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  3. <!-- This file documents the GNU linker LD
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  17. <title>LD: PowerPC64 ELF64</title>
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  29. <link href="SPU-ELF.html#SPU-ELF" rel="next" title="SPU ELF">
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    64. 

  64. <a name="PowerPC64-ELF64"></a>
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  65. <div class="header">
    66. 

  66. <p>
    67. 

  67. Next: <a href="SPU-ELF.html#SPU-ELF" accesskey="n" rel="next">SPU ELF</a>, Previous: <a href="PowerPC-ELF32.html#PowerPC-ELF32" accesskey="p" rel="prev">PowerPC ELF32</a>, Up: <a href="Machine-Dependent.html#Machine-Dependent" accesskey="u" rel="up">Machine Dependent</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
    68. 

  68. </div>
    69. 

  69. <hr>
    70. 

  70. <a name="ld-and-PowerPC64-64_002dbit-ELF-Support"></a>
    71. 

  71. <h3 class="section">4.13 <code>ld</code> and PowerPC64 64-bit ELF Support</h3>
    72. 

  72. 

  73. <a name="index-PowerPC64-ELF64-options"></a>
    74. 

  74. <dl compact="compact">
    75. 

  75. <dd><a name="index-PowerPC64-stub-grouping"></a>
    76. 

  76. <a name="index-_002d_002dstub_002dgroup_002dsize"></a>
    77. 

  77. </dd>
    78. 

  78. <dt><samp>--stub-group-size</samp></dt>
    79. 

  79. <dd><p>Long branch stubs, PLT call stubs  and TOC adjusting stubs are placed
    80. 

  80. by <code>ld</code> in stub sections located between groups of input sections.
    81. 

  81. &lsquo;<samp>--stub-group-size</samp>&rsquo; specifies the maximum size of a group of input
    82. 

  82. sections handled by one stub section.  Since branch offsets are signed,
    83. 

  83. a stub section may serve two groups of input sections, one group before
    84. 

  84. the stub section, and one group after it.  However, when using
    85. 

  85. conditional branches that require stubs, it may be better (for branch
    86. 

  86. prediction) that stub sections only serve one group of input sections.
    87. 

  87. A negative value for &lsquo;<samp>N</samp>&rsquo; chooses this scheme, ensuring that
    88. 

  88. branches to stubs always use a negative offset.  Two special values of
    89. 

  89. &lsquo;<samp>N</samp>&rsquo; are recognized, &lsquo;<samp>1</samp>&rsquo; and &lsquo;<samp>-1</samp>&rsquo;.  These both instruct
    90. 

  90. <code>ld</code> to automatically size input section groups for the branch types
    91. 

  91. detected, with the same behaviour regarding stub placement as other
    92. 

  92. positive or negative values of &lsquo;<samp>N</samp>&rsquo; respectively.
    93. 

  93. </p>
    94. 

  94. <p>Note that &lsquo;<samp>--stub-group-size</samp>&rsquo; does not split input sections.  A
    95. 

  95. single input section larger than the group size specified will of course
    96. 

  96. create a larger group (of one section).  If input sections are too
    97. 

  97. large, it may not be possible for a branch to reach its stub.
    98. 

  98. </p>
    99. 

  99. <a name="index-PowerPC64-stub-symbols"></a>
    100. 

  100. <a name="index-_002d_002demit_002dstub_002dsyms-1"></a>
    101. 

  101. </dd>
    102. 

  102. <dt><samp>--emit-stub-syms</samp></dt>
    103. 

  103. <dd><p>This option causes <code>ld</code> to label linker stubs with a local
    104. 

  104. symbol that encodes the stub type and destination.
    105. 

  105. </p>
    106. 

  106. <a name="index-PowerPC64-dot-symbols"></a>
    107. 

  107. <a name="index-_002d_002ddotsyms"></a>
    108. 

  108. <a name="index-_002d_002dno_002ddotsyms"></a>
    109. 

  109. </dd>
    110. 

  110. <dt><samp>--dotsyms, --no-dotsyms</samp></dt>
    111. 

  111. <dd><p>These two options control how <code>ld</code> interprets version patterns
    112. 

  112. in a version script.  Older PowerPC64 compilers emitted both a
    113. 

  113. function descriptor symbol with the same name as the function, and a
    114. 

  114. code entry symbol with the name prefixed by a dot (&lsquo;<samp>.</samp>&rsquo;).  To
    115. 

  115. properly version a function &lsquo;<samp>foo</samp>&rsquo;, the version script thus needs
    116. 

  116. to control both &lsquo;<samp>foo</samp>&rsquo; and &lsquo;<samp>.foo</samp>&rsquo;.  The option
    117. 

  117. &lsquo;<samp>--dotsyms</samp>&rsquo;, on by default, automatically adds the required
    118. 

  118. dot-prefixed patterns.  Use &lsquo;<samp>--no-dotsyms</samp>&rsquo; to disable this
    119. 

  119. feature.
    120. 

  120. </p>
    121. 

  121. <a name="index-PowerPC64-TLS-optimization"></a>
    122. 

  122. <a name="index-_002d_002dno_002dtls_002doptimize-1"></a>
    123. 

  123. </dd>
    124. 

  124. <dt><samp>--no-tls-optimize</samp></dt>
    125. 

  125. <dd><p>PowerPC64 <code>ld</code> normally performs some optimization of code
    126. 

  126. sequences used to access Thread-Local Storage.  Use this option to
    127. 

  127. disable the optimization.
    128. 

  128. </p>
    129. 

  129. <a name="index-PowerPC64-OPD-optimization"></a>
    130. 

  130. <a name="index-_002d_002dno_002dopd_002doptimize"></a>
    131. 

  131. </dd>
    132. 

  132. <dt><samp>--no-opd-optimize</samp></dt>
    133. 

  133. <dd><p>PowerPC64 <code>ld</code> normally removes <code>.opd</code> section entries
    134. 

  134. corresponding to deleted link-once functions, or functions removed by
    135. 

  135. the action of &lsquo;<samp>--gc-sections</samp>&rsquo; or linker script <code>/DISCARD/</code>.
    136. 

  136. Use this option to disable <code>.opd</code> optimization.
    137. 

  137. </p>
    138. 

  138. <a name="index-PowerPC64-OPD-spacing"></a>
    139. 

  139. <a name="index-_002d_002dnon_002doverlapping_002dopd"></a>
    140. 

  140. </dd>
    141. 

  141. <dt><samp>--non-overlapping-opd</samp></dt>
    142. 

  142. <dd><p>Some PowerPC64 compilers have an option to generate compressed
    143. 

  143. <code>.opd</code> entries spaced 16 bytes apart, overlapping the third word,
    144. 

  144. the static chain pointer (unused in C) with the first word of the next
    145. 

  145. entry.  This option expands such entries to the full 24 bytes.
    146. 

  146. </p>
    147. 

  147. <a name="index-PowerPC64-TOC-optimization"></a>
    148. 

  148. <a name="index-_002d_002dno_002dtoc_002doptimize"></a>
    149. 

  149. </dd>
    150. 

  150. <dt><samp>--no-toc-optimize</samp></dt>
    151. 

  151. <dd><p>PowerPC64 <code>ld</code> normally removes unused <code>.toc</code> section
    152. 

  152. entries.  Such entries are detected by examining relocations that
    153. 

  153. reference the TOC in code sections.  A reloc in a deleted code section
    154. 

  154. marks a TOC word as unneeded, while a reloc in a kept code section
    155. 

  155. marks a TOC word as needed.  Since the TOC may reference itself, TOC
    156. 

  156. relocs are also examined.  TOC words marked as both needed and
    157. 

  157. unneeded will of course be kept.  TOC words without any referencing
    158. 

  158. reloc are assumed to be part of a multi-word entry, and are kept or
    159. 

  159. discarded as per the nearest marked preceding word.  This works
    160. 

  160. reliably for compiler generated code, but may be incorrect if assembly
    161. 

  161. code is used to insert TOC entries.  Use this option to disable the
    162. 

  162. optimization.
    163. 

  163. </p>
    164. 

  164. <a name="index-PowerPC64-multi_002dTOC"></a>
    165. 

  165. <a name="index-_002d_002dno_002dmulti_002dtoc"></a>
    166. 

  166. </dd>
    167. 

  167. <dt><samp>--no-multi-toc</samp></dt>
    168. 

  168. <dd><p>If given any toc option besides <code>-mcmodel=medium</code> or
    169. 

  169. <code>-mcmodel=large</code>, PowerPC64 GCC generates code for a TOC model
    170. 

  170. where TOC
    171. 

  171. entries are accessed with a 16-bit offset from r2.  This limits the
    172. 

  172. total TOC size to 64K.  PowerPC64 <code>ld</code> extends this limit by
    173. 

  173. grouping code sections such that each group uses less than 64K for its
    174. 

  174. TOC entries, then inserts r2 adjusting stubs between inter-group
    175. 

  175. calls.  <code>ld</code> does not split apart input sections, so cannot
    176. 

  176. help if a single input file has a <code>.toc</code> section that exceeds
    177. 

  177. 64K, most likely from linking multiple files with <code>ld -r</code>.
    178. 

  178. Use this option to turn off this feature.
    179. 

  179. </p>
    180. 

  180. <a name="index-PowerPC64-TOC-sorting"></a>
    181. 

  181. <a name="index-_002d_002dno_002dtoc_002dsort"></a>
    182. 

  182. </dd>
    183. 

  183. <dt><samp>--no-toc-sort</samp></dt>
    184. 

  184. <dd><p>By default, <code>ld</code> sorts TOC sections so that those whose file
    185. 

  185. happens to have a section called <code>.init</code> or <code>.fini</code> are
    186. 

  186. placed first, followed by TOC sections referenced by code generated
    187. 

  187. with PowerPC64 gcc&rsquo;s <code>-mcmodel=small</code>, and lastly TOC sections
    188. 

  188. referenced only by code generated with PowerPC64 gcc&rsquo;s
    189. 

  189. <code>-mcmodel=medium</code> or <code>-mcmodel=large</code> options.  Doing this
    190. 

  190. results in better TOC grouping for multi-TOC.  Use this option to turn
    191. 

  191. off this feature.
    192. 

  192. </p>
    193. 

  193. <a name="index-PowerPC64-PLT-stub-alignment"></a>
    194. 

  194. <a name="index-_002d_002dplt_002dalign"></a>
    195. 

  195. <a name="index-_002d_002dno_002dplt_002dalign"></a>
    196. 

  196. </dd>
    197. 

  197. <dt><samp>--plt-align</samp></dt>
    198. 

  198. <dt><samp>--no-plt-align</samp></dt>
    199. 

  199. <dd><p>Use these options to control whether individual PLT call stubs are
    200. 

  200. padded so that they don&rsquo;t cross a 32-byte boundary, or to the
    201. 

  201. specified power of two boundary when using <code>--plt-align=</code>.  Note
    202. 

  202. that this isn&rsquo;t alignment in the usual sense.  By default PLT call
    203. 

  203. stubs are packed tightly.
    204. 

  204. </p>
    205. 

  205. <a name="index-PowerPC64-PLT-call-stub-static-chain"></a>
    206. 

  206. <a name="index-_002d_002dplt_002dstatic_002dchain"></a>
    207. 

  207. <a name="index-_002d_002dno_002dplt_002dstatic_002dchain"></a>
    208. 

  208. </dd>
    209. 

  209. <dt><samp>--plt-static-chain</samp></dt>
    210. 

  210. <dt><samp>--no-plt-static-chain</samp></dt>
    211. 

  211. <dd><p>Use these options to control whether PLT call stubs load the static
    212. 

  212. chain pointer (r11).  <code>ld</code> defaults to not loading the static
    213. 

  213. chain since there is never any need to do so on a PLT call.
    214. 

  214. </p>
    215. 

  215. <a name="index-PowerPC64-PLT-call-stub-thread-safety"></a>
    216. 

  216. <a name="index-_002d_002dplt_002dthread_002dsafe"></a>
    217. 

  217. <a name="index-_002d_002dno_002dplt_002dthread_002dsafe"></a>
    218. 

  218. </dd>
    219. 

  219. <dt><samp>--plt-thread-safe</samp></dt>
    220. 

  220. <dt><samp>--no-thread-safe</samp></dt>
    221. 

  221. <dd><p>With power7&rsquo;s weakly ordered memory model, it is possible when using
    222. 

  222. lazy binding for ld.so to update a plt entry in one thread and have
    223. 

  223. another thread see the individual plt entry words update in the wrong
    224. 

  224. order, despite ld.so carefully writing in the correct order and using
    225. 

  225. memory write barriers.  To avoid this we need some sort of read
    226. 

  226. barrier in the call stub, or use LD_BIND_NOW=1.  By default, <code>ld</code>
    227. 

  227. looks for calls to commonly used functions that create threads, and if
    228. 

  228. seen, adds the necessary barriers.  Use these options to change the
    229. 

  229. default behaviour.
    230. 

  230. </p></dd>
    231. 

  231. </dl>
    232. 

  232. 

  233. 

  234. 

  235. <hr>
    236. 

  236. <div class="header">
    237. 

  237. <p>
    238. 

  238. Next: <a href="SPU-ELF.html#SPU-ELF" accesskey="n" rel="next">SPU ELF</a>, Previous: <a href="PowerPC-ELF32.html#PowerPC-ELF32" accesskey="p" rel="prev">PowerPC ELF32</a>, Up: <a href="Machine-Dependent.html#Machine-Dependent" accesskey="u" rel="up">Machine Dependent</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
    239. 

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    240. 

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    245. 

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