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<p class="MsoNormal">To quantify this, a quick experiment showed a line-0 directive occupied 5 bytes, of which 2 bytes were setting the column to zero. Not setting the column reduces the size increase by 40%.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Good to know - does that include changing the file too? (On either side of the equation/data)<br>
Though I expect some of the cost of these new line table entries includes changing it back to something else on the other side - in some cases I imagine it'll be changing it back to the same file/line, potentially. Though I suppose if we don't change the file,
we'll save both the change, and the change back. Column probably changes anyway so it doesn't cost us anything on the change "back" perhaps.<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Right, my accounting was incomplete, and my example used unrealistically small line numbers. Let's try again being a little more analytical.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">First off, I'm happy to make the simplifying assumption that column changes more-or-less every statement, and that we should accrue only one column-change per
line-0 directive. (Offhand I don't know how Clang decides to set the column numbers.) Setting the column is a "standard" opcode, so occupies 1 byte plus the ULEB operand., which in this case will be 1 byte, for a total of 2 bytes. This cost is going to
accrue for every line-0 directive, unless we optimize it away.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Setting the file is also a standard opcode, again 1 byte plus the ULEB operand, which will usually be 1 byte (unless the file number > 127) so again assume
2 bytes. This will happen twice per line-0 directive in a scope outside actual file #1, which is less common but surely does happen.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Setting the line number is more 'fun.' We'll simplify this to assume the line number is outside the line-range for special opcodes, so setting it to 0 and
back requires standard opcodes. This time it's a 1-byte opcode plus an SLEB operand, because you don't "set" the line, you "advance" the line. The max value you can encode as SLEB in 2 bytes is 8191, if I'm counting the bits correctly. If we further assume
most line numbers are in the range 127..8191 then setting the line number to 0 and back will require 3 bytes each. In addition we'll need a 1-byte special opcode to advance the address and "emit" the line record.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">So, assuming we fix the file-number problem (trivially, see my inline comment) and we're up past line 128, then a line-0 directive will cost:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">2 bytes to set column=0<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">3 bytes to advance line to 0 [e.g. advance by -200, if we were at line 200]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">1 byte special opcode to emit the record<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">3 bytes to advance line to 201 [or whatever]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">and the rest of it would be like usual.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">That adds up to 9 bytes per directive (usually), and if we optimize away the column number, we save 2 bytes, or 22% of the increase (not 40%, sigh). But, I
think, still worth doing.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> David Blaikie [mailto:dblaikie@gmail.com]
<br>
<b>Sent:</b> Wednesday, January 27, 2016 10:09 AM<br>
<b>To:</b> reviews+D16569+public+6623e660a2ba7fa7@reviews.llvm.org; Robinson, Paul<br>
<b>Cc:</b> Frédéric Riss; Adrian Prantl; Eric Christopher; llvm-commits<br>
<b>Subject:</b> Re: [PATCH] D16569: Emit line 0 line information for interesting 'orphan' instructions<o:p></o:p></span></p>
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<p class="MsoNormal">On Wed, Jan 27, 2016 at 10:06 AM, Paul Robinson via llvm-commits <<a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">probinson added a comment.<br>
<br>
In <a href="http://reviews.llvm.org/D16569#337412" target="_blank">http://reviews.llvm.org/D16569#337412</a>, @probinson wrote:<br>
<br>
> Second, I'd hope that the column number is irrelevant and so we shouldn't bother explicitly setting it to zero. That's probably something that would have to be handled where the line-table gets encoded, rather than here, and so can be deferred to a follow-up.<br>
<br>
<br>
To quantify this, a quick experiment showed a line-0 directive occupied 5 bytes, of which 2 bytes were setting the column to zero. Not setting the column reduces the size increase by 40%.<o:p></o:p></p>
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<p class="MsoNormal">Good to know - does that include changing the file too? (On either side of the equation/data)<br>
Though I expect some of the cost of these new line table entries includes changing it back to something else on the other side - in some cases I imagine it'll be changing it back to the same file/line, potentially. Though I suppose if we don't change the file,
we'll save both the change, and the change back. Column probably changes anyway so it doesn't cost us anything on the change "back" perhaps.<br>
<br>
- Dave<o:p></o:p></p>
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<a href="http://reviews.llvm.org/D16569" target="_blank">http://reviews.llvm.org/D16569</a><br>
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