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<a href='https://github.com/akkartik/mu/blob/main/baremetal/302stack_allocate.subx'>https://github.com/akkartik/mu/blob/main/baremetal/302stack_allocate.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="subxComment"># A function which pushes n zeros on the stack.</span>
<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>
<span id="L3" class="LineNr"> 3 </span><span class="subxComment"># vars on the stack).</span>
<span id="L4" class="LineNr"> 4 </span>
<span id="L5" class="LineNr"> 5 </span>== code
<span id="L6" class="LineNr"> 6 </span>
<span id="L7" class="LineNr"> 7 </span><span class="CommentedCode">#? Entry:</span>
<span id="L8" class="LineNr"> 8 </span><span class="CommentedCode">#? # . prologue</span>
<span id="L9" class="LineNr"> 9 </span><span class="CommentedCode">#? 89/<- %ebp 4/r32/esp</span>
<span id="L10" class="LineNr">10 </span><span class="CommentedCode">#? #</span>
<span id="L11" class="LineNr">11 </span><span class="CommentedCode">#? 68/push 0xfcfdfeff/imm32</span>
<span id="L12" class="LineNr">12 </span><span class="CommentedCode">#? b8/copy-to-eax 0x34353637/imm32</span>
<span id="L13" class="LineNr">13 </span><span class="CommentedCode">#? $dump-stack0:</span>
<span id="L14" class="LineNr">14 </span><span class="CommentedCode">#? (push-n-zero-bytes 4)</span>
<span id="L15" class="LineNr">15 </span><span class="CommentedCode">#? 68/push 0x20/imm32</span>
<span id="L16" class="LineNr">16 </span><span class="CommentedCode">#? $dump-stack9:</span>
<span id="L17" class="LineNr">17 </span><span class="CommentedCode">#? b8/copy-to-eax 1/imm32/exit</span>
<span id="L18" class="LineNr">18 </span><span class="CommentedCode">#? cd/syscall 0x80/imm8</span>
<span id="L19" class="LineNr">19 </span>
<span id="L20" class="LineNr">20 </span><span class="subxComment"># This is not a regular function, so it won't be idiomatic.</span>
<span id="L21" class="LineNr">21 </span><span class="subxComment"># Registers must be properly restored.</span>
<span id="L22" class="LineNr">22 </span><span class="subxComment"># Registers can be spilled, but that modifies the stack and needs to be</span>
<span id="L23" class="LineNr">23 </span><span class="subxComment"># cleaned up.</span>
<span id="L24" class="LineNr">24 </span>
<span id="L25" class="LineNr">25 </span><span class="subxComment"># Overhead:</span>
<span id="L26" class="LineNr">26 </span><span class="subxComment"># 62 + n*6 instructions to push n bytes.</span>
<span id="L27" class="LineNr">27 </span><span class="subxComment"># If we just emitted code to push n zeroes, it would be:</span>
<span id="L28" class="LineNr">28 </span><span class="subxComment"># 5 bytes for 4 zero bytes, so 1.25 bytes per zero. And that's not even</span>
<span id="L29" class="LineNr">29 </span><span class="subxComment"># instructions.</span>
<span id="L30" class="LineNr">30 </span><span class="subxComment"># But on the other hand it would destroy the instruction cache, where this</span>
<span id="L31" class="LineNr">31 </span><span class="subxComment"># approach requires 15 instructions, fixed.</span>
<span id="L32" class="LineNr">32 </span>
<span id="L33" class="LineNr">33 </span><span class="subxComment"># n must be positive</span>
<span id="L34" class="LineNr">34 </span><span class="subxFunction">push-n-zero-bytes</span>: <span class="subxComment"># n: int</span>
<span id="L35" class="LineNr">35 </span><span class="Constant">$push-n-zero-bytes:prologue</span>:
<span id="L36" class="LineNr">36 </span> 89/<- *<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>
<span id="L37" class="LineNr">37 </span> 89/<- %ebp 4/r32/esp
<span id="L38" class="LineNr">38 </span><span class="Constant">$push-n-zero-bytes:copy-ra</span>:
<span id="L39" class="LineNr">39 </span> <span class="subxComment"># -- esp = ebp</span>
<span id="L40" class="LineNr">40 </span> 89/<- *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L60'>Push-n-zero-bytes-eax</a></span> 0/r32/eax
<span id="L41" class="LineNr">41 </span> 8b/-> *esp 0/r32/eax
<span id="L42" class="LineNr">42 </span> 2b/subtract *(ebp+4) 4/r32/esp
<span id="L43" class="LineNr">43 </span> <span class="subxComment"># -- esp+n = ebp</span>
<span id="L44" class="LineNr">44 </span> 89/<- *esp 0/r32/eax
<span id="L45" class="LineNr">45 </span> 8b/-> *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L60'>Push-n-zero-bytes-eax</a></span> 0/r32/eax
<span id="L46" class="LineNr">46 </span><span class="Constant">$push-n-zero-bytes:bulk-cleaning</span>:
<span id="L47" class="LineNr">47 </span> 89/<- *<span class="SpecialChar"><a href='302stack_allocate.subx.html#L58'>Push-n-zero-bytes-esp</a></span> 4/r32/esp
<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
<span id="L49" class="LineNr">49 </span> 81 0/subop/add *(ebp+4) 4/imm32
<span id="L50" class="LineNr">50 </span> (<a href='120allocate.subx.html#L6'>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>
<span id="L51" class="LineNr">51 </span><span class="Constant">$push-n-zero-bytes:epilogue</span>:
<span id="L52" class="LineNr">52 </span> 8b/-> *<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>
<span id="L53" class="LineNr">53 </span> c3/return
<span id="L54" class="LineNr">54 </span>
<span id="L55" class="LineNr">55 </span>== data
<span id="L56" class="LineNr">56 </span><span class="SpecialChar">Push-n-zero-bytes-ebp</span>: <span class="subxComment"># (addr int)</span>
<span id="L57" class="LineNr">57 </span> 0/imm32
<span id="L58" class="LineNr">58 </span><span class="SpecialChar">Push-n-zero-bytes-esp</span>: <span class="subxComment"># (addr int)</span>
<span id="L59" class="LineNr">59 </span> 0/imm32
<span id="L60" class="LineNr">60 </span><span class="SpecialChar">Push-n-zero-bytes-eax</span>:
<span id="L61" class="LineNr">61 </span> 0/imm32
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