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<a href='https://github.com/akkartik/mu/blob/main/linux/bootstrap/035compute_segment_address.cc'>https://github.com/akkartik/mu/blob/main/linux/bootstrap/035compute_segment_address.cc</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="Comment">//: ELF binaries have finicky rules about the precise alignment each segment</span>
<span id="L2" class="LineNr"> 2 </span><span class="Comment">//: should start at. They depend on the amount of code in a program.</span>
<span id="L3" class="LineNr"> 3 </span><span class="Comment">//: We shouldn't expect people to adjust segment addresses everytime they make</span>
<span id="L4" class="LineNr"> 4 </span><span class="Comment">//: a change to their programs.</span>
<span id="L5" class="LineNr"> 5 </span><span class="Comment">//: Let's start taking the given segment addresses as guidelines, and adjust</span>
<span id="L6" class="LineNr"> 6 </span><span class="Comment">//: them as necessary.</span>
<span id="L7" class="LineNr"> 7 </span><span class="Comment">//: This gives up a measure of control in placing code and data.</span>
<span id="L8" class="LineNr"> 8 </span>
<span id="L9" class="LineNr"> 9 </span><span class="Normal">void</span> <a href='035compute_segment_address.cc.html#L9'>test_segment_name</a><span class="Delimiter">()</span> <span class="Delimiter">{</span>
<span id="L10" class="LineNr">10 </span> <a href='011run.cc.html#L82'>run</a><span class="Delimiter">(</span>
<span id="L11" class="LineNr">11 </span> <span class="Constant">"== code 0x09000000</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L12" class="LineNr">12 </span> <span class="Constant">"05/add-to-EAX 0x0d0c0b0a/imm32</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L13" class="LineNr">13 </span> <span class="Comment">// code starts at 0x09000000 + p_offset, which is 0x54 for a single-segment binary</span>
<span id="L14" class="LineNr">14 </span> <span class="Delimiter">);</span>
<span id="L15" class="LineNr">15 </span> <a href='003trace.cc.html#L290'>CHECK_TRACE_CONTENTS</a><span class="Delimiter">(</span>
<span id="L16" class="LineNr">16 </span> <span class="Constant">"load: 0x09000054 -> 05</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L17" class="LineNr">17 </span> <span class="Constant">"load: 0x09000055 -> 0a</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L18" class="LineNr">18 </span> <span class="Constant">"load: 0x09000056 -> 0b</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L19" class="LineNr">19 </span> <span class="Constant">"load: 0x09000057 -> 0c</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L20" class="LineNr">20 </span> <span class="Constant">"load: 0x09000058 -> 0d</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L21" class="LineNr">21 </span> <span class="Constant">"run: add imm32 0x0d0c0b0a to <a href='010vm.cc.html#L10'>EAX</a></span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L22" class="LineNr">22 </span> <span class="Constant">"run: storing 0x0d0c0b0a</span><span class="cSpecial">\n</span><span class="Constant">"</span>
<span id="L23" class="LineNr">23 </span> <span class="Delimiter">);</span>
<span id="L24" class="LineNr">24 </span><span class="Delimiter">}</span>
<span id="L25" class="LineNr">25 </span>
<span id="L26" class="LineNr">26 </span><span class="Comment">//: compute segment address</span>
<span id="L27" class="LineNr">27 </span>
<span id="L28" class="LineNr">28 </span><span class="Delimiter">:(before "End Transforms")</span>
<span id="L29" class="LineNr">29 </span><a href='031transforms.cc.html#L4'>Transform</a><span class="Delimiter">.</span>push_back<span class="Delimiter">(</span><a href='035compute_segment_address.cc.html#L32'>compute_segment_starts</a><span class="Delimiter">);</span>
<span id="L30" class="LineNr">30 </span>
<span id="L31" class="LineNr">31 </span><span class="Delimiter">:(code)</span>
<span id="L32" class="LineNr">32 </span><span class="Normal">void</span> <a href='035compute_segment_address.cc.html#L32'>compute_segment_starts</a><span class="Delimiter">(</span>program& p<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L33" class="LineNr">33 </span> <a href='003trace.cc.html#L96'>trace</a><span class="Delimiter">(</span><span class="Constant">3</span><span class="Delimiter">,</span> <span class="Constant">"transform"</span><span class="Delimiter">)</span> << <span class="Constant">"-- compute segment addresses"</span> << end<span class="Delimiter">();</span>
<span id="L34" class="LineNr">34 </span> <span class="Normal">uint32_t</span> p_offset = <span class="Comment">/*</span><span class="Comment">size of ehdr</span><span class="Comment">*/</span><span class="Constant">0x34</span> + <a href='001help.cc.html#L168'>SIZE</a><span class="Delimiter">(</span>p<span class="Delimiter">.</span><a href='011run.cc.html#L106'>segments</a><span class="Delimiter">)</span>*<span class="Constant">0x20</span><span class="Comment">/*</span><span class="Comment">size of each phdr</span><span class="Comment">*/</span><span class="Delimiter">;</span>
<span id="L35" class="LineNr">35 </span> <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">size_t</span> i = <span class="Constant">0</span><span class="Delimiter">;</span> i < p<span class="Delimiter">.</span><a href='011run.cc.html#L106'>segments</a><span class="Delimiter">.</span>size<span class="Delimiter">();</span> ++i<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L36" class="LineNr">36 </span> segment& curr = p<span class="Delimiter">.</span><a href='011run.cc.html#L106'>segments</a><span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">);</span>
<span id="L37" class="LineNr">37 </span> <span class="Normal">if</span> <span class="Delimiter">(</span>curr<span class="Delimiter">.</span>start >= <span class="Constant">0x08000000</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L38" class="LineNr">38 </span> <span class="Comment">// valid address for user space, so assume we're creating a real ELF binary, not just running a test</span>
<span id="L39" class="LineNr">39 </span> curr<span class="Delimiter">.</span>start &= <span class="Constant">0xfffff000</span><span class="Delimiter">;</span> <span class="Comment">// same number of zeros as the p_align used when emitting the ELF binary</span>
<span id="L40" class="LineNr">40 </span> curr<span class="Delimiter">.</span>start |= <span class="Delimiter">(</span>p_offset & <span class="Constant">0xfff</span><span class="Delimiter">);</span>
<span id="L41" class="LineNr">41 </span> <a href='003trace.cc.html#L96'>trace</a><span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">"transform"</span><span class="Delimiter">)</span> << <span class="Constant">"segment "</span> << i << <span class="Constant">" begins at address 0x"</span> << <a href='010vm.cc.html#L408'>HEXWORD</a> << curr<span class="Delimiter">.</span>start << end<span class="Delimiter">();</span>
<span id="L42" class="LineNr">42 </span> <span class="Delimiter">}</span>
<span id="L43" class="LineNr">43 </span> p_offset += size_of<span class="Delimiter">(</span>curr<span class="Delimiter">);</span>
<span id="L44" class="LineNr">44 </span> assert<span class="Delimiter">(</span>p_offset < <a href='010vm.cc.html#L98'>SEGMENT_ALIGNMENT</a><span class="Delimiter">);</span> <span class="Comment">// for now we get less and less available space in each successive segment</span>
<span id="L45" class="LineNr">45 </span> <span class="Delimiter">}</span>
<span id="L46" class="LineNr">46 </span><span class="Delimiter">}</span>
<span id="L47" class="LineNr">47 </span>
<span id="L48" class="LineNr">48 </span><span class="Normal">uint32_t</span> size_of<span class="Delimiter">(</span><span class="Normal">const</span> segment& s<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L49" class="LineNr">49 </span> <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
<span id="L50" class="LineNr">50 </span> <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">int</span> i = <span class="Constant">0</span><span class="Delimiter">;</span> i < <a href='001help.cc.html#L168'>SIZE</a><span class="Delimiter">(</span>s<span class="Delimiter">.</span><a href='011run.cc.html#L113'>lines</a><span class="Delimiter">);</span> ++i<span class="Delimiter">)</span>
<span id="L51" class="LineNr">51 </span> sum += <a href='035compute_segment_address.cc.html#L56'>num_bytes</a><span class="Delimiter">(</span>s<span class="Delimiter">.</span><a href='011run.cc.html#L113'>lines</a><span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
<span id="L52" class="LineNr">52 </span> <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
<span id="L53" class="LineNr">53 </span><span class="Delimiter">}</span>
<span id="L54" class="LineNr">54 </span>
<span id="L55" class="LineNr">55 </span><span class="Comment">// Assumes all bitfields are packed.</span>
<span id="L56" class="LineNr">56 </span><span class="Normal">uint32_t</span> <a href='035compute_segment_address.cc.html#L56'>num_bytes</a><span class="Delimiter">(</span><span class="Normal">const</span> line& inst<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L57" class="LineNr">57 </span> <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
<span id="L58" class="LineNr">58 </span> <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">int</span> i = <span class="Constant">0</span><span class="Delimiter">;</span> i < <a href='001help.cc.html#L168'>SIZE</a><span class="Delimiter">(</span>inst<span class="Delimiter">.</span><a href='011run.cc.html#L122'>words</a><span class="Delimiter">);</span> ++i<span class="Delimiter">)</span>
<span id="L59" class="LineNr">59 </span> sum += size_of<span class="Delimiter">(</span>inst<span class="Delimiter">.</span><a href='011run.cc.html#L122'>words</a><span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
<span id="L60" class="LineNr">60 </span> <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
<span id="L61" class="LineNr">61 </span><span class="Delimiter">}</span>
<span id="L62" class="LineNr">62 </span>
<span id="L63" class="LineNr">63 </span><span class="Normal">int</span> size_of<span class="Delimiter">(</span><span class="Normal">const</span> word& w<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L64" class="LineNr">64 </span> <span class="Normal">if</span> <span class="Delimiter">(</span>has_argument_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">"disp32"</span><span class="Delimiter">)</span> || has_argument_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">"imm32"</span><span class="Delimiter">))</span>
<span id="L65" class="LineNr">65 </span> <span class="Identifier">return</span> <span class="Constant">4</span><span class="Delimiter">;</span>
<span id="L66" class="LineNr">66 </span> <span class="Normal">else</span> <span class="Normal">if</span> <span class="Delimiter">(</span>has_argument_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">"disp16"</span><span class="Delimiter">))</span>
<span id="L67" class="LineNr">67 </span> <span class="Identifier">return</span> <span class="Constant">2</span><span class="Delimiter">;</span>
<span id="L68" class="LineNr">68 </span> <span class="Comment">// End size_of(word w) Special-cases</span>
<span id="L69" class="LineNr">69 </span> <span class="Normal">else</span>
<span id="L70" class="LineNr">70 </span> <span class="Identifier">return</span> <span class="Constant">1</span><span class="Delimiter">;</span>
<span id="L71" class="LineNr">71 </span><span class="Delimiter">}</span>
<span id="L72" class="LineNr">72 </span>
<span id="L73" class="LineNr">73 </span><span class="Comment">//: Dependencies:</span>
<span id="L74" class="LineNr">74 </span><span class="Comment">//: - We'd like to compute segment addresses before setting up global variables,</span>
<span id="L75" class="LineNr">75 </span><span class="Comment">//: because computing addresses for global variables requires knowing where</span>
<span id="L76" class="LineNr">76 </span><span class="Comment">//: the data segment starts.</span>
<span id="L77" class="LineNr">77 </span><span class="Comment">//: - We'd like to finish expanding labels before computing segment addresses,</span>
<span id="L78" class="LineNr">78 </span><span class="Comment">//: because it would make computing the sizes of segments more self-contained</span>
<span id="L79" class="LineNr">79 </span><span class="Comment">//: (num_bytes).</span>
<span id="L80" class="LineNr">80 </span><span class="Comment">//:</span>
<span id="L81" class="LineNr">81 </span><span class="Comment">//: Decision: compute segment addresses before expanding labels, by being</span>
<span id="L82" class="LineNr">82 </span><span class="Comment">//: aware in this layer of certain argument types that will eventually occupy</span>
<span id="L83" class="LineNr">83 </span><span class="Comment">//: multiple bytes.</span>
<span id="L84" class="LineNr">84 </span><span class="Comment">//:</span>
<span id="L85" class="LineNr">85 </span><span class="Comment">//: The layer to expand labels later hooks into num_bytes() to teach this</span>
<span id="L86" class="LineNr">86 </span><span class="Comment">//: layer that labels occupy zero space in the binary.</span>
</pre>
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