p>For proper relaxation, both the <code>tlsgd</code>, <code>literal</code> and
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<code>lituse</code> relocations must be in the same extended basic block.
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That is, the relocation with the lowest address must be executed
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first at runtime.
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</p>
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</dd>
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<dt><code>!tlsldm</code></dt>
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<dt><code>!tlsldm!<var>N</var></code></dt>
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<dd><p>Used with an <code>lda</code> instruction to load the address of a TLS
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descriptor for the current module in the GOT.
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</p>
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<p>Similar in other respects to <code>tlsgd</code>.
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</p>
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</dd>
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<dt><code>!gotdtprel</code></dt>
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<dd><p>Used with an <code>ldq</code> instruction to load the offset of the TLS
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symbol within its module’s thread-local storage block. Also known
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as the dynamic thread pointer offset or dtp-relative offset.
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</p>
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</dd>
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<dt><code>!dtprelhi</code></dt>
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<dt><code>!dtprello</code></dt>
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<dt><code>!dtprel</code></dt>
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<dd><p>Like <code>gprel</code> relocations except they compute dtp-relative offsets.
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</p>
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</dd>
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<dt><code>!gottprel</code></dt>
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<dd><p>Used with an <code>ldq</code> instruction to load the offset of the TLS
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symbol from the thread pointer. Also known as the tp-relative offset.
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</p>
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</dd>
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<dt><code>!tprelhi</code></dt>
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<dt><code>!tprello</code></dt>
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<dt><code>!tprel</code></dt>
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<dd><p>Like <code>gprel</code> relocations except they compute tp-relative offsets.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Alpha_002dRegs.html#Alpha_002dRegs" accesskey="p" rel="prev">Alpha-Regs</a>, Up: <a href="Alpha-Syntax.html#Alpha-Syntax" accesskey="u" rel="up">Alpha Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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