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| <h3 class="section">4.1 <samp><span class="command">ld</span></samp> and the H8/300</h3> |
| |
| <p><a name="index-H8_002f300-support-622"></a>For the H8/300, <samp><span class="command">ld</span></samp> can perform these global optimizations when |
| you specify the ‘<samp><span class="samp">--relax</span></samp>’ command-line option. |
| |
| |
| <a name="index-relaxing-on-H8_002f300-623"></a> |
| <dl><dt><em>relaxing address modes</em><dd><samp><span class="command">ld</span></samp> finds all <code>jsr</code> and <code>jmp</code> instructions whose |
| targets are within eight bits, and turns them into eight-bit |
| program-counter relative <code>bsr</code> and <code>bra</code> instructions, |
| respectively. |
| |
| <p><a name="index-synthesizing-on-H8_002f300-624"></a><br><dt><em>synthesizing instructions</em><dd><!-- FIXME: specifically mov.b, or any mov instructions really? -> mov.b only, at least on H8, H8H, H8S --> |
| <samp><span class="command">ld</span></samp> finds all <code>mov.b</code> instructions which use the |
| sixteen-bit absolute address form, but refer to the top |
| page of memory, and changes them to use the eight-bit address form. |
| (That is: the linker turns ‘<samp><span class="samp">mov.b </span><code>@</code><var>aa</var><span class="samp">:16</span></samp>’ into |
| ‘<samp><span class="samp">mov.b </span><code>@</code><var>aa</var><span class="samp">:8</span></samp>’ whenever the address <var>aa</var> is in the |
| top page of memory). |
| |
| <p><samp><span class="command">ld</span></samp> finds all <code>mov</code> instructions which use the register |
| indirect with 32-bit displacement addressing mode, but use a small |
| displacement inside 16-bit displacement range, and changes them to use |
| the 16-bit displacement form. (That is: the linker turns ‘<samp><span class="samp">mov.b |
| </span><code>@</code><var>d</var><span class="samp">:32,ERx</span></samp>’ into ‘<samp><span class="samp">mov.b </span><code>@</code><var>d</var><span class="samp">:16,ERx</span></samp>’ |
| whenever the displacement <var>d</var> is in the 16 bit signed integer |
| range. Only implemented in ELF-format ld). |
| |
| <br><dt><em>bit manipulation instructions</em><dd><samp><span class="command">ld</span></samp> finds all bit manipulation instructions like <code>band, bclr, |
| biand, bild, bior, bist, bixor, bld, bnot, bor, bset, bst, btst, bxor</code> |
| which use 32 bit and 16 bit absolute address form, but refer to the top |
| page of memory, and changes them to use the 8 bit address form. |
| (That is: the linker turns ‘<samp><span class="samp">bset #xx:3,</span><code>@</code><var>aa</var><span class="samp">:32</span></samp>’ into |
| ‘<samp><span class="samp">bset #xx:3,</span><code>@</code><var>aa</var><span class="samp">:8</span></samp>’ whenever the address <var>aa</var> is in |
| the top page of memory). |
| |
| <br><dt><em>system control instructions</em><dd><samp><span class="command">ld</span></samp> finds all <code>ldc.w, stc.w</code> instructions which use the |
| 32 bit absolute address form, but refer to the top page of memory, and |
| changes them to use 16 bit address form. |
| (That is: the linker turns ‘<samp><span class="samp">ldc.w </span><code>@</code><var>aa</var><span class="samp">:32,ccr</span></samp>’ into |
| ‘<samp><span class="samp">ldc.w </span><code>@</code><var>aa</var><span class="samp">:16,ccr</span></samp>’ whenever the address <var>aa</var> is in |
| the top page of memory). |
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