[llvm-dev] [LAA] RtCheck on pointers of different address spaces.
Devadasan, Christudasan via llvm-dev
llvm-dev at lists.llvm.org
Sun Jul 26 18:22:22 PDT 2020
Thanks for looking into it Stefanos and Florian.
Truly appreciate your effort with the analysis and initiating the fix.
Regards,
CD
From: Stefanos Baziotis <stefanos.baziotis at gmail.com>
Sent: Monday, July 27, 2020 4:17 AM
To: Florian Hahn <florian_hahn at apple.com>
Cc: Devadasan, Christudasan <Christudasan.Devadasan at amd.com>; LLVM Dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [LAA] RtCheck on pointers of different address spaces.
[CAUTION: External Email]
Hi,
Florian thanks for creating the bug report. Actually, I couldn't reproduce the bug with -O3, but with -globals-aa -loop-load-elim I can. Not sure why.
> Attached the testcase.
Thanks for that!
> > `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.
Exactly, that was my point :)
Now, a little info from my trying to find the source of the bug for anyone interested:
In the test case we have 4 pointers as you said:
%tmp4 = bitcast %struct.barney* %tmp3 to i64* -- AS1
%tmp12 = bitcast %struct.wombat* %tmp11 to i64* -- AS1
%tmp16 = getelementptr inbounds [4000 x float], [4000 x float] addrspace(3)* @global.1, i32 0, i32 %tmp15 -- AS2
%tmp = getelementptr inbounds [4000 x float], [4000 x float] addrspace(3)* @global.1, i32 0, i32 %arg -- AS2
First of all, the claim above holds for this case too: groupChecks() won't try to group pointers that are
not in the same alias set because they won't be in the same EqClass in DepCands. While groupChecks()
here tries to group %tmp4 and %tmp16 together, this is _not because_ they're in the same EqClass
but because of a bug.
You see, groupChecks() starts by assigning an index to each pointer in Pointers (using PositionMap).
But if you put a debug print in that loop, you'll see that %tmp16 is _not_ in Pointers. So, there's no
index assigned to it.
But, %tmp16 is in the same EqClass with %tmp. And %tmp _is_ in Pointers. Because of the latter,
it means we'll come across it in the next loop. Which in turn means that we'll come across any pointer
in %tmp's EqClass (will try to `addPointer()` it to some Group). So, at some point, we'll try to handle %tmp16.
But remember, %tmp16 does not have an entry in PositionMap. So, this line:
unsigned Pointer = PositionMap[MI->getPointer()];
will make Pointer = 0 (you get 0 when it's not found in this DenseMap).
Now, then we proceed by calling addPointer() which has this line:
const SCEV *Start = RtCheck.Pointers[Index].Start;
Remember, Index here is equal to Pointer previously. So, 0. But if you remember, when
we assigned indexes to pointers (in the PositionMap loop), we started from 0. So, _some pointer_
has an index of 0. And that happens to be %tmp4. Calling then getMinFromExprs() eventually
results in the crash you saw.
I'm not sure how we should solve that. I mean, it would be good if we used
1-based indexing somehow and assert that we never get a 0 back when
we query PositionMap but that's not the root of the problem.
P.S. The reason that %tmp16 doesn't make it to Pointers is because in createCheckForAccess(),
which is called from canCheckPtrAtRT(), we couldn't find its bounds.
Cheers,
Stefanos
Στις Δευ, 27 Ιουλ 2020 στις 12:34 π.μ., ο/η Florian Hahn <florian_hahn at apple.com<mailto:florian_hahn at apple.com>> έγραψε:
Thanks for sharing the reproducer. I reduced it a bit and filed a bug report https://bugs.llvm.org/show_bug.cgi?id=46854<https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.llvm.org%2Fshow_bug.cgi%3Fid%3D46854&data=02%7C01%7CChristudasan.Devadasan%40amd.com%7C3c166e3fc5d145ab590508d831b5c58e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637314004075060708&sdata=4BJLUkH4Ez%2BmRwnk%2FZE0kWZWCcsBxczn0MgbTqFhN0E%3D&reserved=0>
Cheers,
Florian
On Jul 26, 2020, at 18:52, Devadasan, Christudasan via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
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<mailto: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<mailto:stefanos.baziotis at gmail.com>>
Sent: Sunday, July 26, 2020 6:09 PM
To: Devadasan, Christudasan <Christudasan.Devadasan at amd.com<mailto:Christudasan.Devadasan at amd.com>>
Cc: LLVM Dev <llvm-dev at lists.llvm.org<mailto: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%7C3c166e3fc5d145ab590508d831b5c58e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637314004075070704&sdata=3Fx51NaDT1WAwLXeWXR1%2FoOx%2FpTCfbVAgOh85MhU4Ks%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%7C3c166e3fc5d145ab590508d831b5c58e%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637314004075070704&sdata=zkdapYGZIcF9y9LZm2wEZi0Dnge8dzHTb6qjjU%2BEaFQ%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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