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190 lines
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<title>PowerPC64 ELF64 - Untitled</title>
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<meta name="description" content="Untitled">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Machine-Dependent.html#Machine-Dependent" title="Machine Dependent">
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<link rel="prev" href="PowerPC-ELF32.html#PowerPC-ELF32" title="PowerPC ELF32">
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<link rel="next" href="S_002f390-ELF.html#S_002f390-ELF" title="S/390 ELF">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU linker LD
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(GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10))
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version 2.33.1.
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled ``GNU Free Documentation License''.-->
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</head>
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<body>
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<div class="node">
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<a name="PowerPC64-ELF64"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="S_002f390-ELF.html#S_002f390-ELF">S/390 ELF</a>,
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Previous: <a rel="previous" accesskey="p" href="PowerPC-ELF32.html#PowerPC-ELF32">PowerPC ELF32</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Dependent.html#Machine-Dependent">Machine Dependent</a>
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<hr>
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</div>
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<h3 class="section">4.12 <samp><span class="command">ld</span></samp> and PowerPC64 64-bit ELF Support</h3>
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<p><a name="index-PowerPC64-ELF64-options-714"></a>
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<a name="index-PowerPC64-stub-grouping-715"></a>
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<a name="index-g_t_002d_002dstub_002dgroup_002dsize-716"></a>
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<dl><dt><samp><span class="option">--stub-group-size</span></samp><dd>Long branch stubs, PLT call stubs and TOC adjusting stubs are placed
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by <samp><span class="command">ld</span></samp> in stub sections located between groups of input sections.
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‘<samp><span class="samp">--stub-group-size</span></samp>’ specifies the maximum size of a group of input
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sections handled by one stub section. Since branch offsets are signed,
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a stub section may serve two groups of input sections, one group before
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the stub section, and one group after it. However, when using
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conditional branches that require stubs, it may be better (for branch
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prediction) that stub sections only serve one group of input sections.
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A negative value for ‘<samp><span class="samp">N</span></samp>’ chooses this scheme, ensuring that
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branches to stubs always use a negative offset. Two special values of
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‘<samp><span class="samp">N</span></samp>’ are recognized, ‘<samp><span class="samp">1</span></samp>’ and ‘<samp><span class="samp">-1</span></samp>’. These both instruct
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<samp><span class="command">ld</span></samp> to automatically size input section groups for the branch types
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detected, with the same behaviour regarding stub placement as other
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positive or negative values of ‘<samp><span class="samp">N</span></samp>’ respectively.
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<p>Note that ‘<samp><span class="samp">--stub-group-size</span></samp>’ does not split input sections. A
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single input section larger than the group size specified will of course
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create a larger group (of one section). If input sections are too
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large, it may not be possible for a branch to reach its stub.
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<p><a name="index-PowerPC64-stub-symbols-717"></a><a name="index-g_t_002d_002demit_002dstub_002dsyms-718"></a><br><dt><samp><span class="option">--emit-stub-syms</span></samp><dd>This option causes <samp><span class="command">ld</span></samp> to label linker stubs with a local
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symbol that encodes the stub type and destination.
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<p><a name="index-PowerPC64-dot-symbols-719"></a><a name="index-g_t_002d_002ddotsyms-720"></a><a name="index-g_t_002d_002dno_002ddotsyms-721"></a><br><dt><samp><span class="option">--dotsyms</span></samp><dt><samp><span class="option">--no-dotsyms</span></samp><dd>These two options control how <samp><span class="command">ld</span></samp> interprets version patterns
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in a version script. Older PowerPC64 compilers emitted both a
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function descriptor symbol with the same name as the function, and a
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code entry symbol with the name prefixed by a dot (‘<samp><span class="samp">.</span></samp>’). To
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properly version a function ‘<samp><span class="samp">foo</span></samp>’, the version script thus needs
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to control both ‘<samp><span class="samp">foo</span></samp>’ and ‘<samp><span class="samp">.foo</span></samp>’. The option
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‘<samp><span class="samp">--dotsyms</span></samp>’, on by default, automatically adds the required
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dot-prefixed patterns. Use ‘<samp><span class="samp">--no-dotsyms</span></samp>’ to disable this
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feature.
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<p><a name="index-PowerPC64-register-save_002frestore-functions-722"></a><a name="index-g_t_002d_002dsave_002drestore_002dfuncs-723"></a><a name="index-g_t_002d_002dno_002dsave_002drestore_002dfuncs-724"></a><br><dt><samp><span class="option">--save-restore-funcs</span></samp><dt><samp><span class="option">--no-save-restore-funcs</span></samp><dd>These two options control whether PowerPC64 <samp><span class="command">ld</span></samp> automatically
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provides out-of-line register save and restore functions used by
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‘<samp><span class="samp">-Os</span></samp>’ code. The default is to provide any such referenced
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function for a normal final link, and to not do so for a relocatable
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link.
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<p><a name="index-PowerPC64-TLS-optimization-725"></a><a name="index-g_t_002d_002dno_002dtls_002doptimize-726"></a><br><dt><samp><span class="option">--no-tls-optimize</span></samp><dd>PowerPC64 <samp><span class="command">ld</span></samp> normally performs some optimization of code
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sequences used to access Thread-Local Storage. Use this option to
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disable the optimization.
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<p><a name="index-PowerPC64-_005f_005ftls_005fget_005faddr-optimization-727"></a><a name="index-g_t_002d_002dtls_002dget_002daddr_002doptimize-728"></a><a name="index-g_t_002d_002dno_002dtls_002dget_002daddr_002doptimize-729"></a><br><dt><samp><span class="option">--tls-get-addr-optimize</span></samp><dt><samp><span class="option">--no-tls-get-addr-optimize</span></samp><dd>These options control whether PowerPC64 <samp><span class="command">ld</span></samp> uses a special
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stub to call __tls_get_addr. PowerPC64 glibc 2.22 and later support
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an optimization that allows the second and subsequent calls to
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<code>__tls_get_addr</code> for a given symbol to be resolved by the special
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stub without calling in to glibc. By default the linker enables this
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option when glibc advertises the availability of __tls_get_addr_opt.
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Forcing this option on when using an older glibc won't do much besides
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slow down your applications, but may be useful if linking an
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application against an older glibc with the expectation that it will
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normally be used on systems having a newer glibc.
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<p><a name="index-PowerPC64-OPD-optimization-730"></a><a name="index-g_t_002d_002dno_002dopd_002doptimize-731"></a><br><dt><samp><span class="option">--no-opd-optimize</span></samp><dd>PowerPC64 <samp><span class="command">ld</span></samp> normally removes <code>.opd</code> section entries
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corresponding to deleted link-once functions, or functions removed by
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the action of ‘<samp><span class="samp">--gc-sections</span></samp>’ or linker script <code>/DISCARD/</code>.
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Use this option to disable <code>.opd</code> optimization.
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<p><a name="index-PowerPC64-OPD-spacing-732"></a><a name="index-g_t_002d_002dnon_002doverlapping_002dopd-733"></a><br><dt><samp><span class="option">--non-overlapping-opd</span></samp><dd>Some PowerPC64 compilers have an option to generate compressed
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<code>.opd</code> entries spaced 16 bytes apart, overlapping the third word,
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the static chain pointer (unused in C) with the first word of the next
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entry. This option expands such entries to the full 24 bytes.
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<p><a name="index-PowerPC64-TOC-optimization-734"></a><a name="index-g_t_002d_002dno_002dtoc_002doptimize-735"></a><br><dt><samp><span class="option">--no-toc-optimize</span></samp><dd>PowerPC64 <samp><span class="command">ld</span></samp> normally removes unused <code>.toc</code> section
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entries. Such entries are detected by examining relocations that
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reference the TOC in code sections. A reloc in a deleted code section
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marks a TOC word as unneeded, while a reloc in a kept code section
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marks a TOC word as needed. Since the TOC may reference itself, TOC
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relocs are also examined. TOC words marked as both needed and
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unneeded will of course be kept. TOC words without any referencing
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reloc are assumed to be part of a multi-word entry, and are kept or
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discarded as per the nearest marked preceding word. This works
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reliably for compiler generated code, but may be incorrect if assembly
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code is used to insert TOC entries. Use this option to disable the
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optimization.
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<p><a name="index-PowerPC64-multi_002dTOC-736"></a><a name="index-g_t_002d_002dno_002dmulti_002dtoc-737"></a><br><dt><samp><span class="option">--no-multi-toc</span></samp><dd>If given any toc option besides <code>-mcmodel=medium</code> or
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<code>-mcmodel=large</code>, PowerPC64 GCC generates code for a TOC model
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where TOC
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entries are accessed with a 16-bit offset from r2. This limits the
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total TOC size to 64K. PowerPC64 <samp><span class="command">ld</span></samp> extends this limit by
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grouping code sections such that each group uses less than 64K for its
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TOC entries, then inserts r2 adjusting stubs between inter-group
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calls. <samp><span class="command">ld</span></samp> does not split apart input sections, so cannot
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help if a single input file has a <code>.toc</code> section that exceeds
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64K, most likely from linking multiple files with <samp><span class="command">ld -r</span></samp>.
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Use this option to turn off this feature.
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<p><a name="index-PowerPC64-TOC-sorting-738"></a><a name="index-g_t_002d_002dno_002dtoc_002dsort-739"></a><br><dt><samp><span class="option">--no-toc-sort</span></samp><dd>By default, <samp><span class="command">ld</span></samp> sorts TOC sections so that those whose file
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happens to have a section called <code>.init</code> or <code>.fini</code> are
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placed first, followed by TOC sections referenced by code generated
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with PowerPC64 gcc's <code>-mcmodel=small</code>, and lastly TOC sections
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referenced only by code generated with PowerPC64 gcc's
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<code>-mcmodel=medium</code> or <code>-mcmodel=large</code> options. Doing this
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results in better TOC grouping for multi-TOC. Use this option to turn
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off this feature.
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<p><a name="index-PowerPC64-PLT-stub-alignment-740"></a><a name="index-g_t_002d_002dplt_002dalign-741"></a><a name="index-g_t_002d_002dno_002dplt_002dalign-742"></a><br><dt><samp><span class="option">--plt-align</span></samp><dt><samp><span class="option">--no-plt-align</span></samp><dd>Use these options to control whether individual PLT call stubs are
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aligned to a 32-byte boundary, or to the specified power of two
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boundary when using <code>--plt-align=</code>. A negative value may be
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specified to pad PLT call stubs so that they do not cross the
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specified power of two boundary (or the minimum number of boundaries
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if a PLT stub is so large that it must cross a boundary). By default
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PLT call stubs are aligned to 32-byte boundaries.
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<p><a name="index-PowerPC64-PLT-call-stub-static-chain-743"></a><a name="index-g_t_002d_002dplt_002dstatic_002dchain-744"></a><a name="index-g_t_002d_002dno_002dplt_002dstatic_002dchain-745"></a><br><dt><samp><span class="option">--plt-static-chain</span></samp><dt><samp><span class="option">--no-plt-static-chain</span></samp><dd>Use these options to control whether PLT call stubs load the static
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chain pointer (r11). <code>ld</code> defaults to not loading the static
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chain since there is never any need to do so on a PLT call.
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<p><a name="index-PowerPC64-PLT-call-stub-thread-safety-746"></a><a name="index-g_t_002d_002dplt_002dthread_002dsafe-747"></a><a name="index-g_t_002d_002dno_002dplt_002dthread_002dsafe-748"></a><br><dt><samp><span class="option">--plt-thread-safe</span></samp><dt><samp><span class="option">--no-plt-thread-safe</span></samp><dd>With power7's weakly ordered memory model, it is possible when using
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lazy binding for ld.so to update a plt entry in one thread and have
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another thread see the individual plt entry words update in the wrong
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order, despite ld.so carefully writing in the correct order and using
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memory write barriers. To avoid this we need some sort of read
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barrier in the call stub, or use LD_BIND_NOW=1. By default, <code>ld</code>
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looks for calls to commonly used functions that create threads, and if
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seen, adds the necessary barriers. Use these options to change the
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default behaviour.
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<p><a name="index-PowerPC64-ELFv2-PLT-localentry-optimization-749"></a><a name="index-g_t_002d_002dplt_002dlocalentry-750"></a><a name="index-g_t_002d_002dno_002dplt_002dlocalentry-751"></a><br><dt><samp><span class="option">--plt-localentry</span></samp><dt><samp><span class="option">--no-localentry</span></samp><dd>ELFv2 functions with localentry:0 are those with a single entry point,
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ie. global entry == local entry, and that have no requirement on r2
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(the TOC/GOT pointer) or r12, and guarantee r2 is unchanged on return.
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Such an external function can be called via the PLT without saving r2
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or restoring it on return, avoiding a common load-hit-store for small
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functions. The optimization is attractive, with up to 40% reduction
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in execution time for a small function, but can result in symbol
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interposition failures. Also, minor changes in a shared library,
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including system libraries, can cause a function that was localentry:0
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to become localentry:8. This will result in a dynamic loader
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complaint and failure to run. The option is experimental, use with
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care. <samp><span class="option">--no-plt-localentry</span></samp> is the default.
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</dl>
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</body></html>
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