Kuperstein, Michael M
2013-May-21 15:23 UTC
[LLVMdev] Mixing noalias and regular arguments
Hi all, I'm trying to understand the semantics of noalias arguments, and I'm not entirely sure I got it correctly. To the best of my understanding, if an argument is declared noalias, "This indicates that pointer values based on the argument do not alias pointer values which are not based on it" implies, among other things, that it cannot alias any other argument, even if that argument is NOT declared noalias. However, currently, BasicAliasAnalysis doesn't recognize this case explicitly. Sometimes it will work for other reasons (e.g. if it knows the other argument does not get captured), but it's relatively easy to get circumstances where the result is MayAlias. I'm attaching a patch that addresses this. Can anyone offer an opinion on the basic issue and, assuming this is the desired behavior, on the patch? Thanks, Michael --------------------------------------------------------------------- Intel Israel (74) Limited This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130521/31c4effd/attachment.html> -------------- next part -------------- A non-text attachment was scrubbed... Name: noalias.diff Type: application/octet-stream Size: 2878 bytes Desc: noalias.diff URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130521/31c4effd/attachment.obj>
Kuperstein, Michael M
2013-May-26 06:31 UTC
[LLVMdev] Mixing noalias and regular arguments
Ping? (Is there a code owner for AA, btw?) From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Kuperstein, Michael M Sent: Tuesday, May 21, 2013 18:23 To: LLVMdev at cs.uiuc.edu Cc: Raoux, Thomas F Subject: [LLVMdev] Mixing noalias and regular arguments Hi all, I'm trying to understand the semantics of noalias arguments, and I'm not entirely sure I got it correctly. To the best of my understanding, if an argument is declared noalias, "This indicates that pointer values based on the argument do not alias pointer values which are not based on it" implies, among other things, that it cannot alias any other argument, even if that argument is NOT declared noalias. However, currently, BasicAliasAnalysis doesn't recognize this case explicitly. Sometimes it will work for other reasons (e.g. if it knows the other argument does not get captured), but it's relatively easy to get circumstances where the result is MayAlias. I'm attaching a patch that addresses this. Can anyone offer an opinion on the basic issue and, assuming this is the desired behavior, on the patch? Thanks, Michael --------------------------------------------------------------------- Intel Israel (74) Limited This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. --------------------------------------------------------------------- Intel Israel (74) Limited This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130526/ba195ced/attachment.html>
Kuperstein, Michael M wrote:> Ping?Pong! Sorry for the slow review, I had this patch starred but hadn't got around to it. Yes, the rationale and implementation are correct.> (Is there a code owner for AA, btw?)(It falls back on the more general code owner who is Chris Lattner in this case, "Everything not covered by someone else".) +/// isNoAliasArgument - Return true if this is an argument with the noalias +/// attribute. +bool isNoAliasArgument(const Value* V); "const Value* V" should be "const Value *V". + // Arguments can't alias with noalias arguments + if ((isa<Argument>(O1) && isNoAliasArgument(O2)) || + (isa<Argument>(O2) && isNoAliasArgument(O1))) + return NoAlias; Fold this into the logic right above it: // Arguments can't alias with local allocations or noalias calls // in the same function. if (((isa<Argument>(O1) && (isa<AllocaInst>(O2) || isNoAliasCall(O2))) || (isa<Argument>(O2) && (isa<AllocaInst>(O1) || isNoAliasCall(O1))))) return NoAlias; by factoring out the combined tests "isa<AllocaInst>(V) || isNoAliasCall(V) || isNoAliasArgument(V)" into a function. +/// isNoAliasArgument - Return true if this is an argument with the noalias +/// attribute. +bool llvm::isNoAliasArgument(const Value* V) +{ + if (const Argument *A = dyn_cast<Argument>(V)) Please be consistent, "const Value* V" vs. "const Argument *A". This file puts the star on the right of the space. Thanks for fixing this. Please commit once you've addressed the above! Nick> *From:*llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] > *On Behalf Of *Kuperstein, Michael M > *Sent:* Tuesday, May 21, 2013 18:23 > *To:* LLVMdev at cs.uiuc.edu > *Cc:* Raoux, Thomas F > *Subject:* [LLVMdev] Mixing noalias and regular arguments > > Hi all, > > I’m trying to understand the semantics of noalias arguments, and I’m not > entirely sure I got it correctly. > > To the best of my understanding, if an argument is declared noalias, > “This indicates that pointer values based on the argument do not alias > pointer values which are not based on it” implies, among other things, > that it cannot alias any other argument, even if that argument is NOT > declared noalias. > > However, currently, BasicAliasAnalysis doesn’t recognize this case > explicitly. Sometimes it will work for other reasons (e.g. if it knows > the other argument does not get captured), but it’s relatively easy to > get circumstances where the result is MayAlias. > > I’m attaching a patch that addresses this. > > Can anyone offer an opinion on the basic issue and, assuming this is the > desired behavior, on the patch? > > Thanks, > > Michael