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| <h3 class="section">4.10 <samp><span class="command">ld</span></samp> and PowerPC64 64-bit ELF Support</h3> |
| |
| <p><a name="index-PowerPC64-ELF64-options-636"></a> |
| |
| <a name="index-PowerPC64-stub-grouping-637"></a> |
| <a name="index-g_t_002d_002dstub_002dgroup_002dsize-638"></a> |
| <dl><dt><samp><span class="option">--stub-group-size</span></samp><dd>Long branch stubs, PLT call stubs and TOC adjusting stubs are placed |
| by <samp><span class="command">ld</span></samp> in stub sections located between groups of input sections. |
| ‘<samp><span class="samp">--stub-group-size</span></samp>’ specifies the maximum size of a group of input |
| sections handled by one stub section. Since branch offsets are signed, |
| a stub section may serve two groups of input sections, one group before |
| the stub section, and one group after it. However, when using |
| conditional branches that require stubs, it may be better (for branch |
| prediction) that stub sections only serve one group of input sections. |
| A negative value for ‘<samp><span class="samp">N</span></samp>’ chooses this scheme, ensuring that |
| branches to stubs always use a negative offset. Two special values of |
| ‘<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 |
| <samp><span class="command">ld</span></samp> to automatically size input section groups for the branch types |
| detected, with the same behaviour regarding stub placement as other |
| positive or negative values of ‘<samp><span class="samp">N</span></samp>’ respectively. |
| |
| <p>Note that ‘<samp><span class="samp">--stub-group-size</span></samp>’ does not split input sections. A |
| single input section larger than the group size specified will of course |
| create a larger group (of one section). If input sections are too |
| large, it may not be possible for a branch to reach its stub. |
| |
| <p><a name="index-PowerPC64-stub-symbols-639"></a><a name="index-g_t_002d_002demit_002dstub_002dsyms-640"></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 |
| symbol that encodes the stub type and destination. |
| |
| <p><a name="index-PowerPC64-dot-symbols-641"></a><a name="index-g_t_002d_002ddotsyms-642"></a><a name="index-g_t_002d_002dno_002ddotsyms-643"></a><br><dt><samp><span class="option">--dotsyms, --no-dotsyms</span></samp><dd>These two options control how <samp><span class="command">ld</span></samp> interprets version patterns |
| in a version script. Older PowerPC64 compilers emitted both a |
| function descriptor symbol with the same name as the function, and a |
| code entry symbol with the name prefixed by a dot (‘<samp><span class="samp">.</span></samp>’). To |
| properly version a function ‘<samp><span class="samp">foo</span></samp>’, the version script thus needs |
| to control both ‘<samp><span class="samp">foo</span></samp>’ and ‘<samp><span class="samp">.foo</span></samp>’. The option |
| ‘<samp><span class="samp">--dotsyms</span></samp>’, on by default, automatically adds the required |
| dot-prefixed patterns. Use ‘<samp><span class="samp">--no-dotsyms</span></samp>’ to disable this |
| feature. |
| |
| <p><a name="index-PowerPC64-TLS-optimization-644"></a><a name="index-g_t_002d_002dno_002dtls_002doptimize-645"></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 |
| sequences used to access Thread-Local Storage. Use this option to |
| disable the optimization. |
| |
| <p><a name="index-PowerPC64-OPD-optimization-646"></a><a name="index-g_t_002d_002dno_002dopd_002doptimize-647"></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 |
| corresponding to deleted link-once functions, or functions removed by |
| the action of ‘<samp><span class="samp">--gc-sections</span></samp>’ or linker script <code>/DISCARD/</code>. |
| Use this option to disable <code>.opd</code> optimization. |
| |
| <p><a name="index-PowerPC64-OPD-spacing-648"></a><a name="index-g_t_002d_002dnon_002doverlapping_002dopd-649"></a><br><dt><samp><span class="option">--non-overlapping-opd</span></samp><dd>Some PowerPC64 compilers have an option to generate compressed |
| <code>.opd</code> entries spaced 16 bytes apart, overlapping the third word, |
| the static chain pointer (unused in C) with the first word of the next |
| entry. This option expands such entries to the full 24 bytes. |
| |
| <p><a name="index-PowerPC64-TOC-optimization-650"></a><a name="index-g_t_002d_002dno_002dtoc_002doptimize-651"></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 |
| entries. Such entries are detected by examining relocations that |
| reference the TOC in code sections. A reloc in a deleted code section |
| marks a TOC word as unneeded, while a reloc in a kept code section |
| marks a TOC word as needed. Since the TOC may reference itself, TOC |
| relocs are also examined. TOC words marked as both needed and |
| unneeded will of course be kept. TOC words without any referencing |
| reloc are assumed to be part of a multi-word entry, and are kept or |
| discarded as per the nearest marked preceding word. This works |
| reliably for compiler generated code, but may be incorrect if assembly |
| code is used to insert TOC entries. Use this option to disable the |
| optimization. |
| |
| <p><a name="index-PowerPC64-multi_002dTOC-652"></a><a name="index-g_t_002d_002dno_002dmulti_002dtoc-653"></a><br><dt><samp><span class="option">--no-multi-toc</span></samp><dd>If given any toc option besides <code>-mcmodel=medium</code> or |
| <code>-mcmodel=large</code>, PowerPC64 GCC generates code for a TOC model |
| where TOC |
| entries are accessed with a 16-bit offset from r2. This limits the |
| total TOC size to 64K. PowerPC64 <samp><span class="command">ld</span></samp> extends this limit by |
| grouping code sections such that each group uses less than 64K for its |
| TOC entries, then inserts r2 adjusting stubs between inter-group |
| calls. <samp><span class="command">ld</span></samp> does not split apart input sections, so cannot |
| help if a single input file has a <code>.toc</code> section that exceeds |
| 64K, most likely from linking multiple files with <samp><span class="command">ld -r</span></samp>. |
| Use this option to turn off this feature. |
| |
| <p><a name="index-PowerPC64-TOC-sorting-654"></a><a name="index-g_t_002d_002dno_002dtoc_002dsort-655"></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 |
| happens to have a section called <code>.init</code> or <code>.fini</code> are |
| placed first, followed by TOC sections referenced by code generated |
| with PowerPC64 gcc's <code>-mcmodel=small</code>, and lastly TOC sections |
| referenced only by code generated with PowerPC64 gcc's |
| <code>-mcmodel=medium</code> or <code>-mcmodel=large</code> options. Doing this |
| results in better TOC grouping for multi-TOC. Use this option to turn |
| off this feature. |
| |
| <p><a name="index-PowerPC64-PLT-stub-alignment-656"></a><a name="index-g_t_002d_002dplt_002dalign-657"></a><a name="index-g_t_002d_002dno_002dplt_002dalign-658"></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 |
| aligned to a 32-byte boundary, or to the specified power of two |
| boundary when using <code>--plt-align=</code>. By default PLT call stubs |
| are packed tightly. |
| |
| <p><a name="index-PowerPC64-PLT-call-stub-static-chain-659"></a><a name="index-g_t_002d_002dplt_002dstatic_002dchain-660"></a><a name="index-g_t_002d_002dno_002dplt_002dstatic_002dchain-661"></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 |
| chain pointer (r11). <code>ld</code> defaults to not loading the static |
| chain since there is never any need to do so on a PLT call. |
| |
| <p><a name="index-PowerPC64-PLT-call-stub-thread-safety-662"></a><a name="index-g_t_002d_002dplt_002dthread_002dsafe-663"></a><a name="index-g_t_002d_002dno_002dplt_002dthread_002dsafe-664"></a><br><dt><samp><span class="option">--plt-thread-safe</span></samp><dt><samp><span class="option">--no-thread-safe</span></samp><dd>With power7's weakly ordered memory model, it is possible when using |
| lazy binding for ld.so to update a plt entry in one thread and have |
| another thread see the individual plt entry words update in the wrong |
| order, despite ld.so carefully writing in the correct order and using |
| memory write barriers. To avoid this we need some sort of read |
| barrier in the call stub, or use LD_BIND_NOW=1. By default, <code>ld</code> |
| looks for calls to commonly used functions that create threads, and if |
| seen, adds the necessary barriers. Use these options to change the |
| default behaviour. |
| </dl> |
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