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| <h3 class="section">4.14 <code>ld</code> and Xtensa Processors</h3> |
| |
| <p><a name="index-Xtensa-processors-672"></a>The default <samp><span class="command">ld</span></samp> behavior for Xtensa processors is to interpret |
| <code>SECTIONS</code> commands so that lists of explicitly named sections in a |
| specification with a wildcard file will be interleaved when necessary to |
| keep literal pools within the range of PC-relative load offsets. For |
| example, with the command: |
| |
| <pre class="smallexample"> SECTIONS |
| { |
| .text : { |
| *(.literal .text) |
| } |
| } |
| </pre> |
| <p class="noindent"><samp><span class="command">ld</span></samp> may interleave some of the <code>.literal</code> |
| and <code>.text</code> sections from different object files to ensure that the |
| literal pools are within the range of PC-relative load offsets. A valid |
| interleaving might place the <code>.literal</code> sections from an initial |
| group of files followed by the <code>.text</code> sections of that group of |
| files. Then, the <code>.literal</code> sections from the rest of the files |
| and the <code>.text</code> sections from the rest of the files would follow. |
| |
| <p><a name="index-g_t_0040option_007b_002d_002drelax_007d-on-Xtensa-673"></a><a name="index-relaxing-on-Xtensa-674"></a>Relaxation is enabled by default for the Xtensa version of <samp><span class="command">ld</span></samp> and |
| provides two important link-time optimizations. The first optimization |
| is to combine identical literal values to reduce code size. A redundant |
| literal will be removed and all the <code>L32R</code> instructions that use it |
| will be changed to reference an identical literal, as long as the |
| location of the replacement literal is within the offset range of all |
| the <code>L32R</code> instructions. The second optimization is to remove |
| unnecessary overhead from assembler-generated “longcall” sequences of |
| <code>L32R</code>/<code>CALLX</code><var>n</var> when the target functions are within |
| range of direct <code>CALL</code><var>n</var> instructions. |
| |
| <p>For each of these cases where an indirect call sequence can be optimized |
| to a direct call, the linker will change the <code>CALLX</code><var>n</var> |
| instruction to a <code>CALL</code><var>n</var> instruction, remove the <code>L32R</code> |
| instruction, and remove the literal referenced by the <code>L32R</code> |
| instruction if it is not used for anything else. Removing the |
| <code>L32R</code> instruction always reduces code size but can potentially |
| hurt performance by changing the alignment of subsequent branch targets. |
| By default, the linker will always preserve alignments, either by |
| switching some instructions between 24-bit encodings and the equivalent |
| density instructions or by inserting a no-op in place of the <code>L32R</code> |
| instruction that was removed. If code size is more important than |
| performance, the <samp><span class="option">--size-opt</span></samp> option can be used to prevent the |
| linker from widening density instructions or inserting no-ops, except in |
| a few cases where no-ops are required for correctness. |
| |
| <p>The following Xtensa-specific command-line options can be used to |
| control the linker: |
| |
| <p><a name="index-Xtensa-options-675"></a> |
| <dl> |
| <dt><samp><span class="option">--size-opt</span></samp><dd>When optimizing indirect calls to direct calls, optimize for code size |
| more than performance. With this option, the linker will not insert |
| no-ops or widen density instructions to preserve branch target |
| alignment. There may still be some cases where no-ops are required to |
| preserve the correctness of the code. |
| </dl> |
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