Kostya Serebryany
2013-Nov-19 17:45 UTC
[LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
On Tue, Nov 19, 2013 at 9:05 PM, Kuperstein, Michael M < michael.m.kuperstein at intel.com> wrote:> My $0.02 - I'm not sure the transformation introduces a data race. > > To the best of my understanding, the point of the C++11/C11 memory model > is to allow a wide array of compiler transformations - including > speculative loads - for non-atomic variables. > I believe what's most likely happening (without looking at the Mozilla > source) is that the original program contains a C++ data race, and the > transformation exposes it to TSan. >The original program is race-free. I've posted a minimized reproducer that actually triggers a tsan false positive at O1 here: https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c5> > -----Original Message----- > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On > Behalf Of Evgeniy Stepanov > Sent: Tuesday, November 19, 2013 18:55 > To: Kostya Serebryany > Cc: LLVM Developers Mailing List > Subject: Re: [LLVMdev] Curiosity about transform changes under Sanitizers > (Was: [PATCH] Disable branch folding with MemorySanitizer) > > The root cause of those issues is the fact that sanitizers verify > C++-level semantics with LLVM IR level instrumentation. For example, > speculative loads are OK in IR if it can be proved that the load won't > trap, but in C++ it would be a data race. > > > On Tue, Nov 19, 2013 at 8:38 PM, Kostya Serebryany <kcc at google.com> wrote: > > > > > > > > On Tue, Nov 19, 2013 at 8:25 PM, David Blaikie <dblaikie at gmail.com> > wrote: > >> > >> Just moving this branch of the thread out of the review because I > >> don't want to derail the review thread... > >> > >> Kostya - why are these two cases not optimization bugs in general? > >> (why do they only affect sanitizers?) > > > > > > The recent case from mozilla > > (https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2) is > > a legal optimization -- it hoists a safe load (i.e. a load which is > > known not to > > fail) out of conditional branch. > > It reduces the number of basic blocks and branches, and so I think > > it's good in general. > > I can't imagine a case where this optimization will break a valid > program. > > Which is the second one you are referring to? > > > > --kcc > > > >> > >> > >> > >> > >> On Mon, Nov 18, 2013 at 8:37 PM, Kostya Serebryany <kcc at google.com> > wrote: > >>> > >>> And we've been just informed by the mozilla folks about yet another > >>> case of optimization being hostile to sanitizers: > >>> hoisting a safe load out of conditional branch introduces a race > >>> which tsan happily reports. > >>> https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > >>> > >>> --kcc > >>> > >>> > >>> On Tue, Nov 19, 2013 at 8:06 AM, Kostya Serebryany <kcc at google.com> > >>> wrote: > >>>> > >>>> > >>>> > >>>> > >>>> On Tue, Nov 19, 2013 at 1:27 AM, David Blaikie <dblaikie at gmail.com> > >>>> wrote: > >>>>> > >>>>> Do we have precedence for this kind of change (where sanitizers > >>>>> affect optimizations in arbitrary internal ways - not simply by > >>>>> enabling/disabling certain passes)? > >>>> > >>>> > >>>> Yes. AddressSanitizer and ThreadSanitizer disable load widening > >>>> that would create a partially out-of-bounds or a racy access. > >>>> See lib/Analysis/MemoryDependenceAnalysis.cpp (search for > >>>> Attribute::SanitizeAddress and Attribute::SanitizeThread). > >>>> This case with MemorySanitizer is slightly different because we are > >>>> not fighting a false positive, but rather a debug-info-damaging > optimization. > >>>> > >>>> --kcc > >>>> > >>>>> > >>>>> If not, does this need some deeper discussion about alternatives > >>>>> (is it important that we be able to produce equivalent code > >>>>> without the sanitizers enabled?)? > >>>>> > >>>>> > >>>>> On Mon, Nov 18, 2013 at 7:02 AM, Evgeniy Stepanov > >>>>> <eugenis at google.com> > >>>>> wrote: > >>>>>> > >>>>>> Branch folding optimization often leads to confusing MSan reports > >>>>>> due to lost debug info. > >>>>>> For example, > >>>>>> 1: if (x < 0) > >>>>>> 2: if (y < 0) > >>>>>> 3: do_something(); > >>>>>> is transformed into something like > >>>>>> %0 = and i32 %y, %x > >>>>>> %1 = icmp slt i32 %0, 0 > >>>>>> br i1 %1, label %if.then2, label %if.end3 where all 3 > >>>>>> instructions are associated with line 1. > >>>>>> > >>>>>> This patch disables folding of conditional branches in functions > >>>>>> with sanitize_memory attribute. > >>>>>> > >>>>>> http://llvm-reviews.chandlerc.com/D2214 > >>>>>> > >>>>>> Files: > >>>>>> lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> > >>>>>> Index: lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> ================================================================> >>>>>> => >>>>>> --- lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> +++ lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> @@ -1967,6 +1967,13 @@ > >>>>>> bool llvm::FoldBranchToCommonDest(BranchInst *BI) { > >>>>>> BasicBlock *BB = BI->getParent(); > >>>>>> > >>>>>> + // This optimization results in confusing MemorySanitizer > reports. > >>>>>> Use of > >>>>>> + // uninitialized value in this branch instruction is reported > >>>>>> + with > >>>>>> the > >>>>>> + // predecessor's debug location. > >>>>>> + if (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory) && > >>>>>> + BI->isConditional()) > >>>>>> + return false; > >>>>>> + > >>>>>> Instruction *Cond = 0; > >>>>>> if (BI->isConditional()) > >>>>>> Cond = dyn_cast<Instruction>(BI->getCondition()); > >>>>>> Index: test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> ================================================================> >>>>>> => >>>>>> --- test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> +++ test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> @@ -0,0 +1,29 @@ > >>>>>> +; RUN: opt < %s -simplifycfg -S | FileCheck %s > >>>>>> + > >>>>>> +declare void @callee() > >>>>>> + > >>>>>> +; Test that conditional branches are not folded with > sanitize_memory. > >>>>>> +define void @caller(i32 %x, i32 %y) sanitize_memory { ; CHECK: > >>>>>> +define void @caller(i32 [[X:%.*]], i32 [[Y:%.*]]) ; CHECK: icmp > >>>>>> +slt i32 {{.*}}[[X]] ; CHECK: icmp slt i32 {{.*}}[[Y]] ; CHECK: > >>>>>> +ret void > >>>>>> + > >>>>>> +entry: > >>>>>> + %cmp = icmp slt i32 %x, 0 > >>>>>> + br i1 %cmp, label %if.then, label %if.end3 > >>>>>> + > >>>>>> +if.then: ; preds = %entry > >>>>>> + %cmp1 = icmp slt i32 %y, 0 > >>>>>> + br i1 %cmp1, label %if.then2, label %if.end > >>>>>> + > >>>>>> +if.then2: ; preds > %if.then > >>>>>> + call void @callee() > >>>>>> + br label %if.end > >>>>>> + > >>>>>> +if.end: ; preds > >>>>>> %if.then2, %if.then > >>>>>> + br label %if.end3 > >>>>>> + > >>>>>> +if.end3: ; preds > %if.end, > >>>>>> %entry > >>>>>> + ret void > >>>>>> +} > >>>>>> > >>>>>> _______________________________________________ > >>>>>> llvm-commits mailing list > >>>>>> llvm-commits at cs.uiuc.edu > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>>> > >>>>> > >>>>> > >>>>> _______________________________________________ > >>>>> llvm-commits mailing list > >>>>> llvm-commits at cs.uiuc.edu > >>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>> > >>>> > >>> > >> > > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > --------------------------------------------------------------------- > 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/20131119/594d174f/attachment.html>
Kuperstein, Michael M
2013-Nov-19 17:56 UTC
[LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
What I'm trying to say is that according to my understanding of the C++11 memory model, even in that small reproducer, the store to g and the load from g are in fact a data race. (This is regardless of the fact the load is protected by a branch that is not taken.) From: Kostya Serebryany [mailto:kcc at google.com] Sent: Tuesday, November 19, 2013 19:46 To: Kuperstein, Michael M Cc: Evgeniy Stepanov; LLVM Developers Mailing List Subject: Re: [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer) On Tue, Nov 19, 2013 at 9:05 PM, Kuperstein, Michael M <michael.m.kuperstein at intel.com<mailto:michael.m.kuperstein at intel.com>> wrote: My $0.02 - I'm not sure the transformation introduces a data race. To the best of my understanding, the point of the C++11/C11 memory model is to allow a wide array of compiler transformations - including speculative loads - for non-atomic variables. I believe what's most likely happening (without looking at the Mozilla source) is that the original program contains a C++ data race, and the transformation exposes it to TSan. The original program is race-free. I've posted a minimized reproducer that actually triggers a tsan false positive at O1 here: https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c5 -----Original Message----- From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu>] On Behalf Of Evgeniy Stepanov Sent: Tuesday, November 19, 2013 18:55 To: Kostya Serebryany Cc: LLVM Developers Mailing List Subject: Re: [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer) The root cause of those issues is the fact that sanitizers verify C++-level semantics with LLVM IR level instrumentation. For example, speculative loads are OK in IR if it can be proved that the load won't trap, but in C++ it would be a data race. On Tue, Nov 19, 2013 at 8:38 PM, Kostya Serebryany <kcc at google.com<mailto:kcc at google.com>> wrote:> > > > On Tue, Nov 19, 2013 at 8:25 PM, David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> wrote: >> >> Just moving this branch of the thread out of the review because I >> don't want to derail the review thread... >> >> Kostya - why are these two cases not optimization bugs in general? >> (why do they only affect sanitizers?) > > > The recent case from mozilla > (https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2) is > a legal optimization -- it hoists a safe load (i.e. a load which is > known not to > fail) out of conditional branch. > It reduces the number of basic blocks and branches, and so I think > it's good in general. > I can't imagine a case where this optimization will break a valid program. > Which is the second one you are referring to? > > --kcc > >> >> >> >> >> On Mon, Nov 18, 2013 at 8:37 PM, Kostya Serebryany <kcc at google.com<mailto:kcc at google.com>> wrote: >>> >>> And we've been just informed by the mozilla folks about yet another >>> case of optimization being hostile to sanitizers: >>> hoisting a safe load out of conditional branch introduces a race >>> which tsan happily reports. >>> https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 >>> >>> --kcc >>> >>> >>> On Tue, Nov 19, 2013 at 8:06 AM, Kostya Serebryany <kcc at google.com<mailto:kcc at google.com>> >>> wrote: >>>> >>>> >>>> >>>> >>>> On Tue, Nov 19, 2013 at 1:27 AM, David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>> >>>> wrote: >>>>> >>>>> Do we have precedence for this kind of change (where sanitizers >>>>> affect optimizations in arbitrary internal ways - not simply by >>>>> enabling/disabling certain passes)? >>>> >>>> >>>> Yes. AddressSanitizer and ThreadSanitizer disable load widening >>>> that would create a partially out-of-bounds or a racy access. >>>> See lib/Analysis/MemoryDependenceAnalysis.cpp (search for >>>> Attribute::SanitizeAddress and Attribute::SanitizeThread). >>>> This case with MemorySanitizer is slightly different because we are >>>> not fighting a false positive, but rather a debug-info-damaging optimization. >>>> >>>> --kcc >>>> >>>>> >>>>> If not, does this need some deeper discussion about alternatives >>>>> (is it important that we be able to produce equivalent code >>>>> without the sanitizers enabled?)? >>>>> >>>>> >>>>> On Mon, Nov 18, 2013 at 7:02 AM, Evgeniy Stepanov >>>>> <eugenis at google.com<mailto:eugenis at google.com>> >>>>> wrote: >>>>>> >>>>>> Branch folding optimization often leads to confusing MSan reports >>>>>> due to lost debug info. >>>>>> For example, >>>>>> 1: if (x < 0) >>>>>> 2: if (y < 0) >>>>>> 3: do_something(); >>>>>> is transformed into something like >>>>>> %0 = and i32 %y, %x >>>>>> %1 = icmp slt i32 %0, 0 >>>>>> br i1 %1, label %if.then2, label %if.end3 where all 3 >>>>>> instructions are associated with line 1. >>>>>> >>>>>> This patch disables folding of conditional branches in functions >>>>>> with sanitize_memory attribute. >>>>>> >>>>>> http://llvm-reviews.chandlerc.com/D2214 >>>>>> >>>>>> Files: >>>>>> lib/Transforms/Utils/SimplifyCFG.cpp >>>>>> test/Transforms/SimplifyCFG/branch-fold-msan.ll >>>>>> >>>>>> Index: lib/Transforms/Utils/SimplifyCFG.cpp >>>>>> ================================================================>>>>>> =>>>>>> --- lib/Transforms/Utils/SimplifyCFG.cpp >>>>>> +++ lib/Transforms/Utils/SimplifyCFG.cpp >>>>>> @@ -1967,6 +1967,13 @@ >>>>>> bool llvm::FoldBranchToCommonDest(BranchInst *BI) { >>>>>> BasicBlock *BB = BI->getParent(); >>>>>> >>>>>> + // This optimization results in confusing MemorySanitizer reports. >>>>>> Use of >>>>>> + // uninitialized value in this branch instruction is reported >>>>>> + with >>>>>> the >>>>>> + // predecessor's debug location. >>>>>> + if (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory) && >>>>>> + BI->isConditional()) >>>>>> + return false; >>>>>> + >>>>>> Instruction *Cond = 0; >>>>>> if (BI->isConditional()) >>>>>> Cond = dyn_cast<Instruction>(BI->getCondition()); >>>>>> Index: test/Transforms/SimplifyCFG/branch-fold-msan.ll >>>>>> ================================================================>>>>>> =>>>>>> --- test/Transforms/SimplifyCFG/branch-fold-msan.ll >>>>>> +++ test/Transforms/SimplifyCFG/branch-fold-msan.ll >>>>>> @@ -0,0 +1,29 @@ >>>>>> +; RUN: opt < %s -simplifycfg -S | FileCheck %s >>>>>> + >>>>>> +declare void @callee() >>>>>> + >>>>>> +; Test that conditional branches are not folded with sanitize_memory. >>>>>> +define void @caller(i32 %x, i32 %y) sanitize_memory { ; CHECK: >>>>>> +define void @caller(i32 [[X:%.*]], i32 [[Y:%.*]]) ; CHECK: icmp >>>>>> +slt i32 {{.*}}[[X]] ; CHECK: icmp slt i32 {{.*}}[[Y]] ; CHECK: >>>>>> +ret void >>>>>> + >>>>>> +entry: >>>>>> + %cmp = icmp slt i32 %x, 0 >>>>>> + br i1 %cmp, label %if.then, label %if.end3 >>>>>> + >>>>>> +if.then: ; preds = %entry >>>>>> + %cmp1 = icmp slt i32 %y, 0 >>>>>> + br i1 %cmp1, label %if.then2, label %if.end >>>>>> + >>>>>> +if.then2: ; preds = %if.then >>>>>> + call void @callee() >>>>>> + br label %if.end >>>>>> + >>>>>> +if.end: ; preds >>>>>> %if.then2, %if.then >>>>>> + br label %if.end3 >>>>>> + >>>>>> +if.end3: ; preds = %if.end, >>>>>> %entry >>>>>> + ret void >>>>>> +} >>>>>> >>>>>> _______________________________________________ >>>>>> llvm-commits mailing list >>>>>> llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu> >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits >>>>>> >>>>> >>>>> >>>>> _______________________________________________ >>>>> llvm-commits mailing list >>>>> llvm-commits at cs.uiuc.edu<mailto:llvm-commits at cs.uiuc.edu> >>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits >>>>> >>>> >>> >> > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu> http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >_______________________________________________ LLVM Developers mailing list LLVMdev at cs.uiuc.edu<mailto:LLVMdev at cs.uiuc.edu> http://llvm.cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev --------------------------------------------------------------------- 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/20131119/fd82810f/attachment.html>
Kostya Serebryany
2013-Nov-19 17:58 UTC
[LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
On Tue, Nov 19, 2013 at 9:56 PM, Kuperstein, Michael M < michael.m.kuperstein at intel.com> wrote:> What I’m trying to say is that according to my understanding of the > C++11 memory model, even in that small reproducer, the store to g and the > load from g are in fact a data race. > > (This is regardless of the fact the load is protected by a branch that is > not taken.) >My understanding of the standard is quite the opposite.> > > *From:* Kostya Serebryany [mailto:kcc at google.com] > *Sent:* Tuesday, November 19, 2013 19:46 > *To:* Kuperstein, Michael M > *Cc:* Evgeniy Stepanov; LLVM Developers Mailing List > > *Subject:* Re: [LLVMdev] Curiosity about transform changes under > Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer) > > > > > > > > On Tue, Nov 19, 2013 at 9:05 PM, Kuperstein, Michael M < > michael.m.kuperstein at intel.com> wrote: > > My $0.02 - I'm not sure the transformation introduces a data race. > > To the best of my understanding, the point of the C++11/C11 memory model > is to allow a wide array of compiler transformations - including > speculative loads - for non-atomic variables. > I believe what's most likely happening (without looking at the Mozilla > source) is that the original program contains a C++ data race, and the > transformation exposes it to TSan. > > > > The original program is race-free. > > I've posted a minimized reproducer that actually triggers a tsan false > positive at O1 here: > > https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c5 > > > > > -----Original Message----- > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On > Behalf Of Evgeniy Stepanov > Sent: Tuesday, November 19, 2013 18:55 > To: Kostya Serebryany > Cc: LLVM Developers Mailing List > Subject: Re: [LLVMdev] Curiosity about transform changes under Sanitizers > (Was: [PATCH] Disable branch folding with MemorySanitizer) > > The root cause of those issues is the fact that sanitizers verify > C++-level semantics with LLVM IR level instrumentation. For example, > speculative loads are OK in IR if it can be proved that the load won't > trap, but in C++ it would be a data race. > > > On Tue, Nov 19, 2013 at 8:38 PM, Kostya Serebryany <kcc at google.com> wrote: > > > > > > > > On Tue, Nov 19, 2013 at 8:25 PM, David Blaikie <dblaikie at gmail.com> > wrote: > >> > >> Just moving this branch of the thread out of the review because I > >> don't want to derail the review thread... > >> > >> Kostya - why are these two cases not optimization bugs in general? > >> (why do they only affect sanitizers?) > > > > > > The recent case from mozilla > > (https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2) is > > a legal optimization -- it hoists a safe load (i.e. a load which is > > known not to > > fail) out of conditional branch. > > It reduces the number of basic blocks and branches, and so I think > > it's good in general. > > I can't imagine a case where this optimization will break a valid > program. > > Which is the second one you are referring to? > > > > --kcc > > > >> > >> > >> > >> > >> On Mon, Nov 18, 2013 at 8:37 PM, Kostya Serebryany <kcc at google.com> > wrote: > >>> > >>> And we've been just informed by the mozilla folks about yet another > >>> case of optimization being hostile to sanitizers: > >>> hoisting a safe load out of conditional branch introduces a race > >>> which tsan happily reports. > >>> https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > >>> > >>> --kcc > >>> > >>> > >>> On Tue, Nov 19, 2013 at 8:06 AM, Kostya Serebryany <kcc at google.com> > >>> wrote: > >>>> > >>>> > >>>> > >>>> > >>>> On Tue, Nov 19, 2013 at 1:27 AM, David Blaikie <dblaikie at gmail.com> > >>>> wrote: > >>>>> > >>>>> Do we have precedence for this kind of change (where sanitizers > >>>>> affect optimizations in arbitrary internal ways - not simply by > >>>>> enabling/disabling certain passes)? > >>>> > >>>> > >>>> Yes. AddressSanitizer and ThreadSanitizer disable load widening > >>>> that would create a partially out-of-bounds or a racy access. > >>>> See lib/Analysis/MemoryDependenceAnalysis.cpp (search for > >>>> Attribute::SanitizeAddress and Attribute::SanitizeThread). > >>>> This case with MemorySanitizer is slightly different because we are > >>>> not fighting a false positive, but rather a debug-info-damaging > optimization. > >>>> > >>>> --kcc > >>>> > >>>>> > >>>>> If not, does this need some deeper discussion about alternatives > >>>>> (is it important that we be able to produce equivalent code > >>>>> without the sanitizers enabled?)? > >>>>> > >>>>> > >>>>> On Mon, Nov 18, 2013 at 7:02 AM, Evgeniy Stepanov > >>>>> <eugenis at google.com> > >>>>> wrote: > >>>>>> > >>>>>> Branch folding optimization often leads to confusing MSan reports > >>>>>> due to lost debug info. > >>>>>> For example, > >>>>>> 1: if (x < 0) > >>>>>> 2: if (y < 0) > >>>>>> 3: do_something(); > >>>>>> is transformed into something like > >>>>>> %0 = and i32 %y, %x > >>>>>> %1 = icmp slt i32 %0, 0 > >>>>>> br i1 %1, label %if.then2, label %if.end3 where all 3 > >>>>>> instructions are associated with line 1. > >>>>>> > >>>>>> This patch disables folding of conditional branches in functions > >>>>>> with sanitize_memory attribute. > >>>>>> > >>>>>> http://llvm-reviews.chandlerc.com/D2214 > >>>>>> > >>>>>> Files: > >>>>>> lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> > >>>>>> Index: lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> ================================================================> >>>>>> => >>>>>> --- lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> +++ lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> @@ -1967,6 +1967,13 @@ > >>>>>> bool llvm::FoldBranchToCommonDest(BranchInst *BI) { > >>>>>> BasicBlock *BB = BI->getParent(); > >>>>>> > >>>>>> + // This optimization results in confusing MemorySanitizer > reports. > >>>>>> Use of > >>>>>> + // uninitialized value in this branch instruction is reported > >>>>>> + with > >>>>>> the > >>>>>> + // predecessor's debug location. > >>>>>> + if (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory) && > >>>>>> + BI->isConditional()) > >>>>>> + return false; > >>>>>> + > >>>>>> Instruction *Cond = 0; > >>>>>> if (BI->isConditional()) > >>>>>> Cond = dyn_cast<Instruction>(BI->getCondition()); > >>>>>> Index: test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> ================================================================> >>>>>> => >>>>>> --- test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> +++ test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> @@ -0,0 +1,29 @@ > >>>>>> +; RUN: opt < %s -simplifycfg -S | FileCheck %s > >>>>>> + > >>>>>> +declare void @callee() > >>>>>> + > >>>>>> +; Test that conditional branches are not folded with > sanitize_memory. > > >>>>>> +define void @caller(i32 %x, i32 %y) sanitize_memory { ; CHECK: > >>>>>> +define void @caller(i32 [[X:%.*]], i32 [[Y:%.*]]) ; CHECK: icmp > >>>>>> +slt i32 {{.*}}[[X]] ; CHECK: icmp slt i32 {{.*}}[[Y]] ; CHECK: > >>>>>> +ret void > > >>>>>> + > >>>>>> +entry: > >>>>>> + %cmp = icmp slt i32 %x, 0 > >>>>>> + br i1 %cmp, label %if.then, label %if.end3 > >>>>>> + > >>>>>> +if.then: ; preds = %entry > >>>>>> + %cmp1 = icmp slt i32 %y, 0 > >>>>>> + br i1 %cmp1, label %if.then2, label %if.end > >>>>>> + > >>>>>> +if.then2: ; preds > %if.then > >>>>>> + call void @callee() > >>>>>> + br label %if.end > >>>>>> + > >>>>>> +if.end: ; preds > >>>>>> %if.then2, %if.then > >>>>>> + br label %if.end3 > >>>>>> + > >>>>>> +if.end3: ; preds > %if.end, > >>>>>> %entry > >>>>>> + ret void > >>>>>> +} > >>>>>> > >>>>>> _______________________________________________ > >>>>>> llvm-commits mailing list > >>>>>> llvm-commits at cs.uiuc.edu > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>>> > >>>>> > >>>>> > >>>>> _______________________________________________ > >>>>> llvm-commits mailing list > >>>>> llvm-commits at cs.uiuc.edu > >>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>> > >>>> > >>> > >> > > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > --------------------------------------------------------------------- > 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/20131119/cc6e567d/attachment.html>
Hal Finkel
2013-Nov-19 18:03 UTC
[LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
----- Original Message -----> From: "Michael M Kuperstein" <michael.m.kuperstein at intel.com> > To: "Kostya Serebryany" <kcc at google.com> > Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu> > Sent: Tuesday, November 19, 2013 11:56:44 AM > Subject: Re: [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with > MemorySanitizer) > > > > > > What I’m trying to say is that according to my understanding of the > C++11 memory model, even in that small reproducer, the store to g > and the load from g are in fact a data race. > > (This is regardless of the fact the load is protected by a branch > that is not taken.)Can you please explain this more? I don't think this makes sense. -Hal> > > > From: Kostya Serebryany [mailto:kcc at google.com] > Sent: Tuesday, November 19, 2013 19:46 > To: Kuperstein, Michael M > Cc: Evgeniy Stepanov; LLVM Developers Mailing List > Subject: Re: [LLVMdev] Curiosity about transform changes under > Sanitizers (Was: [PATCH] Disable branch folding with > MemorySanitizer) > > > > > > > > > > > On Tue, Nov 19, 2013 at 9:05 PM, Kuperstein, Michael M < > michael.m.kuperstein at intel.com > wrote: > > My $0.02 - I'm not sure the transformation introduces a data race. > > To the best of my understanding, the point of the C++11/C11 memory > model is to allow a wide array of compiler transformations - > including speculative loads - for non-atomic variables. > I believe what's most likely happening (without looking at the > Mozilla source) is that the original program contains a C++ data > race, and the transformation exposes it to TSan. > > > > > > The original program is race-free. > > > I've posted a minimized reproducer that actually triggers a tsan > false positive at O1 here: > > > https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c5 > > > > > > > > > > -----Original Message----- > From: llvmdev-bounces at cs.uiuc.edu [mailto: > llvmdev-bounces at cs.uiuc.edu ] On Behalf Of Evgeniy Stepanov > Sent: Tuesday, November 19, 2013 18:55 > To: Kostya Serebryany > Cc: LLVM Developers Mailing List > Subject: Re: [LLVMdev] Curiosity about transform changes under > Sanitizers (Was: [PATCH] Disable branch folding with > MemorySanitizer) > > The root cause of those issues is the fact that sanitizers verify > C++-level semantics with LLVM IR level instrumentation. For example, > speculative loads are OK in IR if it can be proved that the load > won't trap, but in C++ it would be a data race. > > > On Tue, Nov 19, 2013 at 8:38 PM, Kostya Serebryany < kcc at google.com > > wrote: > > > > > > > > On Tue, Nov 19, 2013 at 8:25 PM, David Blaikie < dblaikie at gmail.com > > > wrote: > >> > >> Just moving this branch of the thread out of the review because I > >> don't want to derail the review thread... > >> > >> Kostya - why are these two cases not optimization bugs in general? > >> (why do they only affect sanitizers?) > > > > > > The recent case from mozilla > > ( https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > > ) is > > a legal optimization -- it hoists a safe load (i.e. a load which is > > known not to > > fail) out of conditional branch. > > It reduces the number of basic blocks and branches, and so I think > > it's good in general. > > I can't imagine a case where this optimization will break a valid > > program. > > Which is the second one you are referring to? > > > > --kcc > > > >> > >> > >> > >> > >> On Mon, Nov 18, 2013 at 8:37 PM, Kostya Serebryany < > >> kcc at google.com > wrote: > >>> > >>> And we've been just informed by the mozilla folks about yet > >>> another > >>> case of optimization being hostile to sanitizers: > >>> hoisting a safe load out of conditional branch introduces a race > >>> which tsan happily reports. > >>> https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > >>> > >>> --kcc > >>> > >>> > >>> On Tue, Nov 19, 2013 at 8:06 AM, Kostya Serebryany < > >>> kcc at google.com > > >>> wrote: > >>>> > >>>> > >>>> > >>>> > >>>> On Tue, Nov 19, 2013 at 1:27 AM, David Blaikie < > >>>> dblaikie at gmail.com > > >>>> wrote: > >>>>> > >>>>> Do we have precedence for this kind of change (where sanitizers > >>>>> affect optimizations in arbitrary internal ways - not simply by > >>>>> enabling/disabling certain passes)? > >>>> > >>>> > >>>> Yes. AddressSanitizer and ThreadSanitizer disable load widening > >>>> that would create a partially out-of-bounds or a racy access. > >>>> See lib/Analysis/MemoryDependenceAnalysis.cpp (search for > >>>> Attribute::SanitizeAddress and Attribute::SanitizeThread). > >>>> This case with MemorySanitizer is slightly different because we > >>>> are > >>>> not fighting a false positive, but rather a debug-info-damaging > >>>> optimization. > >>>> > >>>> --kcc > >>>> > >>>>> > >>>>> If not, does this need some deeper discussion about > >>>>> alternatives > >>>>> (is it important that we be able to produce equivalent code > >>>>> without the sanitizers enabled?)? > >>>>> > >>>>> > >>>>> On Mon, Nov 18, 2013 at 7:02 AM, Evgeniy Stepanov > >>>>> < eugenis at google.com > > >>>>> wrote: > >>>>>> > >>>>>> Branch folding optimization often leads to confusing MSan > >>>>>> reports > >>>>>> due to lost debug info. > >>>>>> For example, > >>>>>> 1: if (x < 0) > >>>>>> 2: if (y < 0) > >>>>>> 3: do_something(); > >>>>>> is transformed into something like > >>>>>> %0 = and i32 %y, %x > >>>>>> %1 = icmp slt i32 %0, 0 > >>>>>> br i1 %1, label %if.then2, label %if.end3 where all 3 > >>>>>> instructions are associated with line 1. > >>>>>> > >>>>>> This patch disables folding of conditional branches in > >>>>>> functions > >>>>>> with sanitize_memory attribute. > >>>>>> > >>>>>> http://llvm-reviews.chandlerc.com/D2214 > >>>>>> > >>>>>> Files: > >>>>>> lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> > >>>>>> Index: lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> ================================================================> >>>>>> => >>>>>> --- lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> +++ lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> @@ -1967,6 +1967,13 @@ > >>>>>> bool llvm::FoldBranchToCommonDest(BranchInst *BI) { > >>>>>> BasicBlock *BB = BI->getParent(); > >>>>>> > >>>>>> + // This optimization results in confusing MemorySanitizer > >>>>>> reports. > >>>>>> Use of > >>>>>> + // uninitialized value in this branch instruction is > >>>>>> reported > >>>>>> + with > >>>>>> the > >>>>>> + // predecessor's debug location. > >>>>>> + if > >>>>>> (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory) > >>>>>> && > >>>>>> + BI->isConditional()) > >>>>>> + return false; > >>>>>> + > >>>>>> Instruction *Cond = 0; > >>>>>> if (BI->isConditional()) > >>>>>> Cond = dyn_cast<Instruction>(BI->getCondition()); > >>>>>> Index: test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> ================================================================> >>>>>> => >>>>>> --- test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> +++ test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> @@ -0,0 +1,29 @@ > >>>>>> +; RUN: opt < %s -simplifycfg -S | FileCheck %s > >>>>>> + > >>>>>> +declare void @callee() > >>>>>> + > >>>>>> +; Test that conditional branches are not folded with > >>>>>> sanitize_memory. > > >>>>>> +define void @caller(i32 %x, i32 %y) sanitize_memory { ; > >>>>>> CHECK: > >>>>>> +define void @caller(i32 [[X:%.*]], i32 [[Y:%.*]]) ; CHECK: > >>>>>> icmp > >>>>>> +slt i32 {{.*}}[[X]] ; CHECK: icmp slt i32 {{.*}}[[Y]] ; > >>>>>> CHECK: > >>>>>> +ret void > > > > >>>>>> + > >>>>>> +entry: > >>>>>> + %cmp = icmp slt i32 %x, 0 > >>>>>> + br i1 %cmp, label %if.then, label %if.end3 > >>>>>> + > >>>>>> +if.then: ; preds = %entry > >>>>>> + %cmp1 = icmp slt i32 %y, 0 > >>>>>> + br i1 %cmp1, label %if.then2, label %if.end > >>>>>> + > >>>>>> +if.then2: ; preds = %if.then > >>>>>> + call void @callee() > >>>>>> + br label %if.end > >>>>>> + > >>>>>> +if.end: ; preds > >>>>>> %if.then2, %if.then > >>>>>> + br label %if.end3 > >>>>>> + > >>>>>> +if.end3: ; preds = %if.end, > >>>>>> %entry > >>>>>> + ret void > >>>>>> +} > >>>>>> > >>>>>> _______________________________________________ > >>>>>> llvm-commits mailing list > >>>>>> llvm-commits at cs.uiuc.edu > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>>> > >>>>> > >>>>> > >>>>> _______________________________________________ > >>>>> llvm-commits mailing list > >>>>> llvm-commits at cs.uiuc.edu > >>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>> > >>>> > >>> > >> > > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > --------------------------------------------------------------------- > 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. > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory
Hal Finkel
2013-Nov-19 18:08 UTC
[LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
----- Original Message -----> From: "Kostya Serebryany" <kcc at google.com> > To: "Michael M Kuperstein" <michael.m.kuperstein at intel.com> > Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu> > Sent: Tuesday, November 19, 2013 11:45:39 AM > Subject: Re: [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with > MemorySanitizer) > > > > > > > > > On Tue, Nov 19, 2013 at 9:05 PM, Kuperstein, Michael M < > michael.m.kuperstein at intel.com > wrote: > > > My $0.02 - I'm not sure the transformation introduces a data race. > > To the best of my understanding, the point of the C++11/C11 memory > model is to allow a wide array of compiler transformations - > including speculative loads - for non-atomic variables. > I believe what's most likely happening (without looking at the > Mozilla source) is that the original program contains a C++ data > race, and the transformation exposes it to TSan. > > > > The original program is race-free. > I've posted a minimized reproducer that actually triggers a tsan > false positive at O1 here: > https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c5 > >Here's my problem: int g; int foo(int cond) { if (cond) return g; return 0; } is being transformed into this (at the IR level): int g; int foo(int cond) { return cond ? g : 0; } but I could have just as easily made that transformation at the C++ level as well, and I don't see that as introducing a data race here (in practice). When you have a load that is used only conditionally, TSAN should only record it when the condition is true. IMHO, to do otherwise, is a bug in TSAN. Furthermore, if you try and do this by disabling all optimizations that transform conditionally-executed blocks into selects, you'll have a lot more work to do (because you'll also need to handle every place in the backend that does if conversion). -Hal> > > > > -----Original Message----- > From: llvmdev-bounces at cs.uiuc.edu [mailto: > llvmdev-bounces at cs.uiuc.edu ] On Behalf Of Evgeniy Stepanov > Sent: Tuesday, November 19, 2013 18:55 > To: Kostya Serebryany > Cc: LLVM Developers Mailing List > Subject: Re: [LLVMdev] Curiosity about transform changes under > Sanitizers (Was: [PATCH] Disable branch folding with > MemorySanitizer) > > The root cause of those issues is the fact that sanitizers verify > C++-level semantics with LLVM IR level instrumentation. For example, > speculative loads are OK in IR if it can be proved that the load > won't trap, but in C++ it would be a data race. > > > On Tue, Nov 19, 2013 at 8:38 PM, Kostya Serebryany < kcc at google.com > > wrote: > > > > > > > > On Tue, Nov 19, 2013 at 8:25 PM, David Blaikie < dblaikie at gmail.com > > > wrote: > >> > >> Just moving this branch of the thread out of the review because I > >> don't want to derail the review thread... > >> > >> Kostya - why are these two cases not optimization bugs in general? > >> (why do they only affect sanitizers?) > > > > > > The recent case from mozilla > > ( https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > > ) is > > a legal optimization -- it hoists a safe load (i.e. a load which is > > known not to > > fail) out of conditional branch. > > It reduces the number of basic blocks and branches, and so I think > > it's good in general. > > I can't imagine a case where this optimization will break a valid > > program. > > Which is the second one you are referring to? > > > > --kcc > > > >> > >> > >> > >> > >> On Mon, Nov 18, 2013 at 8:37 PM, Kostya Serebryany < > >> kcc at google.com > wrote: > >>> > >>> And we've been just informed by the mozilla folks about yet > >>> another > >>> case of optimization being hostile to sanitizers: > >>> hoisting a safe load out of conditional branch introduces a race > >>> which tsan happily reports. > >>> https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > >>> > >>> --kcc > >>> > >>> > >>> On Tue, Nov 19, 2013 at 8:06 AM, Kostya Serebryany < > >>> kcc at google.com > > >>> wrote: > >>>> > >>>> > >>>> > >>>> > >>>> On Tue, Nov 19, 2013 at 1:27 AM, David Blaikie < > >>>> dblaikie at gmail.com > > >>>> wrote: > >>>>> > >>>>> Do we have precedence for this kind of change (where sanitizers > >>>>> affect optimizations in arbitrary internal ways - not simply by > >>>>> enabling/disabling certain passes)? > >>>> > >>>> > >>>> Yes. AddressSanitizer and ThreadSanitizer disable load widening > >>>> that would create a partially out-of-bounds or a racy access. > >>>> See lib/Analysis/MemoryDependenceAnalysis.cpp (search for > >>>> Attribute::SanitizeAddress and Attribute::SanitizeThread). > >>>> This case with MemorySanitizer is slightly different because we > >>>> are > >>>> not fighting a false positive, but rather a debug-info-damaging > >>>> optimization. > >>>> > >>>> --kcc > >>>> > >>>>> > >>>>> If not, does this need some deeper discussion about > >>>>> alternatives > >>>>> (is it important that we be able to produce equivalent code > >>>>> without the sanitizers enabled?)? > >>>>> > >>>>> > >>>>> On Mon, Nov 18, 2013 at 7:02 AM, Evgeniy Stepanov > >>>>> < eugenis at google.com > > >>>>> wrote: > >>>>>> > >>>>>> Branch folding optimization often leads to confusing MSan > >>>>>> reports > >>>>>> due to lost debug info. > >>>>>> For example, > >>>>>> 1: if (x < 0) > >>>>>> 2: if (y < 0) > >>>>>> 3: do_something(); > >>>>>> is transformed into something like > >>>>>> %0 = and i32 %y, %x > >>>>>> %1 = icmp slt i32 %0, 0 > >>>>>> br i1 %1, label %if.then2, label %if.end3 where all 3 > >>>>>> instructions are associated with line 1. > >>>>>> > >>>>>> This patch disables folding of conditional branches in > >>>>>> functions > >>>>>> with sanitize_memory attribute. > >>>>>> > >>>>>> http://llvm-reviews.chandlerc.com/D2214 > >>>>>> > >>>>>> Files: > >>>>>> lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> > >>>>>> Index: lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> ================================================================> >>>>>> => >>>>>> --- lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> +++ lib/Transforms/Utils/SimplifyCFG.cpp > >>>>>> @@ -1967,6 +1967,13 @@ > >>>>>> bool llvm::FoldBranchToCommonDest(BranchInst *BI) { > >>>>>> BasicBlock *BB = BI->getParent(); > >>>>>> > >>>>>> + // This optimization results in confusing MemorySanitizer > >>>>>> reports. > >>>>>> Use of > >>>>>> + // uninitialized value in this branch instruction is > >>>>>> reported > >>>>>> + with > >>>>>> the > >>>>>> + // predecessor's debug location. > >>>>>> + if > >>>>>> (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory) > >>>>>> && > >>>>>> + BI->isConditional()) > >>>>>> + return false; > >>>>>> + > >>>>>> Instruction *Cond = 0; > >>>>>> if (BI->isConditional()) > >>>>>> Cond = dyn_cast<Instruction>(BI->getCondition()); > >>>>>> Index: test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> ================================================================> >>>>>> => >>>>>> --- test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> +++ test/Transforms/SimplifyCFG/branch-fold-msan.ll > >>>>>> @@ -0,0 +1,29 @@ > >>>>>> +; RUN: opt < %s -simplifycfg -S | FileCheck %s > >>>>>> + > >>>>>> +declare void @callee() > >>>>>> + > >>>>>> +; Test that conditional branches are not folded with > >>>>>> sanitize_memory. > >>>>>> +define void @caller(i32 %x, i32 %y) sanitize_memory { ; > >>>>>> CHECK: > >>>>>> +define void @caller(i32 [[X:%.*]], i32 [[Y:%.*]]) ; CHECK: > >>>>>> icmp > >>>>>> +slt i32 {{.*}}[[X]] ; CHECK: icmp slt i32 {{.*}}[[Y]] ; > >>>>>> CHECK: > >>>>>> +ret void > > > >>>>>> + > >>>>>> +entry: > >>>>>> + %cmp = icmp slt i32 %x, 0 > >>>>>> + br i1 %cmp, label %if.then, label %if.end3 > >>>>>> + > >>>>>> +if.then: ; preds = %entry > >>>>>> + %cmp1 = icmp slt i32 %y, 0 > >>>>>> + br i1 %cmp1, label %if.then2, label %if.end > >>>>>> + > >>>>>> +if.then2: ; preds = %if.then > >>>>>> + call void @callee() > >>>>>> + br label %if.end > >>>>>> + > >>>>>> +if.end: ; preds > >>>>>> %if.then2, %if.then > >>>>>> + br label %if.end3 > >>>>>> + > >>>>>> +if.end3: ; preds = %if.end, > >>>>>> %entry > >>>>>> + ret void > >>>>>> +} > >>>>>> > >>>>>> _______________________________________________ > >>>>>> llvm-commits mailing list > >>>>>> llvm-commits at cs.uiuc.edu > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>>> > >>>>> > >>>>> > >>>>> _______________________________________________ > >>>>> llvm-commits mailing list > >>>>> llvm-commits at cs.uiuc.edu > >>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > >>>>> > >>>> > >>> > >> > > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > --------------------------------------------------------------------- > 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. > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory
David Blaikie
2013-Nov-19 18:19 UTC
[LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
On Tue, Nov 19, 2013 at 10:08 AM, Hal Finkel <hfinkel at anl.gov> wrote:> ----- Original Message ----- > > From: "Kostya Serebryany" <kcc at google.com> > > To: "Michael M Kuperstein" <michael.m.kuperstein at intel.com> > > Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu> > > Sent: Tuesday, November 19, 2013 11:45:39 AM > > Subject: Re: [LLVMdev] Curiosity about transform changes under > Sanitizers (Was: [PATCH] Disable branch folding with > > MemorySanitizer) > > > > > > > > > > > > > > > > > > On Tue, Nov 19, 2013 at 9:05 PM, Kuperstein, Michael M < > > michael.m.kuperstein at intel.com > wrote: > > > > > > My $0.02 - I'm not sure the transformation introduces a data race. > > > > To the best of my understanding, the point of the C++11/C11 memory > > model is to allow a wide array of compiler transformations - > > including speculative loads - for non-atomic variables. > > I believe what's most likely happening (without looking at the > > Mozilla source) is that the original program contains a C++ data > > race, and the transformation exposes it to TSan. > > > > > > > > The original program is race-free. > > I've posted a minimized reproducer that actually triggers a tsan > > false positive at O1 here: > > https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c5 > > > > > > Here's my problem: > > int g; > int foo(int cond) { > if (cond) > return g; > return 0; > } > > is being transformed into this (at the IR level): > > int g; > int foo(int cond) { > return cond ? g : 0; >This doesn't have the same semantics as the IR transformed 'select', though, does it? In IR 'g' is unconditionally loaded before the select. In C++, 'g' is completely unevaluated if 'cond' is false.> } > > but I could have just as easily made that transformation at the C++ level > as well, and I don't see that as introducing a data race here (in > practice). When you have a load that is used only conditionally, TSAN > should only record it when the condition is true. IMHO, to do otherwise, is > a bug in TSAN. Furthermore, if you try and do this by disabling all > optimizations that transform conditionally-executed blocks into selects, > you'll have a lot more work to do (because you'll also need to handle every > place in the backend that does if conversion). > > -Hal > > > > > > > > > > > -----Original Message----- > > From: llvmdev-bounces at cs.uiuc.edu [mailto: > > llvmdev-bounces at cs.uiuc.edu ] On Behalf Of Evgeniy Stepanov > > Sent: Tuesday, November 19, 2013 18:55 > > To: Kostya Serebryany > > Cc: LLVM Developers Mailing List > > Subject: Re: [LLVMdev] Curiosity about transform changes under > > Sanitizers (Was: [PATCH] Disable branch folding with > > MemorySanitizer) > > > > The root cause of those issues is the fact that sanitizers verify > > C++-level semantics with LLVM IR level instrumentation. For example, > > speculative loads are OK in IR if it can be proved that the load > > won't trap, but in C++ it would be a data race. > > > > > > On Tue, Nov 19, 2013 at 8:38 PM, Kostya Serebryany < kcc at google.com > > > wrote: > > > > > > > > > > > > On Tue, Nov 19, 2013 at 8:25 PM, David Blaikie < dblaikie at gmail.com > > > > wrote: > > >> > > >> Just moving this branch of the thread out of the review because I > > >> don't want to derail the review thread... > > >> > > >> Kostya - why are these two cases not optimization bugs in general? > > >> (why do they only affect sanitizers?) > > > > > > > > > The recent case from mozilla > > > ( https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > > > ) is > > > a legal optimization -- it hoists a safe load (i.e. a load which is > > > known not to > > > fail) out of conditional branch. > > > It reduces the number of basic blocks and branches, and so I think > > > it's good in general. > > > I can't imagine a case where this optimization will break a valid > > > program. > > > Which is the second one you are referring to? > > > > > > --kcc > > > > > >> > > >> > > >> > > >> > > >> On Mon, Nov 18, 2013 at 8:37 PM, Kostya Serebryany < > > >> kcc at google.com > wrote: > > >>> > > >>> And we've been just informed by the mozilla folks about yet > > >>> another > > >>> case of optimization being hostile to sanitizers: > > >>> hoisting a safe load out of conditional branch introduces a race > > >>> which tsan happily reports. > > >>> https://code.google.com/p/thread-sanitizer/issues/detail?id=40#c2 > > >>> > > >>> --kcc > > >>> > > >>> > > >>> On Tue, Nov 19, 2013 at 8:06 AM, Kostya Serebryany < > > >>> kcc at google.com > > > >>> wrote: > > >>>> > > >>>> > > >>>> > > >>>> > > >>>> On Tue, Nov 19, 2013 at 1:27 AM, David Blaikie < > > >>>> dblaikie at gmail.com > > > >>>> wrote: > > >>>>> > > >>>>> Do we have precedence for this kind of change (where sanitizers > > >>>>> affect optimizations in arbitrary internal ways - not simply by > > >>>>> enabling/disabling certain passes)? > > >>>> > > >>>> > > >>>> Yes. AddressSanitizer and ThreadSanitizer disable load widening > > >>>> that would create a partially out-of-bounds or a racy access. > > >>>> See lib/Analysis/MemoryDependenceAnalysis.cpp (search for > > >>>> Attribute::SanitizeAddress and Attribute::SanitizeThread). > > >>>> This case with MemorySanitizer is slightly different because we > > >>>> are > > >>>> not fighting a false positive, but rather a debug-info-damaging > > >>>> optimization. > > >>>> > > >>>> --kcc > > >>>> > > >>>>> > > >>>>> If not, does this need some deeper discussion about > > >>>>> alternatives > > >>>>> (is it important that we be able to produce equivalent code > > >>>>> without the sanitizers enabled?)? > > >>>>> > > >>>>> > > >>>>> On Mon, Nov 18, 2013 at 7:02 AM, Evgeniy Stepanov > > >>>>> < eugenis at google.com > > > >>>>> wrote: > > >>>>>> > > >>>>>> Branch folding optimization often leads to confusing MSan > > >>>>>> reports > > >>>>>> due to lost debug info. > > >>>>>> For example, > > >>>>>> 1: if (x < 0) > > >>>>>> 2: if (y < 0) > > >>>>>> 3: do_something(); > > >>>>>> is transformed into something like > > >>>>>> %0 = and i32 %y, %x > > >>>>>> %1 = icmp slt i32 %0, 0 > > >>>>>> br i1 %1, label %if.then2, label %if.end3 where all 3 > > >>>>>> instructions are associated with line 1. > > >>>>>> > > >>>>>> This patch disables folding of conditional branches in > > >>>>>> functions > > >>>>>> with sanitize_memory attribute. > > >>>>>> > > >>>>>> http://llvm-reviews.chandlerc.com/D2214 > > >>>>>> > > >>>>>> Files: > > >>>>>> lib/Transforms/Utils/SimplifyCFG.cpp > > >>>>>> test/Transforms/SimplifyCFG/branch-fold-msan.ll > > >>>>>> > > >>>>>> Index: lib/Transforms/Utils/SimplifyCFG.cpp > > >>>>>> ================================================================> > >>>>>> => > >>>>>> --- lib/Transforms/Utils/SimplifyCFG.cpp > > >>>>>> +++ lib/Transforms/Utils/SimplifyCFG.cpp > > >>>>>> @@ -1967,6 +1967,13 @@ > > >>>>>> bool llvm::FoldBranchToCommonDest(BranchInst *BI) { > > >>>>>> BasicBlock *BB = BI->getParent(); > > >>>>>> > > >>>>>> + // This optimization results in confusing MemorySanitizer > > >>>>>> reports. > > >>>>>> Use of > > >>>>>> + // uninitialized value in this branch instruction is > > >>>>>> reported > > >>>>>> + with > > >>>>>> the > > >>>>>> + // predecessor's debug location. > > >>>>>> + if > > >>>>>> (BB->getParent()->hasFnAttribute(Attribute::SanitizeMemory) > > >>>>>> && > > >>>>>> + BI->isConditional()) > > >>>>>> + return false; > > >>>>>> + > > >>>>>> Instruction *Cond = 0; > > >>>>>> if (BI->isConditional()) > > >>>>>> Cond = dyn_cast<Instruction>(BI->getCondition()); > > >>>>>> Index: test/Transforms/SimplifyCFG/branch-fold-msan.ll > > >>>>>> ================================================================> > >>>>>> => > >>>>>> --- test/Transforms/SimplifyCFG/branch-fold-msan.ll > > >>>>>> +++ test/Transforms/SimplifyCFG/branch-fold-msan.ll > > >>>>>> @@ -0,0 +1,29 @@ > > >>>>>> +; RUN: opt < %s -simplifycfg -S | FileCheck %s > > >>>>>> + > > >>>>>> +declare void @callee() > > >>>>>> + > > >>>>>> +; Test that conditional branches are not folded with > > >>>>>> sanitize_memory. > > >>>>>> +define void @caller(i32 %x, i32 %y) sanitize_memory { ; > > >>>>>> CHECK: > > >>>>>> +define void @caller(i32 [[X:%.*]], i32 [[Y:%.*]]) ; CHECK: > > >>>>>> icmp > > >>>>>> +slt i32 {{.*}}[[X]] ; CHECK: icmp slt i32 {{.*}}[[Y]] ; > > >>>>>> CHECK: > > >>>>>> +ret void > > > > > > >>>>>> + > > >>>>>> +entry: > > >>>>>> + %cmp = icmp slt i32 %x, 0 > > >>>>>> + br i1 %cmp, label %if.then, label %if.end3 > > >>>>>> + > > >>>>>> +if.then: ; preds = %entry > > >>>>>> + %cmp1 = icmp slt i32 %y, 0 > > >>>>>> + br i1 %cmp1, label %if.then2, label %if.end > > >>>>>> + > > >>>>>> +if.then2: ; preds = %if.then > > >>>>>> + call void @callee() > > >>>>>> + br label %if.end > > >>>>>> + > > >>>>>> +if.end: ; preds > > >>>>>> %if.then2, %if.then > > >>>>>> + br label %if.end3 > > >>>>>> + > > >>>>>> +if.end3: ; preds = %if.end, > > >>>>>> %entry > > >>>>>> + ret void > > >>>>>> +} > > >>>>>> > > >>>>>> _______________________________________________ > > >>>>>> llvm-commits mailing list > > >>>>>> llvm-commits at cs.uiuc.edu > > >>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > > >>>>>> > > >>>>> > > >>>>> > > >>>>> _______________________________________________ > > >>>>> llvm-commits mailing list > > >>>>> llvm-commits at cs.uiuc.edu > > >>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > > >>>>> > > >>>> > > >>> > > >> > > > > > > > > > _______________________________________________ > > > LLVM Developers mailing list > > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > --------------------------------------------------------------------- > > 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. > > > > > > > > _______________________________________________ > > LLVM Developers mailing list > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > > > > -- > Hal Finkel > Assistant Computational Scientist > Leadership Computing Facility > Argonne National Laboratory > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20131119/833989b2/attachment.html>
Possibly Parallel Threads
- [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
- [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
- [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
- [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)
- [LLVMdev] Curiosity about transform changes under Sanitizers (Was: [PATCH] Disable branch folding with MemorySanitizer)