[llvm] [InstCombine] Remove redundant alignment assumptions. (PR #123348)
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Mon Sep 1 09:38:32 PDT 2025
https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/123348
>From 59474a53c7116d95d6abe538bf97d78bf14bfef5 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 29 Aug 2025 15:07:44 +0100
Subject: [PATCH 1/3] [InstCombine] Add tests with foldable and redundant align
assumptions.
---
.../Transforms/InstCombine/assume-align.ll | 55 ++++++++++++++++++-
1 file changed, 54 insertions(+), 1 deletion(-)
diff --git a/llvm/test/Transforms/InstCombine/assume-align.ll b/llvm/test/Transforms/InstCombine/assume-align.ll
index f0e0257433086..64537ea95ba05 100644
--- a/llvm/test/Transforms/InstCombine/assume-align.ll
+++ b/llvm/test/Transforms/InstCombine/assume-align.ll
@@ -1,5 +1,5 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals smart
-; RUN: opt -S -passes=instcombine,simplifycfg < %s 2>&1 | FileCheck %s
+; RUN: opt -S -passes='instcombine,simplifycfg' < %s 2>&1 | FileCheck %s
declare void @llvm.assume(i1 noundef)
@@ -135,6 +135,17 @@ define ptr @fold_assume_align_pow2_of_loaded_pointer_into_align_metadata(ptr %p)
ret ptr %p2
}
+define ptr @fold_assume_align_i32_pow2_of_loaded_pointer_into_align_metadata(ptr %p) {
+; CHECK-LABEL: @fold_assume_align_i32_pow2_of_loaded_pointer_into_align_metadata(
+; CHECK-NEXT: [[P2:%.*]] = load ptr, ptr [[P:%.*]], align 8
+; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[P2]], i32 8) ]
+; CHECK-NEXT: ret ptr [[P2]]
+;
+ %p2 = load ptr, ptr %p
+ call void @llvm.assume(i1 true) [ "align"(ptr %p2, i32 8) ]
+ ret ptr %p2
+}
+
define ptr @dont_fold_assume_align_pow2_of_loaded_pointer_into_align_metadata_due_to_call(ptr %p) {
; CHECK-LABEL: @dont_fold_assume_align_pow2_of_loaded_pointer_into_align_metadata_due_to_call(
; CHECK-NEXT: [[P2:%.*]] = load ptr, ptr [[P:%.*]], align 8
@@ -249,7 +260,49 @@ define ptr @redundant_assume_align_8_via_asume(ptr %p) {
ret ptr %p
}
+define void @redundant_arg_passed_to_intrinsic(ptr %dst, ptr %src) {
+; CHECK-LABEL: @redundant_arg_passed_to_intrinsic(
+; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[DST:%.*]], i32 8) ]
+; CHECK-NEXT: call void @bar()
+; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[SRC:%.*]], i32 8) ]
+; CHECK-NEXT: call void @llvm.memmove.p0.p0.i64(ptr noundef nonnull align 8 dereferenceable(16) [[DST]], ptr noundef nonnull align 8 dereferenceable(16) [[SRC]], i64 16, i1 false)
+; CHECK-NEXT: ret void
+;
+ call void @llvm.assume(i1 true) [ "align"(ptr %dst, i32 8) ]
+ call void @bar()
+ call void @llvm.assume(i1 true) [ "align"(ptr %src, i32 8) ]
+ call void @llvm.memmove.p0.p0.i64(ptr align 8 %dst, ptr %src, i64 16, i1 false)
+ ret void
+}
+
+define void @test_store(ptr %ptr) {
+; CHECK-LABEL: @test_store(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[PTR:%.*]], i64 2) ]
+; CHECK-NEXT: store i16 0, ptr [[PTR]], align 1
+; CHECK-NEXT: ret void
+;
+entry:
+ call void @llvm.assume(i1 true) [ "align"(ptr %ptr, i64 2) ]
+ store i16 0, ptr %ptr, align 1
+ ret void
+}
+
declare void @foo(ptr)
+declare void @bar()
+
+; !align must have a constant integer alignment.
+define ptr @dont_fold_assume_align_not_constant_of_loaded_pointer_into_align_metadata(ptr %p, i64 %align) {
+; CHECK-LABEL: @dont_fold_assume_align_not_constant_of_loaded_pointer_into_align_metadata(
+; CHECK-NEXT: [[P2:%.*]] = load ptr, ptr [[P:%.*]], align 8
+; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[P2]], i64 [[ALIGN:%.*]]) ]
+; CHECK-NEXT: ret ptr [[P2]]
+;
+ %p2 = load ptr, ptr %p
+ call void @llvm.assume(i1 true) [ "align"(ptr %p2, i64 %align) ]
+ ret ptr %p2
+}
+
;.
; CHECK: [[META0]] = !{i64 8}
;.
>From a5c35a2940a7b6848ecfe23139f1cd484ca7926c Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 29 Aug 2025 15:05:15 +0100
Subject: [PATCH 2/3] [InstCombine] Remove redundant alignment assumptions.
---
.../InstCombine/InstCombineCalls.cpp | 33 ++++++++++++++++---
.../Transforms/InstCombine/assume-align.ll | 3 --
2 files changed, 29 insertions(+), 7 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 42b65dde67255..899ea2d0147be 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3385,12 +3385,13 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
// TODO: apply range metadata for range check patterns?
}
- // Separate storage assumptions apply to the underlying allocations, not any
- // particular pointer within them. When evaluating the hints for AA purposes
- // we getUnderlyingObject them; by precomputing the answers here we can
- // avoid having to do so repeatedly there.
for (unsigned Idx = 0; Idx < II->getNumOperandBundles(); Idx++) {
OperandBundleUse OBU = II->getOperandBundleAt(Idx);
+
+ // Separate storage assumptions apply to the underlying allocations, not
+ // any particular pointer within them. When evaluating the hints for AA
+ // purposes we getUnderlyingObject them; by precomputing the answers here
+ // we can avoid having to do so repeatedly there.
if (OBU.getTagName() == "separate_storage") {
assert(OBU.Inputs.size() == 2);
auto MaybeSimplifyHint = [&](const Use &U) {
@@ -3404,6 +3405,30 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
MaybeSimplifyHint(OBU.Inputs[0]);
MaybeSimplifyHint(OBU.Inputs[1]);
}
+
+ // Try to fold alignment assumption into a load's !align metadata, if the
+ // assumption is valid in the load's context and remove redundant ones.
+ if (OBU.getTagName() == "align" && OBU.Inputs.size() == 2) {
+ RetainedKnowledge RK = getKnowledgeFromBundle(
+ *cast<AssumeInst>(II), II->bundle_op_info_begin()[Idx]);
+ if (!RK || RK.AttrKind != Attribute::Alignment ||
+ !isPowerOf2_64(RK.ArgValue))
+ continue;
+
+ // Don't try to remove align assumptions for pointers derived from
+ // arguments. We might lose information if the function gets inline and
+ // the align argument attribute disappears.
+ Value *UO = getUnderlyingObject(RK.WasOn);
+ if (!UO || isa<Argument>(UO))
+ continue;
+
+ KnownBits Known = computeKnownBits(RK.WasOn, nullptr);
+ unsigned TZ = std::min(Known.countMinTrailingZeros(), 63u);
+ if ((1ULL << TZ) < RK.ArgValue)
+ continue;
+ auto *New = CallBase::removeOperandBundle(II, OBU.getTagID());
+ return New;
+ }
}
// Convert nonnull assume like:
diff --git a/llvm/test/Transforms/InstCombine/assume-align.ll b/llvm/test/Transforms/InstCombine/assume-align.ll
index 64537ea95ba05..80441c428cbff 100644
--- a/llvm/test/Transforms/InstCombine/assume-align.ll
+++ b/llvm/test/Transforms/InstCombine/assume-align.ll
@@ -186,7 +186,6 @@ define ptr @dont_fold_assume_align_zero_of_loaded_pointer_into_align_metadata(pt
define ptr @redundant_assume_align_1(ptr %p) {
; CHECK-LABEL: @redundant_assume_align_1(
; CHECK-NEXT: [[P2:%.*]] = load ptr, ptr [[P:%.*]], align 8
-; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[P2]], i32 1) ]
; CHECK-NEXT: call void @foo(ptr [[P2]])
; CHECK-NEXT: ret ptr [[P2]]
;
@@ -200,7 +199,6 @@ define ptr @redundant_assume_align_1(ptr %p) {
define ptr @redundant_assume_align_8_via_align_metadata(ptr %p) {
; CHECK-LABEL: @redundant_assume_align_8_via_align_metadata(
; CHECK-NEXT: [[P2:%.*]] = load ptr, ptr [[P:%.*]], align 8, !align [[META0:![0-9]+]]
-; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[P2]], i32 8) ]
; CHECK-NEXT: call void @foo(ptr [[P2]])
; CHECK-NEXT: ret ptr [[P2]]
;
@@ -295,7 +293,6 @@ declare void @bar()
define ptr @dont_fold_assume_align_not_constant_of_loaded_pointer_into_align_metadata(ptr %p, i64 %align) {
; CHECK-LABEL: @dont_fold_assume_align_not_constant_of_loaded_pointer_into_align_metadata(
; CHECK-NEXT: [[P2:%.*]] = load ptr, ptr [[P:%.*]], align 8
-; CHECK-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[P2]], i64 [[ALIGN:%.*]]) ]
; CHECK-NEXT: ret ptr [[P2]]
;
%p2 = load ptr, ptr %p
>From 44f742f821d426fb4b7dbbebbbfd1c96bf3954e3 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Mon, 1 Sep 2025 17:36:20 +0100
Subject: [PATCH 3/3] !fixup address comments, update tests
---
.../InstCombine/InstCombineCalls.cpp | 11 ++++----
llvm/test/Analysis/BasicAA/featuretest.ll | 26 ++++++-------------
2 files changed, 14 insertions(+), 23 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 899ea2d0147be..077c54ae9ed1c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3406,8 +3406,7 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
MaybeSimplifyHint(OBU.Inputs[1]);
}
- // Try to fold alignment assumption into a load's !align metadata, if the
- // assumption is valid in the load's context and remove redundant ones.
+ // Try to remove redundant alignment assumptions.
if (OBU.getTagName() == "align" && OBU.Inputs.size() == 2) {
RetainedKnowledge RK = getKnowledgeFromBundle(
*cast<AssumeInst>(II), II->bundle_op_info_begin()[Idx]);
@@ -3422,12 +3421,14 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
if (!UO || isa<Argument>(UO))
continue;
- KnownBits Known = computeKnownBits(RK.WasOn, nullptr);
+ // Compute known bits for the pointer, passing nullptr as context to
+ // avoid computeKnownBits using the assumption we are about to remove
+ // for reasoning.
+ KnownBits Known = computeKnownBits(RK.WasOn, /*CtxI=*/nullptr);
unsigned TZ = std::min(Known.countMinTrailingZeros(), 63u);
if ((1ULL << TZ) < RK.ArgValue)
continue;
- auto *New = CallBase::removeOperandBundle(II, OBU.getTagID());
- return New;
+ return CallBase::removeOperandBundle(II, OBU.getTagID());
}
}
diff --git a/llvm/test/Analysis/BasicAA/featuretest.ll b/llvm/test/Analysis/BasicAA/featuretest.ll
index fd5d2eff10f83..bf028ef49d3a4 100644
--- a/llvm/test/Analysis/BasicAA/featuretest.ll
+++ b/llvm/test/Analysis/BasicAA/featuretest.ll
@@ -15,24 +15,14 @@ declare void @llvm.assume(i1)
; operations on another array. Important for scientific codes.
;
define i32 @different_array_test(i64 %A, i64 %B) {
-; NO_ASSUME-LABEL: @different_array_test(
-; NO_ASSUME-NEXT: [[ARRAY11:%.*]] = alloca [100 x i32], align 4
-; NO_ASSUME-NEXT: [[ARRAY22:%.*]] = alloca [200 x i32], align 4
-; NO_ASSUME-NEXT: call void @llvm.assume(i1 true) [ "align"(ptr [[ARRAY11]], i32 4) ]
-; NO_ASSUME-NEXT: call void @external(ptr nonnull [[ARRAY11]])
-; NO_ASSUME-NEXT: call void @external(ptr nonnull [[ARRAY22]])
-; NO_ASSUME-NEXT: [[POINTER2:%.*]] = getelementptr i32, ptr [[ARRAY22]], i64 [[B:%.*]]
-; NO_ASSUME-NEXT: store i32 7, ptr [[POINTER2]], align 4
-; NO_ASSUME-NEXT: ret i32 0
-;
-; USE_ASSUME-LABEL: @different_array_test(
-; USE_ASSUME-NEXT: [[ARRAY11:%.*]] = alloca [100 x i32], align 4
-; USE_ASSUME-NEXT: [[ARRAY22:%.*]] = alloca [200 x i32], align 4
-; USE_ASSUME-NEXT: call void @external(ptr nonnull [[ARRAY11]])
-; USE_ASSUME-NEXT: call void @external(ptr nonnull [[ARRAY22]])
-; USE_ASSUME-NEXT: [[POINTER2:%.*]] = getelementptr i32, ptr [[ARRAY22]], i64 [[B:%.*]]
-; USE_ASSUME-NEXT: store i32 7, ptr [[POINTER2]], align 4
-; USE_ASSUME-NEXT: ret i32 0
+; CHECK-LABEL: @different_array_test(
+; CHECK-NEXT: [[ARRAY11:%.*]] = alloca [100 x i32], align 4
+; CHECK-NEXT: [[ARRAY22:%.*]] = alloca [200 x i32], align 4
+; CHECK-NEXT: call void @external(ptr nonnull [[ARRAY11]])
+; CHECK-NEXT: call void @external(ptr nonnull [[ARRAY22]])
+; CHECK-NEXT: [[POINTER2:%.*]] = getelementptr i32, ptr [[ARRAY22]], i64 [[B:%.*]]
+; CHECK-NEXT: store i32 7, ptr [[POINTER2]], align 4
+; CHECK-NEXT: ret i32 0
;
%Array1 = alloca i32, i32 100
%Array2 = alloca i32, i32 200
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