llvm dev - Nov 2013 - [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
>

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.

-----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
>
_______________________________________________
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----- Original Message -----
> From: "Evgeniy Stepanov" <eugeni.stepanov at gmail.com>
> To: "Kostya Serebryany" <kcc at google.com>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Tuesday, November 19, 2013 10:55:11 AM
> 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.
So you're saying that *if* the load had been unconditional in the original
C++ program, then it would have been a data race? That does not sound right: the
data race is not a property of the load itself, it is a property of the use of
that data.

 -Hal
> 
> 
> 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
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

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
> >
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