Skip to content

[GVN] Teach GVN simple masked load/store forwarding #157689

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 6 commits into from
Oct 3, 2025
Merged

[GVN] Teach GVN simple masked load/store forwarding #157689

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
6 commits
Select commit Hold shift + click to select a range
11ac166
[GVN] Teach GVN simple masked load/store forwarding
MDevereau Sep 5, 2025
c335ed6
Use PatternMatch
MDevereau Sep 10, 2025
0d7107f
Remove intrinsic declarations
MDevereau Sep 10, 2025
8f7fe4a
Respond to david-arm's review comments
MDevereau Sep 23, 2025
a3a6f9f
Remove pointer check and extra test
MDevereau Sep 29, 2025
6693704
Add bail and test for type mismatch
MDevereau Sep 30, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
8 changes: 8 additions & 0 deletions llvm/include/llvm/IR/PatternMatch.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2781,6 +2781,14 @@ m_MaskedLoad(const Opnd0 &Op0, const Opnd1 &Op1, const Opnd2 &Op2,
return m_Intrinsic<Intrinsic::masked_load>(Op0, Op1, Op2, Op3);
}

/// Matches MaskedStore Intrinsic.
template <typename Opnd0, typename Opnd1, typename Opnd2, typename Opnd3>
inline typename m_Intrinsic_Ty<Opnd0, Opnd1, Opnd2, Opnd3>::Ty
m_MaskedStore(const Opnd0 &Op0, const Opnd1 &Op1, const Opnd2 &Op2,
const Opnd3 &Op3) {
return m_Intrinsic<Intrinsic::masked_store>(Op0, Op1, Op2, Op3);
}

/// Matches MaskedGather Intrinsic.
template <typename Opnd0, typename Opnd1, typename Opnd2, typename Opnd3>
inline typename m_Intrinsic_Ty<Opnd0, Opnd1, Opnd2, Opnd3>::Ty
Expand Down
2 changes: 2 additions & 0 deletions llvm/include/llvm/Transforms/Scalar/GVN.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@ class OptimizationRemarkEmitter;
class PHINode;
class TargetLibraryInfo;
class Value;
class IntrinsicInst;
/// A private "module" namespace for types and utilities used by GVN. These
/// are implementation details and should not be used by clients.
namespace LLVM_LIBRARY_VISIBILITY_NAMESPACE gvn {
Expand Down Expand Up @@ -349,6 +350,7 @@ class GVNPass : public PassInfoMixin<GVNPass> {

// Helper functions of redundant load elimination.
bool processLoad(LoadInst *L);
bool processMaskedLoad(IntrinsicInst *I);
bool processNonLocalLoad(LoadInst *L);
bool processAssumeIntrinsic(AssumeInst *II);

Expand Down
33 changes: 33 additions & 0 deletions llvm/lib/Transforms/Scalar/GVN.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -2287,6 +2287,35 @@ bool GVNPass::processLoad(LoadInst *L) {
return true;
}

// Attempt to process masked loads which have loaded from
// masked stores with the same mask
bool GVNPass::processMaskedLoad(IntrinsicInst *I) {
if (!MD)
return false;
MemDepResult Dep = MD->getDependency(I);
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Looking at processLoad it seems like you need an additional check:

  if (!MD)
    return false;

Would be good to add a test for this too.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I've added the test masked-load-store-no-mem-dep.ll to deal with this.

Instruction *DepInst = Dep.getInst();
if (!DepInst || !Dep.isLocal() || !Dep.isDef())
return false;

Value *Mask = I->getOperand(2);
Value *Passthrough = I->getOperand(3);
Value *StoreVal;
if (!match(DepInst, m_MaskedStore(m_Value(StoreVal), m_Value(), m_Value(),
m_Specific(Mask))) ||
StoreVal->getType() != I->getType())
return false;

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

If a dependency is local then it's either a Clobber or a Def, and it looks like the code below is assuming a Def. Would be good to clarify this. If so, I think you have to bail out for the clobber case as that's more complicated - see AnalyzeLoadAvailability.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I added a bail-out if it's not a Def

// Remove the load but generate a select for the passthrough
Value *OpToForward = llvm::SelectInst::Create(Mask, StoreVal, Passthrough, "",
I->getIterator());

ICF->removeUsersOf(I);
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I don't if ordering matters, but I noticed that in processLoads we do this the other way around:

  ICF->removeUsersOf(L);
  L->replaceAllUsesWith(AvailableValue);

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Done.

I->replaceAllUsesWith(OpToForward);
salvageAndRemoveInstruction(I);
Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Do we also need to call

  if (MSSAU)
    MSSAU->removeMemoryAccess(L);

and

  if (MD && AvailableValue->getType()->isPtrOrPtrVectorTy())
    MD->invalidateCachedPointerInfo(AvailableValue);

similar to processLoads?

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more. 

if (MSSAU)
  MSSAU->removeMemoryAccess(L);

This is indirectly called by

salvageAndRemoveInstruction(I)
//...
removeInstruction(I)

I've added the Pointer stuff though.

++NumGVNLoad;
return true;
}

/// Return a pair the first field showing the value number of \p Exp and the
/// second field showing whether it is a value number newly created.
std::pair<uint32_t, bool>
Expand Down Expand Up @@ -2734,6 +2763,10 @@ bool GVNPass::processInstruction(Instruction *I) {
return false;
}

if (match(I, m_Intrinsic<Intrinsic::masked_load>()) &&
processMaskedLoad(cast<IntrinsicInst>(I)))
return true;

// For conditional branches, we can perform simple conditional propagation on
// the condition value itself.
if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
Expand Down
34 changes: 34 additions & 0 deletions llvm/test/Transforms/GVN/masked-load-store-no-mem-dep.ll
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,34 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -passes=gvn -S -enable-gvn-memdep=true < %s | FileCheck %s
; RUN: opt -passes=gvn -S -enable-gvn-memdep=false < %s | FileCheck %s --check-prefix=MEMDEPFALSE

define <4 x float> @forward_binop_with_sel(ptr %0, ptr %1, i32 %a, i32 %b, <4 x float> %passthrough) {
; CHECK-LABEL: @forward_binop_with_sel(
; CHECK-NEXT: [[MASK:%.*]] = tail call <4 x i1> @llvm.get.active.lane.mask.v4i1.i32(i32 [[A:%.*]], i32 [[B:%.*]])
; CHECK-NEXT: [[LOAD_0_0:%.*]] = call <4 x float> @llvm.masked.load.v4f32.p0(ptr [[TMP0:%.*]], i32 1, <4 x i1> [[MASK]], <4 x float> zeroinitializer)
; CHECK-NEXT: [[GEP_0_16:%.*]] = getelementptr i8, ptr [[TMP0]], i32 16
; CHECK-NEXT: [[LOAD_0_16:%.*]] = call <4 x float> @llvm.masked.load.v4f32.p0(ptr [[GEP_0_16]], i32 1, <4 x i1> [[MASK]], <4 x float> zeroinitializer)
; CHECK-NEXT: [[FMUL:%.*]] = fmul <4 x float> [[LOAD_0_0]], [[LOAD_0_16]]
; CHECK-NEXT: call void @llvm.masked.store.v4f32.p0(<4 x float> [[FMUL]], ptr [[TMP1:%.*]], i32 1, <4 x i1> [[MASK]])
; CHECK-NEXT: [[TMP3:%.*]] = select <4 x i1> [[MASK]], <4 x float> [[FMUL]], <4 x float> [[PASSTHROUGH:%.*]]
; CHECK-NEXT: ret <4 x float> [[TMP3]]
;
; MEMDEPFALSE-LABEL: @forward_binop_with_sel(
; MEMDEPFALSE-NEXT: [[MASK:%.*]] = tail call <4 x i1> @llvm.get.active.lane.mask.v4i1.i32(i32 [[A:%.*]], i32 [[B:%.*]])
; MEMDEPFALSE-NEXT: [[LOAD_0_0:%.*]] = call <4 x float> @llvm.masked.load.v4f32.p0(ptr [[TMP0:%.*]], i32 1, <4 x i1> [[MASK]], <4 x float> zeroinitializer)
; MEMDEPFALSE-NEXT: [[GEP_0_16:%.*]] = getelementptr i8, ptr [[TMP0]], i32 16
; MEMDEPFALSE-NEXT: [[LOAD_0_16:%.*]] = call <4 x float> @llvm.masked.load.v4f32.p0(ptr [[GEP_0_16]], i32 1, <4 x i1> [[MASK]], <4 x float> zeroinitializer)
; MEMDEPFALSE-NEXT: [[FMUL:%.*]] = fmul <4 x float> [[LOAD_0_0]], [[LOAD_0_16]]
; MEMDEPFALSE-NEXT: call void @llvm.masked.store.v4f32.p0(<4 x float> [[FMUL]], ptr [[TMP1:%.*]], i32 1, <4 x i1> [[MASK]])
; MEMDEPFALSE-NEXT: [[LOAD_1_0:%.*]] = call <4 x float> @llvm.masked.load.v4f32.p0(ptr [[TMP1]], i32 1, <4 x i1> [[MASK]], <4 x float> [[PASSTHROUGH:%.*]])
; MEMDEPFALSE-NEXT: ret <4 x float> [[LOAD_1_0]]
;
%mask = tail call <4 x i1> @llvm.get.active.lane.mask.v4i1.i32(i32 %a, i32 %b)
%load.0.0 = call <4 x float> @llvm.masked.load.v4f32.p0(ptr %0, i32 1, <4 x i1> %mask, <4 x float> zeroinitializer)
%gep.0.16 = getelementptr i8, ptr %0, i32 16
%load.0.16 = call <4 x float> @llvm.masked.load.v4f32.p0(ptr %gep.0.16, i32 1, <4 x i1> %mask, <4 x float> zeroinitializer)
%fmul = fmul <4 x float> %load.0.0, %load.0.16
call void @llvm.masked.store.v4f32.p0(<4 x float> %fmul, ptr %1, i32 1, <4 x i1> %mask)
%load.1.0 = call <4 x float> @llvm.masked.load.v4f32.p0(ptr %1, i32 1, <4 x i1> %mask, <4 x float> %passthrough)
ret <4 x float> %load.1.0
}
Loading