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<p class="MsoNormal">Got it, thanks. I didn’t realize the DenseMap would be recomputing those on a resize instead of caching them internally (although I haven’t though particularly hard about whether that internal caching would even be feasible in the general
case).<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">Nikita Popov <nikita.ppv@gmail.com><br>
<b>Date: </b>Monday, May 18, 2020 at 1:19 PM<br>
<b>To: </b>Shoaib Meenai <smeenai@fb.com><br>
<b>Cc: </b>"llvm-dev@lists.llvm.org" <llvm-dev@lists.llvm.org><br>
<b>Subject: </b>Re: [llvm-dev] Understanding LLD's SymbolTable's use of CachedHashStringRef<o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-left:.5in">On Mon, May 18, 2020 at 10:14 PM Shoaib Meenai via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">I was looking at the SymbolTable code in LLD COFF and ELF recently, and I’m confused by the use of CachedHashStringRef.<br>
<br>
From what I understand, a CachedHashStringRef combines a StringRef with a computed hash. There’s no caching going on in the CachedHashStringRef itself; that is, if you construct CachedHashStringRef("foo"), and then construct a second CachedHashStringRef("foo")
again later, you'll compute the hash for "foo" twice [1]. Instead, once you've constructed a CachedHashStringRef from a StringRef, you can pass that around instead of the StringRef to avoid needing to recompute the hash for it.<br>
<br>
LLD COFF's symbol table structure is a DenseMap<CachedHashStringRef, Symbol *> named symMap [2]. (ELF's is similar, except it maps to a vector index instead of a Symbol * directly for symbol order determinism reasons.) The only accesses to symMap are either
iterating over its values or doing a lookup. In the two cases where a map lookup is done [3][4], the input to the function doing the lookup is just a StringRef, and a CachedHashStringRef is constructed to perform the lookup. What's the advantage of using a
CachedHashStringRef in that case, as opposed to just having a DenseMap<StringRef, Symbol *> directly? With that, the DenseMap would be performing the hash computation for each lookup operation, but the CachedHashStringRef construction we have right now is
doing the same hash computation anyway, so I don't understand the benefit of using it here.<br>
<br>
[1] <a href="https://github.com/llvm/llvm-project/blob/47a0e9f49b903aa4ef821d2c7a679a145ee983f9/llvm/include/llvm/ADT/CachedHashString.h#L35-L36" target="_blank">
https://github.com/llvm/llvm-project/blob/47a0e9f49b903aa4ef821d2c7a679a145ee983f9/llvm/include/llvm/ADT/CachedHashString.h#L35-L36</a><br>
[2] <a href="https://github.com/llvm/llvm-project/blob/3f5f8f39734e88c797b003d4a0002b2eaef1ac17/lld/COFF/SymbolTable.h#L129" target="_blank">
https://github.com/llvm/llvm-project/blob/3f5f8f39734e88c797b003d4a0002b2eaef1ac17/lld/COFF/SymbolTable.h#L129</a><br>
[3] <a href="https://github.com/llvm/llvm-project/blob/3f5f8f39734e88c797b003d4a0002b2eaef1ac17/lld/COFF/SymbolTable.cpp#L458" target="_blank">
https://github.com/llvm/llvm-project/blob/3f5f8f39734e88c797b003d4a0002b2eaef1ac17/lld/COFF/SymbolTable.cpp#L458</a><br>
[4] <a href="https://github.com/llvm/llvm-project/blob/3f5f8f39734e88c797b003d4a0002b2eaef1ac17/lld/COFF/SymbolTable.cpp#L727" target="_blank">
https://github.com/llvm/llvm-project/blob/3f5f8f39734e88c797b003d4a0002b2eaef1ac17/lld/COFF/SymbolTable.cpp#L727</a><o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Not familiar with this code, but a possible reason might be to avoid recomputing string hashes when the DenseMap is resized.<o:p></o:p></p>
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<p class="MsoNormal" style="margin-left:.5in">Nikita<o:p></o:p></p>
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