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| <h3 class="section">4.9 <samp><span class="command">ld</span></samp> and PowerPC 32-bit ELF Support</h3> |
| |
| <p><a name="index-PowerPC-long-branches-614"></a><a name="index-g_t_002d_002drelax-on-PowerPC-615"></a>Branches on PowerPC processors are limited to a signed 26-bit |
| displacement, which may result in <samp><span class="command">ld</span></samp> giving |
| ‘<samp><span class="samp">relocation truncated to fit</span></samp>’ errors with very large programs. |
| ‘<samp><span class="samp">--relax</span></samp>’ enables the generation of trampolines that can access |
| the entire 32-bit address space. These trampolines are inserted at |
| section boundaries, so may not themselves be reachable if an input |
| section exceeds 33M in size. You may combine ‘<samp><span class="samp">-r</span></samp>’ and |
| ‘<samp><span class="samp">--relax</span></samp>’ to add trampolines in a partial link. In that case |
| both branches to undefined symbols and inter-section branches are also |
| considered potentially out of range, and trampolines inserted. |
| |
| <p><a name="index-PowerPC-ELF32-options-616"></a> |
| |
| <a name="index-PowerPC-PLT-617"></a> |
| <a name="index-g_t_002d_002dbss_002dplt-618"></a> |
| <dl><dt><samp><span class="option">--bss-plt</span></samp><dd>Current PowerPC GCC accepts a ‘<samp><span class="samp">-msecure-plt</span></samp>’ option that |
| generates code capable of using a newer PLT and GOT layout that has |
| the security advantage of no executable section ever needing to be |
| writable and no writable section ever being executable. PowerPC |
| <samp><span class="command">ld</span></samp> will generate this layout, including stubs to access the |
| PLT, if all input files (including startup and static libraries) were |
| compiled with ‘<samp><span class="samp">-msecure-plt</span></samp>’. ‘<samp><span class="samp">--bss-plt</span></samp>’ forces the old |
| BSS PLT (and GOT layout) which can give slightly better performance. |
| |
| <p><a name="index-g_t_002d_002dsecure_002dplt-619"></a><br><dt><samp><span class="option">--secure-plt</span></samp><dd><samp><span class="command">ld</span></samp> will use the new PLT and GOT layout if it is linking new |
| ‘<samp><span class="samp">-fpic</span></samp>’ or ‘<samp><span class="samp">-fPIC</span></samp>’ code, but does not do so automatically |
| when linking non-PIC code. This option requests the new PLT and GOT |
| layout. A warning will be given if some object file requires the old |
| style BSS PLT. |
| |
| <p><a name="index-PowerPC-GOT-620"></a><a name="index-g_t_002d_002dsdata_002dgot-621"></a><br><dt><samp><span class="option">--sdata-got</span></samp><dd>The new secure PLT and GOT are placed differently relative to other |
| sections compared to older BSS PLT and GOT placement. The location of |
| <code>.plt</code> must change because the new secure PLT is an initialized |
| section while the old PLT is uninitialized. The reason for the |
| <code>.got</code> change is more subtle: The new placement allows |
| <code>.got</code> to be read-only in applications linked with |
| ‘<samp><span class="samp">-z relro -z now</span></samp>’. However, this placement means that |
| <code>.sdata</code> cannot always be used in shared libraries, because the |
| PowerPC ABI accesses <code>.sdata</code> in shared libraries from the GOT |
| pointer. ‘<samp><span class="samp">--sdata-got</span></samp>’ forces the old GOT placement. PowerPC |
| GCC doesn't use <code>.sdata</code> in shared libraries, so this option is |
| really only useful for other compilers that may do so. |
| |
| <p><a name="index-PowerPC-stub-symbols-622"></a><a name="index-g_t_002d_002demit_002dstub_002dsyms-623"></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-PowerPC-TLS-optimization-624"></a><a name="index-g_t_002d_002dno_002dtls_002doptimize-625"></a><br><dt><samp><span class="option">--no-tls-optimize</span></samp><dd>PowerPC <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. |
| </dl> |
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