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Devadasan, Christudasan via llvm-dev

2020-Jul-26 10:06 UTC

[llvm-dev] [LAA] RtCheck on pointers of different address spaces.

Hello,

I Have a question related to the RT check on pointers during Loop Access
Analysis pass.

There is a testcase with loop code that consist of 4 different memory operations
referring two global objects of different address spaces.
One from global constant (address space 4, addr_size = 64) and the other from
local, LDS (address space 3, addr_size= 32).
(Details of various address spaces available for AMDGPU backend:
https://llvm.org/docs/AMDGPUUsage.html#address-spaces)

With upstream compiler, the testcase fails with a crash (given at the end of the
e-mail) in the opt while trying to generate the RT check for these pointers.
Precisely, with two pointers of different address spaces.
The operand type check fails while trying to insert a 'AddExpr' SCEV
node as their effective type differs for these pointers (One with 32-bit and the
other with 64-bit)

Question: Is this intended to try for the RtCheck on pointers from different
address spaces?
                   The comments given in the code snippet (below) hints they
aren't.

Code snippet from LoopAccessAnalysis.cpp:
-----------------------------------------------------------------------------------------------------------------------
  bool AccessAnalysis::canCheckPtrAtRT(...) {
                  ----------
  // If the pointers that we would use for the bounds comparison have different
  // address spaces, assume the values aren't directly comparable, so we
can't
  // use them for the runtime check. We also have to assume they could
  // overlap. In the future there should be metadata for whether address spaces
  // are disjoint.
  unsigned NumPointers = RtCheck.Pointers.size();
  for (unsigned i = 0; i < NumPointers; ++i) {
   for (unsigned j = i + 1; j < NumPointers; ++j) {
      // Only need to check pointers between two different dependency sets.
          if (RtCheck.Pointers[i].DependencySetId =         
RtCheck.Pointers[j].DependencySetId)
       continue;
      // Only need to check pointers in the same alias set.
      if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)
        continue;

      Value *PtrI = RtCheck.Pointers[i].PointerValue;
      Value *PtrJ = RtCheck.Pointers[j].PointerValue;

      unsigned ASi = PtrI->getType()->getPointerAddressSpace();
      unsigned ASj = PtrJ->getType()->getPointerAddressSpace();
      if (ASi != ASj) {
        LLVM_DEBUG(
            dbgs() << "LAA: Runtime check would require comparison
between"
                      " different address spaces\n");
        return false;
      }
    }
----------------------------------------------------------------------------------------------------
More details about the objects, the pointers and the memory operations:
----------------------------------------------------------------------------------------------------
%struct_var1 = type { <2 x float> }
%struct_var2 = type { %struct_var1 }
%class_var1 = type { i32, i32, i32, %struct_var2*, i32, i32, i32}
%class_var2 = type { %class_var1, i8, i8*, i32 }

Objects:
@Obj1 = external protected local_unnamed_addr addrspace(4)
externally_initialized global %class_var2, align 8
@Obj2 = internal unnamed_addr addrspace(3) constant [4000 x float] undef, align
16

Pointers:

1.       %struct_var1.cast = bitcast %struct_var1* %struct_var2.gep to i64*
(write)       // AS1

2.       %struct_var2.cast = bitcast %struct_var2* %arrayidx74 to i64*
(read-only)        // AS1

3.       %arrayidx1705 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %add125 (read-only) // AS2

4.       %arrayidx274 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %arg1 (read-only)  // AS2

While the pointers 1 & 2 belong to one Alias Set (AS1), pointers 3 & 4
belong to a different set (AS2). It is because, the Global Alias Analysis found
there is no alias between these two sets of pointers.
The crash occurs with the pointers 1 & 4 which are from AS1 and AS2
respectively.
The DependenceSetId will be reset to 1 before processing a new AS and the
pointers in question, ended up having the same DependenceSetId (1), though they
are from different AS.

          
----------------------------------------------------------------------------------------------------------------------------------------------------------
            The load/store (memory operations) using these pointers in the loop.
(extracted only the relevant instructions):
          
----------------------------------------------------------------------------------------------------------------------------------------------------------
            define protected amdgpu_kernel void @test_func(i32 %arg1, i32 %arg2)
{
              entry:
  %arrayidx274 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %arg1
          ---------
  br i1 %cmp1, label %header, label %for.end

header:
  % struct_var2.ld = load %struct_var2*, %struct_var2* addrspace(4)*
getelementptr inbounds (%class_var2, %class_var2 addrspace(4)* @Obj1, i64 0, i32
0, i32 3), align 8
  %struct_var2.gep = getelementptr inbounds %struct_var2, %struct_var2*
%struct_var2.ld, i64 undef, i32 0
  %struct_var1.cast = bitcast %struct_var1* %struct_var2.gep to i64*
  br label %for.body

for.body:
          ---------
   %arrayidx74 = getelementptr inbounds %struct_var2, %struct_var2*
%struct_var2.ld, i64 %idxprom73
   %struct_var2.cast = bitcast %struct_var2* %arrayidx74 to i64*
   %for.body.ld = load i64, i64* %struct_var2.cast, align 8
          ---------
  br i1 %cmp2, label %if.then, label %if.end

if.then:
  %rem = srem i32 1, %arg2
  %add125 = add nuw nsw i32 %rem, 1
  %arrayidx1705 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %add125
  %arrayidx1705.ld = load float, float addrspace(3)* %arrayidx1705, align 4
  %arrayidx274.ld = load float, float addrspace(3)* %arrayidx274, align 4
          ---------
  br label %if.end

if.end:
  store i64 %for.body.ld, i64* %struct_var1.cast, align 8
          ---------
  br i1 %cmp3, label %for.body, label %for.end

for.end:
  br exit

exit:
          }

----------------------------------------------------------------------------------------------------------------------------------------------------------
The actual crash and the back-trace:
----------------------------------------------------------------------------------------------------------------------------------------------------------
opt: $SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:2165: const
llvm::SCEV *llvm::ScalarEvolution::getAddExpr(SmallVectorImpl<const
llvm::SCEV *> &, SCEV::NoWrapFlags, unsigned int): Assertion
`getEffectiveSCEVType(Ops[i]->getType()) == ETy && "SCEVAddExpr
operand types don't match!"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash
backtrace.
Stack dump:
0.      Program arguments: $Tools/bin/opt -O3 test.ll -o out.ll

llvm::ScalarEvolution::getAddExpr(llvm::SmallVectorImpl<llvm::SCEV
const*>&, llvm::SCEV::NoWrapFlags, unsigned int)
$SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:2164:5
llvm::ScalarEvolution::getAddExpr(llvm::SCEV const*, llvm::SCEV const*,
llvm::SCEV::NoWrapFlags, unsigned int)
$SRC/llvm-project/llvm/include/llvm/Analysis/ScalarEvolution.h:526:3
llvm::ScalarEvolution::getMinusSCEV(llvm::SCEV const*, llvm::SCEV const*,
llvm::SCEV::NoWrapFlags, unsigned int)
$SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:3833:3
getMinFromExprs(llvm::SCEV const*, llvm::SCEV const*, llvm::ScalarEvolution*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:275:15
llvm::RuntimeCheckingPtrGroup::addPointer(unsigned int)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:292:15
llvm::RuntimePointerChecking::groupChecks(llvm::EquivalenceClasses<llvm::PointerIntPair<llvm::Value*,
1u, bool, llvm::PointerLikeTypeTraits<llvm::Value*>,
llvm::PointerIntPairInfo<llvm::Value*, 1u,
llvm::PointerLikeTypeTraits<llvm::Value*> > > >&, bool)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:413:13
llvm::RuntimePointerChecking::generateChecks(llvm::EquivalenceClasses<llvm::PointerIntPair<llvm::Value*,
1u, bool, llvm::PointerLikeTypeTraits<llvm::Value*>,
llvm::PointerIntPairInfo<llvm::Value*, 1u,
llvm::PointerLikeTypeTraits<llvm::Value*> > > >&, bool)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:259:12
(anonymous
namespace)::AccessAnalysis::canCheckPtrAtRT(llvm::RuntimePointerChecking&,
llvm::ScalarEvolution*, llvm::Loop*, llvm::DenseMap<llvm::Value const*,
llvm::Value*, llvm::DenseMapInfo<llvm::Value const*>,
llvm::detail::DenseMapPair<llvm::Value const*, llvm::Value*> >
const&, bool)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:830:3
llvm::LoopAccessInfo::analyzeLoop(llvm::AAResults*, llvm::LoopInfo*,
llvm::TargetLibraryInfo const*, llvm::DominatorTree*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2038:8
llvm::LoopAccessInfo::LoopAccessInfo(llvm::Loop*, llvm::ScalarEvolution*,
llvm::TargetLibraryInfo const*, llvm::AAResults*, llvm::DominatorTree*,
llvm::LoopInfo*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2222:1
std::_MakeUniq<llvm::LoopAccessInfo>::__single_object
std::make_unique<llvm::LoopAccessInfo, llvm::Loop*&,
llvm::ScalarEvolution*&, llvm::TargetLibraryInfo const*&,
llvm::AAResults*&, llvm::DominatorTree*&,
llvm::LoopInfo*&>(llvm::Loop*&, llvm::ScalarEvolution*&,
llvm::TargetLibraryInfo const*&, llvm::AAResults*&,
llvm::DominatorTree*&, llvm::LoopInfo*&)
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/bits/unique_ptr.h:821:34
llvm::LoopAccessLegacyAnalysis::getInfo(llvm::Loop*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2275:5
(anonymous
namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)::'lambda'(llvm::Loop&)::operator()(llvm::Loop&)
const
$SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:651:53
llvm::LoopAccessInfo const& llvm::function_ref<llvm::LoopAccessInfo
const& (llvm::Loop&)>::callback_fn<(anonymous
namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)::'lambda'(llvm::Loop&)>(long,
llvm::Loop&) $SRC/llvm-project/llvm/include/llvm/ADT/STLExtras.h:185:5
llvm::function_ref<llvm::LoopAccessInfo const&
(llvm::Loop&)>::operator()(llvm::Loop&) const
$SRC/llvm-project/llvm/include/llvm/ADT/STLExtras.h:203:5
eliminateLoadsAcrossLoops(llvm::Function&, llvm::LoopInfo&,
llvm::DominatorTree&, llvm::BlockFrequencyInfo*, llvm::ProfileSummaryInfo*,
llvm::function_ref<llvm::LoopAccessInfo const& (llvm::Loop&)>)
$SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:618:53
(anonymous namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)
$SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:649:5
llvm::FPPassManager::runOnFunction(llvm::Function&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1587:23
llvm::FPPassManager::runOnModule(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1629:16
(anonymous namespace)::MPPassManager::runOnModule(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1698:23
llvm::legacy::PassManagerImpl::run(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:614:16
llvm::legacy::PassManager::run(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1824:3
main $SRC/llvm-project/llvm/tools/opt/opt.cpp:955:3
__libc_start_main /build/glibc-2ORdQG/glibc-2.27/csu/../csu/libc-start.c:344:0
_start ($Tools/bin/opt+0xc98b6a)
Aborted (core dumped)
-----------------------------------------------------------------------------------------------------------------------------------------------------------------

Regards,
CD
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Stefanos Baziotis via llvm-dev

2020-Jul-26 12:39 UTC

head link

[llvm-dev] [LAA] RtCheck on pointers of different address spaces.

Hi,

There are a lot of things going on here, but given this:
>  The crash occurs with the pointers 1 & 4 which are from AS1 and AS2respectively.

and the trace, I'm not sure how that can happen. `groupChecks()` will only
try to group pointers that
are on the same alias set (because it will only try to group pointers that
are in the same Eq class
in DepCands, which if you see its construction in `processMemAccesses()`,
won't put two pointers
from different alias sets in the same Eq Class because _theoretically_, two
such pointers can't
share an underlying object).

Do maybe have a simplified but complete IR ? Is that a possibility?

Kind regards,
Stefanos Baziotis

Στις Κυρ, 26 Ιουλ 2020 στις 1:06 μ.μ., ο/η Devadasan, Christudasan via
llvm-dev <llvm-dev at lists.llvm.org> έγραψε:
> Hello,
>
>
>
> I Have a question related to the RT check on pointers during Loop Access
> Analysis pass.
>
>
>
> There is a testcase with loop code that consist of 4 different memory
> operations referring two global objects of different address spaces.
>
> One from global constant (address space 4, addr_size = 64) and the other
> from local, LDS (address space 3, addr_size= 32).
>
> (Details of various address spaces available for AMDGPU backend:
> https://llvm.org/docs/AMDGPUUsage.html#address-spaces)
>
>
>
> With upstream compiler, the testcase fails with a crash (given at the end
> of the e-mail) in the opt while trying to generate the RT check for these
> pointers. Precisely, with two pointers of different address spaces.
>
> The operand type check fails while trying to insert a ‘AddExpr’ SCEV node
> as their effective type differs for these pointers (One with 32-bit and the
> other with 64-bit)
>
>
>
> *Question: Is this intended to try for the RtCheck on pointers from
> different address spaces? *
>
>                    The comments given in the code snippet (below) hints
> they aren’t.
>
>
>
> Code snippet from LoopAccessAnalysis.cpp:
>
>
>
-----------------------------------------------------------------------------------------------------------------------
>
>   bool AccessAnalysis::canCheckPtrAtRT(...) {
>
>                   ----------
>
>   // If the pointers that we would use for the bounds comparison have
> different
>
>   // address spaces, assume the values aren't directly comparable, so
we
> can't
>
>   // use them for the runtime check. We also have to assume they could
>
>   // overlap. In the future there should be metadata for whether address
> spaces
>
>   // are disjoint.
>
>   unsigned NumPointers = RtCheck.Pointers.size();
>
>   for (unsigned i = 0; i < NumPointers; ++i) {
>
>    for (unsigned j = i + 1; j < NumPointers; ++j) {
>
>       // Only need to check pointers between two different dependency sets.
>
>           if (RtCheck.Pointers[i].DependencySetId =>
>           RtCheck.Pointers[j].DependencySetId)
>
>        continue;
>
>       // Only need to check pointers in the same alias set.
>
>       if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)
>
>         continue;
>
>
>
>       Value *PtrI = RtCheck.Pointers[i].PointerValue;
>
>       Value *PtrJ = RtCheck.Pointers[j].PointerValue;
>
>
>
>       unsigned ASi = PtrI->getType()->getPointerAddressSpace();
>
>       unsigned ASj = PtrJ->getType()->getPointerAddressSpace();
>
>       if (ASi != ASj) {
>
>         LLVM_DEBUG(
>
>             dbgs() << "LAA: Runtime check would require
comparison between"
>
>                       " different address spaces\n");
>
>         return false;
>
>       }
>
>     }
>
>
>
----------------------------------------------------------------------------------------------------
>
> More details about the objects, the pointers and the memory operations:
>
>
>
----------------------------------------------------------------------------------------------------
>
> %struct_var1 = type { <2 x float> }
>
> %struct_var2 = type { %struct_var1 }
>
> %class_var1 = type { i32, i32, i32, %struct_var2*, i32, i32, i32}
>
> %class_var2 = type { %class_var1, i8, i8*, i32 }
>
>
>
> Objects:
>
> @Obj1 = external protected local_unnamed_addr addrspace(4)
> externally_initialized global %class_var2, align 8
>
> @Obj2 = internal unnamed_addr addrspace(3) constant [4000 x float] undef,
> align 16
>
>
>
> Pointers:
>
> 1.       %struct_var1.cast = bitcast %struct_var1* %struct_var2.gep to
> i64* (write)       // AS1
>
> 2.       %struct_var2.cast = bitcast %struct_var2* %arrayidx74 to i64*
> (read-only)        // AS1
>
> 3.       %arrayidx1705 = getelementptr inbounds [4000 x float], [4000 x
> float] addrspace(3)* @Obj2, i32 0, i32 %add125 (read-only) // AS2
>
> 4.       %arrayidx274 = getelementptr inbounds [4000 x float], [4000 x
> float] addrspace(3)* @Obj2, i32 0, i32 %arg1 (read-only)  // AS2
>
>
>
> While the pointers 1 & 2 belong to one Alias Set (AS1), pointers 3
& 4
> belong to a different set (AS2). It is because, the Global Alias Analysis
> found there is no alias between these two sets of pointers.
>
> The crash occurs with the pointers 1 & 4 which are from AS1 and AS2
> respectively.
>
> The DependenceSetId will be reset to 1 before processing a new AS and the
> pointers in question, ended up having the same DependenceSetId (1), though
> they are from different AS.
>
>
>
>
>           
----------------------------------------------------------------------------------------------------------------------------------------------------------
>
>             The load/store (memory operations) using these pointers in the
> loop. (extracted only the relevant instructions):
>
>
>           
----------------------------------------------------------------------------------------------------------------------------------------------------------
>
>             define protected amdgpu_kernel void @test_func(i32 %arg1, i32
> %arg2) {
>
>               entry:
>
>   *%arrayidx274 = getelementptr inbounds [4000 x float], [4000 x float]
> addrspace(3)* @Obj2, i32 0, i32 %arg1*
>
>           ---------
>
>   br i1 %cmp1, label %header, label %for.end
>
>
>
> header:
>
>   % *struct_var2.ld* = load %struct_var2*, %struct_var2* addrspace(4)*
> getelementptr inbounds (%class_var2, %class_var2 addrspace(4)* @Obj1, i64
> 0, i32 0, i32 3), align 8
>
>   %struct_var2.gep = getelementptr inbounds %struct_var2, %struct_var2*
> %struct_var2.ld, i64 undef, i32 0
>
> *  %struct_var1.cast = bitcast %struct_var1* %struct_var2.gep to i64**
>
>   br label %for.body
>
>
>
> for.body:
>
>           ---------
>
>    %arrayidx74 = getelementptr inbounds %struct_var2, %struct_var2*
> %struct_var2.ld, i64 %idxprom73
>
> *   %struct_var2.cast = bitcast %struct_var2* %arrayidx74 to i64**
>
>    %for.body.ld = load i64, i64* %*struct_var2.cast*, align 8
>
>           ---------
>
>   br i1 %cmp2, label %if.then, label %if.end
>
>
>
> if.then:
>
>   %rem = srem i32 1, %arg2
>
>   %add125 = add nuw nsw i32 %rem, 1
>
> *  %arrayidx1705 = getelementptr inbounds [4000 x float], [4000 x float]
> addrspace(3)* @Obj2, i32 0, i32 %add125*
>
>   %arrayidx1705.ld = load float, float addrspace(3)* %*arrayidx1705*,
> align 4
>
>   %arrayidx274.ld = load float, float addrspace(3)* %*arrayidx274*, align
> 4
>
>           ---------
>
>   br label %if.end
>
>
>
> if.end:
>
>   store i64 %for.body.ld, i64* %*struct_var1.cast*, align 8
>
>           ---------
>
>   br i1 %cmp3, label %for.body, label %for.end
>
>
>
> for.end:
>
>   br exit
>
>
>
> exit:
>
>           }
>
>
>
>
>
----------------------------------------------------------------------------------------------------------------------------------------------------------
>
> The actual crash and the back-trace:
>
>
>
----------------------------------------------------------------------------------------------------------------------------------------------------------
>
> opt: $SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:2165: const
> llvm::SCEV *llvm::ScalarEvolution::getAddExpr(SmallVectorImpl<const
> llvm::SCEV *> &, SCEV::NoWrapFlags, unsigned int): Assertion
> `getEffectiveSCEVType(Ops[i]->getType()) == ETy &&
"SCEVAddExpr operand
> types don't match!"' failed.
>
> PLEASE submit a bug report to https://bugs.llvm.org/ and include the
> crash backtrace.
>
> Stack dump:
>
> 0.      Program arguments: $Tools/bin/opt -O3 test.ll -o out.ll
>
>
>
> llvm::ScalarEvolution::getAddExpr(llvm::SmallVectorImpl<llvm::SCEV
> const*>&, llvm::SCEV::NoWrapFlags, unsigned int)
> $SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:2164:5
>
> llvm::ScalarEvolution::getAddExpr(llvm::SCEV const*, llvm::SCEV const*,
> llvm::SCEV::NoWrapFlags, unsigned int)
> $SRC/llvm-project/llvm/include/llvm/Analysis/ScalarEvolution.h:526:3
>
> llvm::ScalarEvolution::getMinusSCEV(llvm::SCEV const*, llvm::SCEV const*,
> llvm::SCEV::NoWrapFlags, unsigned int)
> $SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:3833:3
>
> getMinFromExprs(llvm::SCEV const*, llvm::SCEV const*,
> llvm::ScalarEvolution*)
> $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:275:15
>
> llvm::RuntimeCheckingPtrGroup::addPointer(unsigned int)
> $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:292:15
>
>
llvm::RuntimePointerChecking::groupChecks(llvm::EquivalenceClasses<llvm::PointerIntPair<llvm::Value*,
> 1u, bool, llvm::PointerLikeTypeTraits<llvm::Value*>,
> llvm::PointerIntPairInfo<llvm::Value*, 1u,
> llvm::PointerLikeTypeTraits<llvm::Value*> > > >&, bool)
> $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:413:13
>
>
llvm::RuntimePointerChecking::generateChecks(llvm::EquivalenceClasses<llvm::PointerIntPair<llvm::Value*,
> 1u, bool, llvm::PointerLikeTypeTraits<llvm::Value*>,
> llvm::PointerIntPairInfo<llvm::Value*, 1u,
> llvm::PointerLikeTypeTraits<llvm::Value*> > > >&, bool)
> $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:259:12
>
> (anonymous
>
namespace)::AccessAnalysis::canCheckPtrAtRT(llvm::RuntimePointerChecking&,
> llvm::ScalarEvolution*, llvm::Loop*, llvm::DenseMap<llvm::Value const*,
> llvm::Value*, llvm::DenseMapInfo<llvm::Value const*>,
> llvm::detail::DenseMapPair<llvm::Value const*, llvm::Value*> >
const&,
> bool) $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:830:3
>
> llvm::LoopAccessInfo::analyzeLoop(llvm::AAResults*, llvm::LoopInfo*,
> llvm::TargetLibraryInfo const*, llvm::DominatorTree*)
> $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2038:8
>
> llvm::LoopAccessInfo::LoopAccessInfo(llvm::Loop*, llvm::ScalarEvolution*,
> llvm::TargetLibraryInfo const*, llvm::AAResults*, llvm::DominatorTree*,
> llvm::LoopInfo*)
> $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2222:1
>
> std::_MakeUniq<llvm::LoopAccessInfo>::__single_object
> std::make_unique<llvm::LoopAccessInfo, llvm::Loop*&,
> llvm::ScalarEvolution*&, llvm::TargetLibraryInfo const*&,
> llvm::AAResults*&, llvm::DominatorTree*&,
llvm::LoopInfo*&>(llvm::Loop*&,
> llvm::ScalarEvolution*&, llvm::TargetLibraryInfo const*&,
> llvm::AAResults*&, llvm::DominatorTree*&, llvm::LoopInfo*&)
>
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/bits/unique_ptr.h:821:34
>
> llvm::LoopAccessLegacyAnalysis::getInfo(llvm::Loop*)
> $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2275:5
>
> (anonymous
>
namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)::'lambda'(llvm::Loop&)::operator()(llvm::Loop&)
> const
> $SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:651:53
>
> llvm::LoopAccessInfo const& llvm::function_ref<llvm::LoopAccessInfo
const&
> (llvm::Loop&)>::callback_fn<(anonymous
>
namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)::'lambda'(llvm::Loop&)>(long,
> llvm::Loop&) $SRC/llvm-project/llvm/include/llvm/ADT/STLExtras.h:185:5
>
> llvm::function_ref<llvm::LoopAccessInfo const&
> (llvm::Loop&)>::operator()(llvm::Loop&) const
> $SRC/llvm-project/llvm/include/llvm/ADT/STLExtras.h:203:5
>
> eliminateLoadsAcrossLoops(llvm::Function&, llvm::LoopInfo&,
> llvm::DominatorTree&, llvm::BlockFrequencyInfo*,
llvm::ProfileSummaryInfo*,
> llvm::function_ref<llvm::LoopAccessInfo const&
(llvm::Loop&)>)
> $SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:618:53
>
> (anonymous
namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)
> $SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:649:5
>
> llvm::FPPassManager::runOnFunction(llvm::Function&)
> $SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1587:23
>
> llvm::FPPassManager::runOnModule(llvm::Module&)
> $SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1629:16
>
> (anonymous namespace)::MPPassManager::runOnModule(llvm::Module&)
> $SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1698:23
>
> llvm::legacy::PassManagerImpl::run(llvm::Module&)
> $SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:614:16
>
> llvm::legacy::PassManager::run(llvm::Module&)
> $SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1824:3
>
> main $SRC/llvm-project/llvm/tools/opt/opt.cpp:955:3
>
> __libc_start_main
> /build/glibc-2ORdQG/glibc-2.27/csu/../csu/libc-start.c:344:0
>
> _start ($Tools/bin/opt+0xc98b6a)
>
> Aborted (core dumped)
>
>
>
-----------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>
>
> Regards,
>
> CD
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Devadasan, Christudasan via llvm-dev

2020-Jul-26 17:52 UTC

head link

[llvm-dev] [LAA] RtCheck on pointers of different address spaces.

Hi Stefanos,

Attached the testcase. I tried to reduce it further, but the problem goes away
when I remove the instructions further.
There is a nested loop and the fault occurs while processing the inner loop
(for.body)
To reproduce the crash:
           opt -O3 testcase.ll -o out.ll
> `groupChecks()` will only try to group pointers that are on the same alias
set.
If that’s true, the RT check should have been prevented for these pointers in
question.

Additional details about the problem:
   The crash first appeared after the following llvm commit to preserve the
Global AA.

commit e6cf796bab7e02d2b8ac7fd495f14f5e21494270
Author: Ryan Santhiraraja <rsanthir at quicinc.com>
Date:   Thu Jul 2 13:53:20 2020 +0100
    Preserve GlobalsAA analysis result in LowerConstantIntrinsics

The Global AA now finds out that these pointers refer different objects and
won’t alias each other, and eventually ended up in different alias sets.
Prior to this commit, none of the alias analysis could appropriate the noAlias
property for these pointers.  (If you revert this patch locally, the testcase
will compile successfully).
The address-space validation (given below) prevented the RT check for these
pointers earlier which I expected even to get triggered with the upstream
compiler today.
But it didn’t occur as they have the same DependenceSetId value. (The
DependenceSetId starts from 1 for each Alias Set and hence pointers of
different AS can have the same Id. If it is intended, I am not sure that the
early continue here, only based on the DependenceSetId, handled all cases)


     // Only need to check pointers between two different dependency sets.

          if (RtCheck.Pointers[i].DependencySetId =
          RtCheck.Pointers[j].DependencySetId)

       continue;

      // Only need to check pointers in the same alias set.

      if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)

        continue;



      Value *PtrI = RtCheck.Pointers[i].PointerValue;

      Value *PtrJ = RtCheck.Pointers[j].PointerValue;



      unsigned ASi = PtrI->getType()->getPointerAddressSpace();

      unsigned ASj = PtrJ->getType()->getPointerAddressSpace();

      if (ASi != ASj) {

        LLVM_DEBUG(

            dbgs() << "LAA: Runtime check would require comparison
between"

                      " different address spaces\n");

        return false;
                   }
Regards,
CD

From: Stefanos Baziotis <stefanos.baziotis at gmail.com>
Sent: Sunday, July 26, 2020 6:09 PM
To: Devadasan, Christudasan <Christudasan.Devadasan at amd.com>
Cc: LLVM Dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [LAA] RtCheck on pointers of different address spaces.

[CAUTION: External Email]
Hi,

There are a lot of things going on here, but given this:
>  The crash occurs with the pointers 1 & 4 which are from AS1 and AS2
respectively.
and the trace, I'm not sure how that can happen. `groupChecks()` will only
try to group pointers that
are on the same alias set (because it will only try to group pointers that are
in the same Eq class
in DepCands, which if you see its construction in `processMemAccesses()`,
won't put two pointers
from different alias sets in the same Eq Class because _theoretically_, two such
pointers can't
share an underlying object).

Do maybe have a simplified but complete IR ? Is that a possibility?

Kind regards,
Stefanos Baziotis

Στις Κυρ, 26 Ιουλ 2020 στις 1:06 μ.μ., ο/η Devadasan, Christudasan via llvm-dev
<llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
έγραψε:
Hello,

I Have a question related to the RT check on pointers during Loop Access
Analysis pass.

There is a testcase with loop code that consist of 4 different memory operations
referring two global objects of different address spaces.
One from global constant (address space 4, addr_size = 64) and the other from
local, LDS (address space 3, addr_size= 32).
(Details of various address spaces available for AMDGPU backend:
https://llvm.org/docs/AMDGPUUsage.html#address-spaces<https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fllvm.org%2Fdocs%2FAMDGPUUsage.html%23address-spaces&data=02%7C01%7CChristudasan.Devadasan%40amd.com%7C4a42db7316004baa429c08d83160f7e7%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637313639833494292&sdata=r7fk%2FU%2FCZLt8RKZNc0lAco6wBJKKDj%2Br621r%2FaBOI6A%3D&reserved=0>)

With upstream compiler, the testcase fails with a crash (given at the end of the
e-mail) in the opt while trying to generate the RT check for these pointers.
Precisely, with two pointers of different address spaces.
The operand type check fails while trying to insert a ‘AddExpr’ SCEV node as
their effective type differs for these pointers (One with 32-bit and the other
with 64-bit)

Question: Is this intended to try for the RtCheck on pointers from different
address spaces?
                   The comments given in the code snippet (below) hints they
aren’t.

Code snippet from LoopAccessAnalysis.cpp:
-----------------------------------------------------------------------------------------------------------------------
  bool AccessAnalysis::canCheckPtrAtRT(...) {
                  ----------
  // If the pointers that we would use for the bounds comparison have different
  // address spaces, assume the values aren't directly comparable, so we
can't
  // use them for the runtime check. We also have to assume they could
  // overlap. In the future there should be metadata for whether address spaces
  // are disjoint.
  unsigned NumPointers = RtCheck.Pointers.size();
  for (unsigned i = 0; i < NumPointers; ++i) {
   for (unsigned j = i + 1; j < NumPointers; ++j) {
      // Only need to check pointers between two different dependency sets.
          if (RtCheck.Pointers[i].DependencySetId =         
RtCheck.Pointers[j].DependencySetId)
       continue;
      // Only need to check pointers in the same alias set.
      if (RtCheck.Pointers[i].AliasSetId != RtCheck.Pointers[j].AliasSetId)
        continue;

      Value *PtrI = RtCheck.Pointers[i].PointerValue;
      Value *PtrJ = RtCheck.Pointers[j].PointerValue;

      unsigned ASi = PtrI->getType()->getPointerAddressSpace();
      unsigned ASj = PtrJ->getType()->getPointerAddressSpace();
      if (ASi != ASj) {
        LLVM_DEBUG(
            dbgs() << "LAA: Runtime check would require comparison
between"
                      " different address spaces\n");
        return false;
      }
    }
----------------------------------------------------------------------------------------------------
More details about the objects, the pointers and the memory operations:
----------------------------------------------------------------------------------------------------
%struct_var1 = type { <2 x float> }
%struct_var2 = type { %struct_var1 }
%class_var1 = type { i32, i32, i32, %struct_var2*, i32, i32, i32}
%class_var2 = type { %class_var1, i8, i8*, i32 }

Objects:
@Obj1 = external protected local_unnamed_addr addrspace(4)
externally_initialized global %class_var2, align 8
@Obj2 = internal unnamed_addr addrspace(3) constant [4000 x float] undef, align
16

Pointers:

1.       %struct_var1.cast = bitcast %struct_var1* %struct_var2.gep to i64*
(write)       // AS1

2.       %struct_var2.cast = bitcast %struct_var2* %arrayidx74 to i64*
(read-only)        // AS1

3.       %arrayidx1705 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %add125 (read-only) // AS2

4.       %arrayidx274 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %arg1 (read-only)  // AS2

While the pointers 1 & 2 belong to one Alias Set (AS1), pointers 3 & 4
belong to a different set (AS2). It is because, the Global Alias Analysis found
there is no alias between these two sets of pointers.
The crash occurs with the pointers 1 & 4 which are from AS1 and AS2
respectively.
The DependenceSetId will be reset to 1 before processing a new AS and the
pointers in question, ended up having the same DependenceSetId (1), though they
are from different AS.

          
----------------------------------------------------------------------------------------------------------------------------------------------------------
            The load/store (memory operations) using these pointers in the loop.
(extracted only the relevant instructions):
          
----------------------------------------------------------------------------------------------------------------------------------------------------------
            define protected amdgpu_kernel void @test_func(i32 %arg1, i32 %arg2)
{
              entry:
  %arrayidx274 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %arg1
          ---------
  br i1 %cmp1, label %header, label %for.end

header:
  % struct_var2.ld = load %struct_var2*, %struct_var2* addrspace(4)*
getelementptr inbounds (%class_var2, %class_var2 addrspace(4)* @Obj1, i64 0, i32
0, i32 3), align 8
  %struct_var2.gep = getelementptr inbounds %struct_var2, %struct_var2*
%struct_var2.ld, i64 undef, i32 0
  %struct_var1.cast = bitcast %struct_var1* %struct_var2.gep to i64*
  br label %for.body

for.body:
          ---------
   %arrayidx74 = getelementptr inbounds %struct_var2, %struct_var2*
%struct_var2.ld, i64 %idxprom73
   %struct_var2.cast = bitcast %struct_var2* %arrayidx74 to i64*
   %for.body.ld = load i64, i64* %struct_var2.cast, align 8
          ---------
  br i1 %cmp2, label %if.then, label %if.end

if.then:
  %rem = srem i32 1, %arg2
  %add125 = add nuw nsw i32 %rem, 1
  %arrayidx1705 = getelementptr inbounds [4000 x float], [4000 x float]
addrspace(3)* @Obj2, i32 0, i32 %add125
  %arrayidx1705.ld = load float, float addrspace(3)* %arrayidx1705, align 4
  %arrayidx274.ld = load float, float addrspace(3)* %arrayidx274, align 4
          ---------
  br label %if.end

if.end:
  store i64 %for.body.ld, i64* %struct_var1.cast, align 8
          ---------
  br i1 %cmp3, label %for.body, label %for.end

for.end:
  br exit

exit:
          }

----------------------------------------------------------------------------------------------------------------------------------------------------------
The actual crash and the back-trace:
----------------------------------------------------------------------------------------------------------------------------------------------------------
opt: $SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:2165: const
llvm::SCEV *llvm::ScalarEvolution::getAddExpr(SmallVectorImpl<const
llvm::SCEV *> &, SCEV::NoWrapFlags, unsigned int): Assertion
`getEffectiveSCEVType(Ops[i]->getType()) == ETy && "SCEVAddExpr
operand types don't match!"' failed.
PLEASE submit a bug report to
https://bugs.llvm.org/<https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.llvm.org%2F&data=02%7C01%7CChristudasan.Devadasan%40amd.com%7C4a42db7316004baa429c08d83160f7e7%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637313639833494292&sdata=L20o6evK45pVbZE4E8csnx6Oc2mBvNihZNNTh0qYMN8%3D&reserved=0>
and include the crash backtrace.
Stack dump:
0.      Program arguments: $Tools/bin/opt -O3 test.ll -o out.ll

llvm::ScalarEvolution::getAddExpr(llvm::SmallVectorImpl<llvm::SCEV
const*>&, llvm::SCEV::NoWrapFlags, unsigned int)
$SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:2164:5
llvm::ScalarEvolution::getAddExpr(llvm::SCEV const*, llvm::SCEV const*,
llvm::SCEV::NoWrapFlags, unsigned int)
$SRC/llvm-project/llvm/include/llvm/Analysis/ScalarEvolution.h:526:3
llvm::ScalarEvolution::getMinusSCEV(llvm::SCEV const*, llvm::SCEV const*,
llvm::SCEV::NoWrapFlags, unsigned int)
$SRC/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:3833:3
getMinFromExprs(llvm::SCEV const*, llvm::SCEV const*, llvm::ScalarEvolution*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:275:15
llvm::RuntimeCheckingPtrGroup::addPointer(unsigned int)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:292:15
llvm::RuntimePointerChecking::groupChecks(llvm::EquivalenceClasses<llvm::PointerIntPair<llvm::Value*,
1u, bool, llvm::PointerLikeTypeTraits<llvm::Value*>,
llvm::PointerIntPairInfo<llvm::Value*, 1u,
llvm::PointerLikeTypeTraits<llvm::Value*> > > >&, bool)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:413:13
llvm::RuntimePointerChecking::generateChecks(llvm::EquivalenceClasses<llvm::PointerIntPair<llvm::Value*,
1u, bool, llvm::PointerLikeTypeTraits<llvm::Value*>,
llvm::PointerIntPairInfo<llvm::Value*, 1u,
llvm::PointerLikeTypeTraits<llvm::Value*> > > >&, bool)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:259:12
(anonymous
namespace)::AccessAnalysis::canCheckPtrAtRT(llvm::RuntimePointerChecking&,
llvm::ScalarEvolution*, llvm::Loop*, llvm::DenseMap<llvm::Value const*,
llvm::Value*, llvm::DenseMapInfo<llvm::Value const*>,
llvm::detail::DenseMapPair<llvm::Value const*, llvm::Value*> >
const&, bool)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:830:3
llvm::LoopAccessInfo::analyzeLoop(llvm::AAResults*, llvm::LoopInfo*,
llvm::TargetLibraryInfo const*, llvm::DominatorTree*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2038:8
llvm::LoopAccessInfo::LoopAccessInfo(llvm::Loop*, llvm::ScalarEvolution*,
llvm::TargetLibraryInfo const*, llvm::AAResults*, llvm::DominatorTree*,
llvm::LoopInfo*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2222:1
std::_MakeUniq<llvm::LoopAccessInfo>::__single_object
std::make_unique<llvm::LoopAccessInfo, llvm::Loop*&,
llvm::ScalarEvolution*&, llvm::TargetLibraryInfo const*&,
llvm::AAResults*&, llvm::DominatorTree*&,
llvm::LoopInfo*&>(llvm::Loop*&, llvm::ScalarEvolution*&,
llvm::TargetLibraryInfo const*&, llvm::AAResults*&,
llvm::DominatorTree*&, llvm::LoopInfo*&)
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/bits/unique_ptr.h:821:34
llvm::LoopAccessLegacyAnalysis::getInfo(llvm::Loop*)
$SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:2275:5
(anonymous
namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)::'lambda'(llvm::Loop&)::operator()(llvm::Loop&)
const
$SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:651:53
llvm::LoopAccessInfo const& llvm::function_ref<llvm::LoopAccessInfo
const& (llvm::Loop&)>::callback_fn<(anonymous
namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)::'lambda'(llvm::Loop&)>(long,
llvm::Loop&) $SRC/llvm-project/llvm/include/llvm/ADT/STLExtras.h:185:5
llvm::function_ref<llvm::LoopAccessInfo const&
(llvm::Loop&)>::operator()(llvm::Loop&) const
$SRC/llvm-project/llvm/include/llvm/ADT/STLExtras.h:203:5
eliminateLoadsAcrossLoops(llvm::Function&, llvm::LoopInfo&,
llvm::DominatorTree&, llvm::BlockFrequencyInfo*, llvm::ProfileSummaryInfo*,
llvm::function_ref<llvm::LoopAccessInfo const& (llvm::Loop&)>)
$SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:618:53
(anonymous namespace)::LoopLoadElimination::runOnFunction(llvm::Function&)
$SRC/llvm-project/llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp:649:5
llvm::FPPassManager::runOnFunction(llvm::Function&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1587:23
llvm::FPPassManager::runOnModule(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1629:16
(anonymous namespace)::MPPassManager::runOnModule(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1698:23
llvm::legacy::PassManagerImpl::run(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:614:16
llvm::legacy::PassManager::run(llvm::Module&)
$SRC/llvm-project/llvm/lib/IR/LegacyPassManager.cpp:1824:3
main $SRC/llvm-project/llvm/tools/opt/opt.cpp:955:3
__libc_start_main /build/glibc-2ORdQG/glibc-2.27/csu/../csu/libc-start.c:344:0
_start ($Tools/bin/opt+0xc98b6a)
Aborted (core dumped)
-----------------------------------------------------------------------------------------------------------------------------------------------------------------

Regards,
CD
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