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<title>Mu - subx/034compute_segment_address.cc</title>
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<a href='https://github.com/akkartik/mu/blob/master/subx/034compute_segment_address.cc'>https://github.com/akkartik/mu/blob/master/subx/034compute_segment_address.cc</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="Comment">//: Start allowing us to not specify precise addresses for the start of each</span>
<span id="L2" class="LineNr"> 2 </span><span class="Comment">//: segment.</span>
<span id="L3" class="LineNr"> 3 </span><span class="Comment">//: This gives up a measure of control in placing code and data.</span>
<span id="L4" class="LineNr"> 4 </span>
<span id="L5" class="LineNr"> 5 </span><span class="Delimiter">:(scenario segment_name)</span>
<span id="L6" class="LineNr"> 6 </span>== <a href='031check_operands.cc.html#L11'>code</a>
<span id="L7" class="LineNr"> 7 </span><span class="PreProc">0</span><span class="Constant">5</span>/add-to-EAX <span class="Constant">0x0d0c0b0a</span>/imm32
<span id="L8" class="LineNr"> 8 </span><span class="Comment"># code starts at 0x08048000 + p_offset, which is 0x54 for a single-segment binary</span>
<span id="L9" class="LineNr"> 9 </span><span class="traceContains">+load: 0x09000054 -> 05</span>
<span id="L10" class="LineNr"> 10 </span><span class="traceContains">+load: 0x09000055 -> 0a</span>
<span id="L11" class="LineNr"> 11 </span><span class="traceContains">+load: 0x09000056 -> 0b</span>
<span id="L12" class="LineNr"> 12 </span><span class="traceContains">+load: 0x09000057 -> 0c</span>
<span id="L13" class="LineNr"> 13 </span><span class="traceContains">+load: 0x09000058 -> 0d</span>
<span id="L14" class="LineNr"> 14 </span><span class="traceContains">+run: add imm32 0x0d0c0b0a to reg EAX</span>
<span id="L15" class="LineNr"> 15 </span><span class="traceContains">+run: storing 0x0d0c0b0a</span>
<span id="L16" class="LineNr"> 16 </span>
<span id="L17" class="LineNr"> 17 </span><span class="Comment">//: Update the parser to handle non-numeric segment name.</span>
<span id="L18" class="LineNr"> 18 </span><span class="Comment">//:</span>
<span id="L19" class="LineNr"> 19 </span><span class="Comment">//: We'll also support repeated segments with non-numeric names.</span>
<span id="L20" class="LineNr"> 20 </span><span class="Comment">//: When we encounter a new reference to an existing segment we'll *prepend*</span>
<span id="L21" class="LineNr"> 21 </span><span class="Comment">//: the new data to existing data for the segment.</span>
<span id="L22" class="LineNr"> 22 </span>
<span id="L23" class="LineNr"> 23 </span><span class="Delimiter">:(before "End Globals")</span>
<span id="L24" class="LineNr"> 24 </span>map<<span class="Comment">/*</span><span class="Comment">name</span><span class="Comment">*/</span>string<span class="Delimiter">,</span> <span class="Normal">int</span>> <span class="Special">Segment_index</span><span class="Delimiter">;</span>
<span id="L25" class="LineNr"> 25 </span><span class="Normal">bool</span> <span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> false</span><span class="Delimiter">;</span> <span class="Comment">// global to permit cross-layer communication</span>
<span id="L26" class="LineNr"> 26 </span><span class="Normal">int</span> <span class="Special">Currently_parsing_segment_index</span> = -<span class="Constant">1</span><span class="Delimiter">;</span> <span class="Comment">// global to permit cross-layer communication</span>
<span id="L27" class="LineNr"> 27 </span><span class="Delimiter">:(before "End Reset")</span>
<span id="L28" class="LineNr"> 28 </span><span class="Special">Segment_index</span><span class="Delimiter">.</span>clear<span class="Delimiter">();</span>
<span id="L29" class="LineNr"> 29 </span><span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> false</span><span class="Delimiter">;</span>
<span id="L30" class="LineNr"> 30 </span><span class="Special">Currently_parsing_segment_index</span> = -<span class="Constant">1</span><span class="Delimiter">;</span>
<span id="L31" class="LineNr"> 31 </span>
<span id="L32" class="LineNr"> 32 </span><span class="Delimiter">:(before "End Segment Parsing Special-cases(segment_title)")</span>
<span id="L33" class="LineNr"> 33 </span><span class="Normal">if</span> <span class="Delimiter">(</span>!starts_with<span class="Delimiter">(</span>segment_title<span class="Delimiter">,</span> <span class="Constant">"0x"</span><span class="Delimiter">))</span> <span class="Delimiter">{</span>
<span id="L34" class="LineNr"> 34 </span> <span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> true</span><span class="Delimiter">;</span>
<span id="L35" class="LineNr"> 35 </span> <span class="Normal">if</span> <span class="Delimiter">(</span>!contains_key<span class="Delimiter">(</span><span class="Special">Segment_index</span><span class="Delimiter">,</span> segment_title<span class="Delimiter">))</span> <span class="Delimiter">{</span>
<span id="L36" class="LineNr"> 36 </span> trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">"parse"</span><span class="Delimiter">)</span> << <span class="Constant">"new segment '"</span> << segment_title << <span class="Constant">"'"</span> << end<span class="Delimiter">();</span>
<span id="L37" class="LineNr"> 37 </span> <span class="Normal">if</span> <span class="Delimiter">(</span>out<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>empty<span class="Delimiter">()</span> && segment_title != <span class="Constant">"code"</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L38" class="LineNr"> 38 </span> raise << <span class="Constant">"first segment must be 'code' but is '"</span> << segment_title << <span class="Constant">"'\n"</span> << end<span class="Delimiter">();</span>
<span id="L39" class="LineNr"> 39 </span> <span class="Identifier">return</span><span class="Delimiter">;</span>
<span id="L40" class="LineNr"> 40 </span> <span class="Delimiter">}</span>
<span id="L41" class="LineNr"> 41 </span> <span class="Normal">if</span> <span class="Delimiter">(</span>SIZE<span class="Delimiter">(</span>out<span class="Delimiter">.</span>segments<span class="Delimiter">)</span> == <span class="Constant">1</span> && segment_title != <span class="Constant">"data"</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L42" class="LineNr"> 42 </span> raise << <span class="Constant">"second segment must be 'data' but is '"</span> << segment_title << <span class="Constant">"'\n"</span> << end<span class="Delimiter">();</span>
<span id="L43" class="LineNr"> 43 </span> <span class="Identifier">return</span><span class="Delimiter">;</span>
<span id="L44" class="LineNr"> 44 </span> <span class="Delimiter">}</span>
<span id="L45" class="LineNr"> 45 </span> put<span class="Delimiter">(</span><span class="Special">Segment_index</span><span class="Delimiter">,</span> segment_title<span class="Delimiter">,</span> SIZE<span class="Delimiter">(</span>out<span class="Delimiter">.</span>segments<span class="Delimiter">));</span>
<span id="L46" class="LineNr"> 46 </span> out<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>push_back<span class="Delimiter">(</span>segment<span class="Delimiter">());</span>
<span id="L47" class="LineNr"> 47 </span> <span class="Delimiter">}</span>
<span id="L48" class="LineNr"> 48 </span> <span class="Normal">else</span> <span class="Delimiter">{</span>
<span id="L49" class="LineNr"> 49 </span> trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">"parse"</span><span class="Delimiter">)</span> << <span class="Constant">"prepending to segment '"</span> << segment_title << <span class="Constant">"'"</span> << end<span class="Delimiter">();</span>
<span id="L50" class="LineNr"> 50 </span> <span class="Delimiter">}</span>
<span id="L51" class="LineNr"> 51 </span> <span class="Special">Currently_parsing_segment_index</span> = get<span class="Delimiter">(</span><span class="Special">Segment_index</span><span class="Delimiter">,</span> segment_title<span class="Delimiter">);</span>
<span id="L52" class="LineNr"> 52 </span><span class="Delimiter">}</span>
<span id="L53" class="LineNr"> 53 </span>
<span id="L54" class="LineNr"> 54 </span><span class="Delimiter">:(before "End flush(p, lines) Special-cases")</span>
<span id="L55" class="LineNr"> 55 </span><span class="Normal">if</span> <span class="Delimiter">(</span><span class="Special">Currently_parsing_named_segment</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L56" class="LineNr"> 56 </span> assert<span class="Delimiter">(</span>!p<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>empty<span class="Delimiter">());</span>
<span id="L57" class="LineNr"> 57 </span> trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">"parse"</span><span class="Delimiter">)</span> << <span class="Constant">"flushing to segment"</span> << end<span class="Delimiter">();</span>
<span id="L58" class="LineNr"> 58 </span> vector<line>& curr_segment_data = p<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Special">Currently_parsing_segment_index</span><span class="Delimiter">).</span>lines<span class="Delimiter">;</span>
<span id="L59" class="LineNr"> 59 </span> curr_segment_data<span class="Delimiter">.</span>insert<span class="Delimiter">(</span>curr_segment_data<span class="Delimiter">.</span>begin<span class="Delimiter">(),</span> lines<span class="Delimiter">.</span>begin<span class="Delimiter">(),</span> lines<span class="Delimiter">.</span>end<span class="Delimiter">());</span>
<span id="L60" class="LineNr"> 60 </span> lines<span class="Delimiter">.</span>clear<span class="Delimiter">();</span>
<span id="L61" class="LineNr"> 61 </span> <span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> false</span><span class="Delimiter">;</span>
<span id="L62" class="LineNr"> 62 </span> <span class="Special">Currently_parsing_segment_index</span> = -<span class="Constant">1</span><span class="Delimiter">;</span>
<span id="L63" class="LineNr"> 63 </span> <span class="Identifier">return</span><span class="Delimiter">;</span>
<span id="L64" class="LineNr"> 64 </span><span class="Delimiter">}</span>
<span id="L65" class="LineNr"> 65 </span>
<span id="L66" class="LineNr"> 66 </span><span class="Delimiter">:(scenario repeated_segment_merges_data)</span>
<span id="L67" class="LineNr"> 67 </span>== <a href='031check_operands.cc.html#L11'>code</a>
<span id="L68" class="LineNr"> 68 </span><span class="PreProc">0</span><span class="Constant">5</span>/add-to-EAX <span class="Constant">0x0d0c0b0a</span>/imm32
<span id="L69" class="LineNr"> 69 </span>== <a href='031check_operands.cc.html#L11'>code</a>
<span id="L70" class="LineNr"> 70 </span>2d/subtract-from-EAX <span class="Constant">0xddccbbaa</span>/imm32
<span id="L71" class="LineNr"> 71 </span><span class="traceContains">+parse: new segment 'code'</span>
<span id="L72" class="LineNr"> 72 </span><span class="traceContains">+parse: prepending to segment 'code'</span>
<span id="L73" class="LineNr"> 73 </span><span class="traceContains">+load: 0x09000054 -> 2d</span>
<span id="L74" class="LineNr"> 74 </span><span class="traceContains">+load: 0x09000055 -> aa</span>
<span id="L75" class="LineNr"> 75 </span><span class="traceContains">+load: 0x09000056 -> bb</span>
<span id="L76" class="LineNr"> 76 </span><span class="traceContains">+load: 0x09000057 -> cc</span>
<span id="L77" class="LineNr"> 77 </span><span class="traceContains">+load: 0x09000058 -> dd</span>
<span id="L78" class="LineNr"> 78 </span><span class="traceContains">+load: 0x09000059 -> 05</span>
<span id="L79" class="LineNr"> 79 </span><span class="traceContains">+load: 0x0900005a -> 0a</span>
<span id="L80" class="LineNr"> 80 </span><span class="traceContains">+load: 0x0900005b -> 0b</span>
<span id="L81" class="LineNr"> 81 </span><span class="traceContains">+load: 0x0900005c -> 0c</span>
<span id="L82" class="LineNr"> 82 </span><span class="traceContains">+load: 0x0900005d -> 0d</span>
<span id="L83" class="LineNr"> 83 </span>
<span id="L84" class="LineNr"> 84 </span><span class="Delimiter">:(scenario error_on_missing_segment_header)</span>
<span id="L85" class="LineNr"> 85 </span><span class="Special">% Hide_errors = true;</span>
<span id="L86" class="LineNr"> 86 </span><span class="PreProc">0</span><span class="Constant">5</span>/add-to-EAX <span class="Constant">0</span>/imm32
<span id="L87" class="LineNr"> 87 </span><span class="traceContains">+error: input does not start with a '==' section header</span>
<span id="L88" class="LineNr"> 88 </span>
<span id="L89" class="LineNr"> 89 </span><span class="Delimiter">:(scenario error_on_first_segment_not_code)</span>
<span id="L90" class="LineNr"> 90 </span><span class="Special">% Hide_errors = true;</span>
<span id="L91" class="LineNr"> 91 </span>== data
<span id="L92" class="LineNr"> 92 </span><span class="PreProc">0</span><span class="Constant">5</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span>
<span id="L93" class="LineNr"> 93 </span><span class="traceContains">+error: first segment must be 'code' but is 'data'</span>
<span id="L94" class="LineNr"> 94 </span>
<span id="L95" class="LineNr"> 95 </span><span class="Delimiter">:(scenario error_on_second_segment_not_data)</span>
<span id="L96" class="LineNr"> 96 </span><span class="Special">% Hide_errors = true;</span>
<span id="L97" class="LineNr"> 97 </span>== <a href='031check_operands.cc.html#L11'>code</a>
<span id="L98" class="LineNr"> 98 </span><span class="PreProc">0</span><span class="Constant">5</span>/add-to-EAX <span class="Constant">0</span>/imm32
<span id="L99" class="LineNr"> 99 </span>== bss
<span id="L100" class="LineNr">100 </span><span class="PreProc">0</span><span class="Constant">5</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span>
<span id="L101" class="LineNr">101 </span><span class="traceContains">+error: second segment must be 'data' but is 'bss'</span>
<span id="L102" class="LineNr">102 </span>
<span id="L103" class="LineNr">103 </span><span class="Comment">//: compute segment address</span>
<span id="L104" class="LineNr">104 </span>
<span id="L105" class="LineNr">105 </span><span class="Delimiter">:(before "End Level-2 Transforms")</span>
<span id="L106" class="LineNr">106 </span><span class="Special">Transform</span><span class="Delimiter">.</span>push_back<span class="Delimiter">(</span>compute_segment_starts<span class="Delimiter">);</span>
<span id="L107" class="LineNr">107 </span>
<span id="L108" class="LineNr">108 </span><span class="Delimiter">:(code)</span>
<span id="L109" class="LineNr">109 </span><span class="Normal">void</span> compute_segment_starts<span class="Delimiter">(</span>program& p<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L110" class="LineNr">110 </span> trace<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">"-- compute segment addresses"</span> << end<span class="Delimiter">();</span>
<span id="L111" class="LineNr">111 </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> + SIZE<span class="Delimiter">(</span>p<span class="Delimiter">.</span>segments<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="L112" class="LineNr">112 </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>segments<span class="Delimiter">.</span>size<span class="Delimiter">();</span> ++i<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L113" class="LineNr">113 </span> segment& curr = p<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">);</span>
<span id="L114" class="LineNr">114 </span> <span class="Normal">if</span> <span class="Delimiter">(</span>curr<span class="Delimiter">.</span>start == <span class="Constant">0</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L115" class="LineNr">115 </span> curr<span class="Delimiter">.</span>start = CODE_SEGMENT + i*SPACE_FOR_SEGMENT + p_offset<span class="Delimiter">;</span>
<span id="L116" class="LineNr">116 </span> trace<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> << HEXWORD << curr<span class="Delimiter">.</span>start << end<span class="Delimiter">();</span>
<span id="L117" class="LineNr">117 </span> <span class="Delimiter">}</span>
<span id="L118" class="LineNr">118 </span> p_offset += size_of<span class="Delimiter">(</span>curr<span class="Delimiter">);</span>
<span id="L119" class="LineNr">119 </span> assert<span class="Delimiter">(</span>p_offset < SEGMENT_ALIGNMENT<span class="Delimiter">);</span> <span class="Comment">// for now we get less and less available space in each successive segment</span>
<span id="L120" class="LineNr">120 </span> <span class="Delimiter">}</span>
<span id="L121" class="LineNr">121 </span><span class="Delimiter">}</span>
<span id="L122" class="LineNr">122 </span>
<span id="L123" class="LineNr">123 </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="L124" class="LineNr">124 </span> <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
<span id="L125" class="LineNr">125 </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 < SIZE<span class="Delimiter">(</span>s<span class="Delimiter">.</span>lines<span class="Delimiter">);</span> ++i<span class="Delimiter">)</span>
<span id="L126" class="LineNr">126 </span> sum += num_bytes<span class="Delimiter">(</span>s<span class="Delimiter">.</span>lines<span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
<span id="L127" class="LineNr">127 </span> <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
<span id="L128" class="LineNr">128 </span><span class="Delimiter">}</span>
<span id="L129" class="LineNr">129 </span>
<span id="L130" class="LineNr">130 </span><span class="Comment">// Assumes all bitfields are packed.</span>
<span id="L131" class="LineNr">131 </span><span class="Normal">uint32_t</span> num_bytes<span class="Delimiter">(</span><span class="Normal">const</span> line& inst<span class="Delimiter">)</span> <span class="Delimiter">{</span>
<span id="L132" class="LineNr">132 </span> <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
<span id="L133" class="LineNr">133 </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 < SIZE<span class="Delimiter">(</span>inst<span class="Delimiter">.</span>words<span class="Delimiter">);</span> ++i<span class="Delimiter">)</span>
<span id="L134" class="LineNr">134 </span> sum += size_of<span class="Delimiter">(</span>inst<span class="Delimiter">.</span>words<span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
<span id="L135" class="LineNr">135 </span> <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
<span id="L136" class="LineNr">136 </span><span class="Delimiter">}</span>
<span id="L137" class="LineNr">137 </span>
<span id="L138" class="LineNr">138 </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="L139" class="LineNr">139 </span> <span class="Normal">if</span> <span class="Delimiter">(</span>has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">"disp32"</span><span class="Delimiter">)</span> || has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">"imm32"</span><span class="Delimiter">))</span>
<span id="L140" class="LineNr">140 </span> <span class="Identifier">return</span> <span class="Constant">4</span><span class="Delimiter">;</span>
<span id="L141" class="LineNr">141 </span> <span class="Normal">else</span> <span class="Normal">if</span> <span class="Delimiter">(</span>has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">"disp16"</span><span class="Delimiter">))</span>
<span id="L142" class="LineNr">142 </span> <span class="Identifier">return</span> <span class="Constant">2</span><span class="Delimiter">;</span>
<span id="L143" class="LineNr">143 </span> <span class="Comment">// End size_of(word w) Special-cases</span>
<span id="L144" class="LineNr">144 </span> <span class="Normal">else</span>
<span id="L145" class="LineNr">145 </span> <span class="Identifier">return</span> <span class="Constant">1</span><span class="Delimiter">;</span>
<span id="L146" class="LineNr">146 </span><span class="Delimiter">}</span>
<span id="L147" class="LineNr">147 </span>
<span id="L148" class="LineNr">148 </span><span class="Comment">//: Dependencies:</span>
<span id="L149" class="LineNr">149 </span><span class="Comment">//: - We'd like to compute segment addresses before setting up global variables,</span>
<span id="L150" class="LineNr">150 </span><span class="Comment">//: because computing addresses for global variables requires knowing where</span>
<span id="L151" class="LineNr">151 </span><span class="Comment">//: the data segment starts.</span>
<span id="L152" class="LineNr">152 </span><span class="Comment">//: - We'd like to finish expanding labels before computing segment addresses,</span>
<span id="L153" class="LineNr">153 </span><span class="Comment">//: because it would make computing the sizes of segments more self-contained</span>
<span id="L154" class="LineNr">154 </span><span class="Comment">//: (num_bytes).</span>
<span id="L155" class="LineNr">155 </span><span class="Comment">//:</span>
<span id="L156" class="LineNr">156 </span><span class="Comment">//: Decision: compute segment addresses before expanding labels, by being</span>
<span id="L157" class="LineNr">157 </span><span class="Comment">//: aware in this layer of certain operand types that will eventually occupy</span>
<span id="L158" class="LineNr">158 </span><span class="Comment">//: multiple bytes.</span>
<span id="L159" class="LineNr">159 </span><span class="Comment">//:</span>
<span id="L160" class="LineNr">160 </span><span class="Comment">//: The layer to expand labels later hooks into num_bytes() to teach this</span>
<span id="L161" class="LineNr">161 </span><span class="Comment">//: layer that labels occupy zero space in the binary.</span>
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