[llvm-dev] [LAA] RtCheck on pointers of different address spaces. (original) (raw)

Devadasan, Christudasan via llvm-dev llvm-dev at lists.llvm.org
Sun Jul 26 03:06:27 PDT 2020

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::PointerLikeTypeTraitsllvm::Value*, llvm::PointerIntPairInfo<llvm::Value*, 1u, llvm::PointerLikeTypeTraitsllvm::Value* > > >&, bool) $SRC/llvm-project/llvm/lib/Analysis/LoopAccessAnalysis.cpp:413:13 llvm::RuntimePointerChecking::generateChecks(llvm::EquivalenceClasses<llvm::PointerIntPair<llvm::Value*, 1u, bool, llvm::PointerLikeTypeTraitsllvm::Value*, llvm::PointerIntPairInfo<llvm::Value*, 1u, llvm::PointerLikeTypeTraitsllvm::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::_MakeUniqllvm::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)

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