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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">There’s also llvm/Support/xxhash.h if the other one falls short.<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="FR">De :</span></b><span lang="FR"> llvm-dev <llvm-dev-bounces@lists.llvm.org>
<b>De la part de</b> David Blaikie via llvm-dev<br>
<b>Envoyé :</b> September 10, 2021 3:28 AM<br>
<b>À :</b> Jeroen Dobbelaere <Jeroen.Dobbelaere@synopsys.com><br>
<b>Cc :</b> llvm-dev@lists.llvm.org<br>
<b>Objet :</b> Re: [llvm-dev] DenseMapInfo::getHashValue() of two sometimes identical pointers<o:p></o:p></span></p>
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<p class="MsoNormal">straight up ^ isn't a very good hash algorithm, as you're seeing - my understanding was even a rudimentary hash algorithm multiplied the previous value by a prime before combining it<br>
<br>
But I think we have a library in llvm for hashing that probably does something pretty good, whatever it is - llvm/ADT/Hashing.h - probably use that? (hash_combine(x, y, z), I guess?)<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Sep 9, 2021 at 11:54 PM Jeroen Dobbelaere 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">Hi all,<br>
<br>
is there a recommended way to implement a 'DenseMapInfo<..>::getHashValue()'<br>
for a class that contains multiple pointers of which some are potentially identical ?<br>
<br>
The standard used xor (^) is canceling out the hash contribution of those identical pointers.<br>
<br>
More concrete: I am adding a 'PtrProvenance' member to MemoryLocation. When I add<br>
the member to the hash computation in the way it seems intended,<br>
<br>
<a href="https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fblob%2Fmain%2Fllvm%2Finclude%2Fllvm%2FAnalysis%2FMemoryLocation.h%23L340-L344&data=04%7C01%7Calexandre.ganea%40ubisoft.com%7Cdf96dd5e086d45c8e7d708d9742c8365%7Ce01bd386fa514210a2a429e5ab6f7ab1%7C0%7C0%7C637668556806959902%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=mPYakZcvSfRQy9ikkdJXev04gORdd1obhlpJGnG4d9E%3D&reserved=0" target="_blank">https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Analysis/MemoryLocation.h#L340-L344</a><br>
<br>
I would do:<br>
<br>
static unsigned getHashValue(const MemoryLocation &Val) {<br>
return DenseMapInfo<const Value *>::getHashValue(Val.Ptr) ^<br>
DenseMapInfo<const Value *>::getHashValue(Val.PtrProvenance) ^ // Handle new member<br>
DenseMapInfo<LocationSize>::getHashValue(Val.Size) ^<br>
DenseMapInfo<AAMDNodes>::getHashValue(Val.AATags);<br>
}<br>
<br>
<br>
But, in the initial case, it is very likely that Val.Ptr == Val.PtrProvenance.<br>
<br>
Is there a recommended (standard) way to fix this ?<br>
<br>
Thanks,<br>
<br>
Jeroen Dobbelaere<br>
<br>
<br>
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