html/302stack_allocate.subx.html

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  56 <a href='https://github.com/akkartik/mu/blob/main/302stack_allocate.subx'>https://github.com/akkartik/mu/blob/main/302stack_allocate.subx</a>
  57 <pre id='vimCodeElement'>
  58 <span id="L1" class="LineNr"> 1 </span><span class="subxComment"># A function which pushes n zeros on the stack.</span>
  59 <span id="L2" class="LineNr"> 2 </span><span class="subxComment"># Really only intended to be called from code generated by mu.subx (for array</span>
  60 <span id="L3" class="LineNr"> 3 </span><span class="subxComment"># vars on the stack).</span>
  61 <span id="L4" class="LineNr"> 4 </span>
  62 <span id="L5" class="LineNr"> 5 </span>== code
  63 <span id="L6" class="LineNr"> 6 </span>
  64 <span id="L7" class="LineNr"> 7 </span><span class="CommentedCode">#? Entry:</span>
  65 <span id="L8" class="LineNr"> 8 </span><span class="CommentedCode">#?     # . prologue</span>
  66 <span id="L9" class="LineNr"> 9 </span><span class="CommentedCode">#?     89/&lt;- %ebp 4/r32/esp</span>
  67 <span id="L10" class="LineNr">10 </span><span class="CommentedCode">#?     #</span>
  68 <span id="L11" class="LineNr">11 </span><span class="CommentedCode">#?     68/push 0xfcfdfeff/imm32</span>
  69 <span id="L12" class="LineNr">12 </span><span class="CommentedCode">#?     b8/copy-to-eax 0x34353637/imm32</span>
  70 <span id="L13" class="LineNr">13 </span><span class="CommentedCode">#? $dump-stack0:</span>
  71 <span id="L14" class="LineNr">14 </span><span class="CommentedCode">#?     (push-n-zero-bytes 4)</span>
  72 <span id="L15" class="LineNr">15 </span><span class="CommentedCode">#?     68/push 0x20/imm32</span>
  73 <span id="L16" class="LineNr">16 </span><span class="CommentedCode">#? $dump-stack9:</span>
  74 <span id="L17" class="LineNr">17 </span><span class="CommentedCode">#?     b8/copy-to-eax 1/imm32/exit</span>
  75 <span id="L18" class="LineNr">18 </span><span class="CommentedCode">#?     cd/syscall 0x80/imm8</span>
  76 <span id="L19" class="LineNr">19 </span>
  77 <span id="L20" class="LineNr">20 </span><span class="subxComment"># This is not a regular function, so it won't be idiomatic.</span>
  78 <span id="L21" class="LineNr">21 </span><span class="subxComment"># Registers must be properly restored.</span>
  79 <span id="L22" class="LineNr">22 </span><span class="subxComment"># Registers can be spilled, but that modifies the stack and needs to be</span>
  80 <span id="L23" class="LineNr">23 </span><span class="subxComment"># cleaned up.</span>
  81 <span id="L24" class="LineNr">24 </span>
  82 <span id="L25" class="LineNr">25 </span><span class="subxComment"># Overhead:</span>
  83 <span id="L26" class="LineNr">26 </span><span class="subxComment">#   62 + n*6 instructions to push n bytes.</span>
  84 <span id="L27" class="LineNr">27 </span><span class="subxComment"># If we just emitted code to push n zeroes, it would be:</span>
  85 <span id="L28" class="LineNr">28 </span><span class="subxComment">#   5 bytes for 4 zero bytes, so 1.25 bytes per zero. And that's not even</span>
  86 <span id="L29" class="LineNr">29 </span><span class="subxComment">#   instructions.</span>
  87 <span id="L30" class="LineNr">30 </span><span class="subxComment"># But on the other hand it would destroy the instruction cache, where this</span>
  88 <span id="L31" class="LineNr">31 </span><span class="subxComment"># approach requires 15 instructions, fixed.</span>
  89 <span id="L32" class="LineNr">32 </span>
  90 <span id="L33" class="LineNr">33 </span><span class="subxComment"># n must be positive</span>
  91 <span id="L34" class="LineNr">34 </span><span class="subxFunction">push-n-zero-bytes</span>:  <span class="subxComment"># n: int</span>
  92 <span id="L35" class="LineNr">35 </span><span class="Constant">$push-n-zero-bytes:prologue</span>:
  93 <span id="L36" class="LineNr">36 </span>    89/&lt;- *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L56'>Push-n-zero-bytes-ebp</a></span> 5/r32/ebp  <span class="subxComment"># spill ebp without affecting stack</span>
  94 <span id="L37" class="LineNr">37 </span>    89/&lt;- %ebp 4/r32/esp
  95 <span id="L38" class="LineNr">38 </span><span class="Constant">$push-n-zero-bytes:copy-ra</span>:
  96 <span id="L39" class="LineNr">39 </span>    <span class="subxComment"># -- esp = ebp</span>
  97 <span id="L40" class="LineNr">40 </span>    89/&lt;- *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L60'>Push-n-zero-bytes-eax</a></span> 0/r32/eax
  98 <span id="L41" class="LineNr">41 </span>    8b/-&gt; *esp 0/r32/eax
  99 <span id="L42" class="LineNr">42 </span>    2b/subtract *(ebp+4) 4/r32/esp
 100 <span id="L43" class="LineNr">43 </span>    <span class="subxComment"># -- esp+n = ebp</span>
 101 <span id="L44" class="LineNr">44 </span>    89/&lt;- *esp 0/r32/eax
 102 <span id="L45" class="LineNr">45 </span>    8b/-&gt; *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L60'>Push-n-zero-bytes-eax</a></span> 0/r32/eax
 103 <span id="L46" class="LineNr">46 </span><span class="Constant">$push-n-zero-bytes:bulk-cleaning</span>:
 104 <span id="L47" class="LineNr">47 </span>    89/&lt;- *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L58'>Push-n-zero-bytes-esp</a></span> 4/r32/esp
 105 <span id="L48" class="LineNr">48 </span>    81 0/subop/add *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L58'>Push-n-zero-bytes-esp</a></span> 4/imm32
 106 <span id="L49" class="LineNr">49 </span>    81 0/subop/add *(ebp+4) 4/imm32
 107 <span id="L50" class="LineNr">50 </span>    (<a href='120allocate.subx.html#L869'>zero-out</a> *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L58'>Push-n-zero-bytes-esp</a></span> *(ebp+4))  <span class="subxComment"># n+4</span>
 108 <span id="L51" class="LineNr">51 </span><span class="Constant">$push-n-zero-bytes:epilogue</span>:
 109 <span id="L52" class="LineNr">52 </span>    8b/-&gt; *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L56'>Push-n-zero-bytes-ebp</a></span> 5/r32/ebp  <span class="subxComment"># restore spill</span>
 110 <span id="L53" class="LineNr">53 </span>    c3/return
 111 <span id="L54" class="LineNr">54 </span>
 112 <span id="L55" class="LineNr">55 </span>== data
 113 <span id="L56" class="LineNr">56 </span><span class="SpecialChar">Push-n-zero-bytes-ebp</span>:  <span class="subxComment"># (addr int)</span>
 114 <span id="L57" class="LineNr">57 </span>  0/imm32
 115 <span id="L58" class="LineNr">58 </span><span class="SpecialChar">Push-n-zero-bytes-esp</span>:  <span class="subxComment"># (addr int)</span>
 116 <span id="L59" class="LineNr">59 </span>  0/imm32
 117 <span id="L60" class="LineNr">60 </span><span class="SpecialChar">Push-n-zero-bytes-eax</span>:
 118 <span id="L61" class="LineNr">61 </span>  0/imm32
 119 </pre>
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