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[InstCombine] Propagate invariant.load metadata across unpacked loads #152186

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Merged
merged 7 commits into from
Aug 14, 2025
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[InstCombine] Propagate invariant.load metadata across unpacked loads #152186

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a3d0e64
[InstCombine] Propagate invariant.load metadata across unpacked loads
VedantParanjape Aug 5, 2025
af85146
Merge branch 'main' into invariant-change
VedantParanjape Aug 5, 2025
71d9921
use enums in mdnode
VedantParanjape Aug 6, 2025
9899e69
adjust AAMetadata
VedantParanjape Aug 6, 2025
9955d70
Revert "adjust AAMetadata"
VedantParanjape Aug 13, 2025
2d4e276
addressed review
VedantParanjape Aug 13, 2025
efe732a
Merge branch 'main' into invariant-change
VedantParanjape Aug 14, 2025
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10 changes: 10 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
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Expand Up @@ -718,6 +718,14 @@ static Instruction *combineLoadToOperationType(InstCombinerImpl &IC,
return nullptr;
}

// Check if the aggregate load has a invariant.load metadata
// If aggregate load has invariant.load metadata, add it to the
// unpacked loads as well.
static void copyInvariantLoadMetadata(LoadInst &LI, LoadInst *NewLoad) {
if (MDNode *MD = LI.getMetadata("invariant.load"))
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Should access by MD_ enum instead of by name. Also isn't this what copyMetadataForLoad is for?

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I don't think that function will correctly propagate metadata for unpacking loads. For example, it copies MD_tbaa as is. But the tbaa nodes would change for scalar loads vs aggregate loads.

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@arsenm now using enums instead of by name.

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The point is still that most of these metadata should propagate, this isn't unique to invariant.load

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We can use copyMetadataForLoad.

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copyMetadataForLoad sounds like only designed for load type casting. And some code that checks NewType->isPointerTy() implicitly assumes the OldType is also a pointer, which sounds incorrect for spliting loads.

    // Note, essentially every kind of metadata should be preserved here! This
    // routine is supposed to clone a load instruction changing *only its type*.
    // The only metadata it makes sense to drop is metadata which is invalidated
    // when the pointer type changes. This should essentially never be the case
    // in LLVM, but we explicitly switch over only known metadata to be
    // conservatively correct. If you are adding metadata to LLVM which pertains
    // to loads, you almost certainly want to add it here.

Shall we add a copyMetadataForSplitLoad?

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copyMetadataForLoad sounds like only designed for load type casting.

You are right. If we have to repeat it multiple times in the future, it is definitely better to have a helper function.

NewLoad->setMetadata("invariant.load", MD);
}

static Instruction *unpackLoadToAggregate(InstCombinerImpl &IC, LoadInst &LI) {
// FIXME: We could probably with some care handle both volatile and atomic
// stores here but it isn't clear that this is important.
Expand All @@ -737,6 +745,7 @@ static Instruction *unpackLoadToAggregate(InstCombinerImpl &IC, LoadInst &LI) {
LoadInst *NewLoad = IC.combineLoadToNewType(LI, ST->getTypeAtIndex(0U),
".unpack");
NewLoad->setAAMetadata(LI.getAAMetadata());
copyInvariantLoadMetadata(LI, NewLoad);
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Suggested change
copyInvariantLoadMetadata(LI, NewLoad);
NewLI->copyMetadata(LI, LLVMContext::MD_invariant_load);

return IC.replaceInstUsesWith(LI, IC.Builder.CreateInsertValue(
PoisonValue::get(T), NewLoad, 0, Name));
}
Expand Down Expand Up @@ -764,6 +773,7 @@ static Instruction *unpackLoadToAggregate(InstCombinerImpl &IC, LoadInst &LI) {
Name + ".unpack");
// Propagate AA metadata. It'll still be valid on the narrowed load.
L->setAAMetadata(LI.getAAMetadata());
copyInvariantLoadMetadata(LI, L);
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Suggested change
copyInvariantLoadMetadata(LI, L);
NewLI->copyMetadata(LI, LLVMContext::MD_invariant_load);

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@VedantParanjape VedantParanjape Aug 13, 2025 •
edited
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@dtcxzyw I have fixed these issues, could you please give a lgtm? I will create another PR for adjusting AA metadata.

V = IC.Builder.CreateInsertValue(V, L, i);
}

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46 changes: 46 additions & 0 deletions llvm/test/Transforms/InstCombine/invariant-metadata-propagation.ll
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@@ -0,0 +1,46 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt -S < %s -passes=instcombine | FileCheck %s

%struct.double2 = type { double, double }
%struct.double1 = type { double }

define %struct.double2 @func1(ptr addrspace(1) %a) {
; CHECK-LABEL: define %struct.double2 @func1(
; CHECK-SAME: ptr addrspace(1) [[A:%.*]]) {
; CHECK-NEXT: [[DOTUNPACK:%.*]] = load double, ptr addrspace(1) [[A]], align 16, !invariant.load [[META0:![0-9]+]]
; CHECK-NEXT: [[TMP1:%.*]] = insertvalue [[STRUCT_DOUBLE2:%.*]] poison, double [[DOTUNPACK]], 0
; CHECK-NEXT: [[DOTELT1:%.*]] = getelementptr inbounds nuw i8, ptr addrspace(1) [[A]], i64 8
; CHECK-NEXT: [[DOTUNPACK2:%.*]] = load double, ptr addrspace(1) [[DOTELT1]], align 8, !invariant.load [[META0]]
; CHECK-NEXT: [[TMP2:%.*]] = insertvalue [[STRUCT_DOUBLE2]] [[TMP1]], double [[DOTUNPACK2]], 1
; CHECK-NEXT: ret [[STRUCT_DOUBLE2]] [[TMP2]]
;
%1 = load %struct.double2, ptr addrspace(1) %a, align 16, !invariant.load !1
ret %struct.double2 %1
}

define %struct.double2 @func2(ptr %a) {
; CHECK-LABEL: define %struct.double2 @func2(
; CHECK-SAME: ptr [[A:%.*]]) {
; CHECK-NEXT: [[DOTUNPACK:%.*]] = load double, ptr [[A]], align 16, !invariant.load [[META0]]
; CHECK-NEXT: [[TMP1:%.*]] = insertvalue [[STRUCT_DOUBLE2:%.*]] poison, double [[DOTUNPACK]], 0
; CHECK-NEXT: [[DOTELT1:%.*]] = getelementptr inbounds nuw i8, ptr [[A]], i64 8
; CHECK-NEXT: [[DOTUNPACK2:%.*]] = load double, ptr [[DOTELT1]], align 8, !invariant.load [[META0]]
; CHECK-NEXT: [[TMP2:%.*]] = insertvalue [[STRUCT_DOUBLE2]] [[TMP1]], double [[DOTUNPACK2]], 1
; CHECK-NEXT: ret [[STRUCT_DOUBLE2]] [[TMP2]]
;
%1 = load %struct.double2, ptr %a, align 16, !invariant.load !1
ret %struct.double2 %1
}

define %struct.double1 @func3(ptr %a) {
; CHECK-LABEL: define %struct.double1 @func3(
; CHECK-SAME: ptr [[A:%.*]]) {
; CHECK-NEXT: [[DOTUNPACK:%.*]] = load double, ptr [[A]], align 16, !invariant.load [[META0]]
; CHECK-NEXT: [[TMP1:%.*]] = insertvalue [[STRUCT_DOUBLE1:%.*]] poison, double [[DOTUNPACK]], 0
; CHECK-NEXT: ret [[STRUCT_DOUBLE1]] [[TMP1]]
;
%1 = load %struct.double1, ptr %a, align 16, !invariant.load !1
ret %struct.double1 %1
}

!1 = !{}