[LSR] Do not consider uses in lifetime intrinsics #149492
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We should ignore uses of pointers in lifetime intrinsics, as these are not actually materialized in the final code, so don't affect register pressure or anything else LSR needs to model. Handling these only results in peculiar rewrites where additional intermediate GEPs are introduced.
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@llvm/pr-subscribers-llvm-transforms Author: Nikita Popov (nikic) ChangesWe should ignore uses of pointers in lifetime intrinsics, as these are not actually materialized in the final code, so don't affect register pressure or anything else LSR needs to model. Handling these only results in peculiar rewrites where additional intermediate GEPs are introduced. Full diff: https://github.com/llvm/llvm-project/pull/149492.diff 2 Files Affected:
diff --git a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
index dc8fa4379752f..4eab10c0bca57 100644
--- a/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -3790,6 +3790,11 @@ LSRInstance::CollectLoopInvariantFixupsAndFormulae() {
continue;
}
+ // Do not consider uses inside lifetime intrinsics. These are not
+ // actually materialized.
+ if (UserInst->isLifetimeStartOrEnd())
+ continue;
+
std::pair<size_t, Immediate> P =
getUse(S, LSRUse::Basic, MemAccessTy());
size_t LUIdx = P.first;
diff --git a/llvm/test/Transforms/LoopStrengthReduce/X86/lifetime-use.ll b/llvm/test/Transforms/LoopStrengthReduce/X86/lifetime-use.ll
new file mode 100644
index 0000000000000..c7a0de22b200b
--- /dev/null
+++ b/llvm/test/Transforms/LoopStrengthReduce/X86/lifetime-use.ll
@@ -0,0 +1,59 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=loop-reduce -mtriple=x86_64-unknown-linux-gnu < %s | FileCheck %s
+
+define void @test(ptr %p, i64 %idx) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: ptr [[P:%.*]], i64 [[IDX:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: [[ALLOCA:%.*]] = alloca [4 x [4 x i32]], align 16
+; CHECK-NEXT: call void @llvm.lifetime.start.p0(i64 64, ptr [[ALLOCA]])
+; CHECK-NEXT: [[TMP0:%.*]] = shl i64 [[IDX]], 6
+; CHECK-NEXT: [[TMP1:%.*]] = add nuw nsw i64 [[TMP0]], 48
+; CHECK-NEXT: [[SCEVGEP:%.*]] = getelementptr i8, ptr [[P]], i64 [[TMP1]]
+; CHECK-NEXT: [[SCEVGEP3:%.*]] = getelementptr nuw i8, ptr [[ALLOCA]], i64 48
+; CHECK-NEXT: br label %[[LOOP:.*]]
+; CHECK: [[LOOP]]:
+; CHECK-NEXT: [[LSR_IV:%.*]] = phi i64 [ [[LSR_IV_NEXT:%.*]], %[[LOOP]] ], [ -8, %[[ENTRY]] ]
+; CHECK-NEXT: [[TMP2:%.*]] = shl nsw i64 [[LSR_IV]], 2
+; CHECK-NEXT: [[SCEVGEP8:%.*]] = getelementptr i8, ptr [[P]], i64 [[TMP2]]
+; CHECK-NEXT: [[SCEVGEP9:%.*]] = getelementptr i8, ptr [[SCEVGEP8]], i64 32
+; CHECK-NEXT: [[TMP3:%.*]] = load i32, ptr [[SCEVGEP9]], align 4
+; CHECK-NEXT: [[SCEVGEP6:%.*]] = getelementptr i8, ptr [[P]], i64 [[LSR_IV]]
+; CHECK-NEXT: [[SCEVGEP7:%.*]] = getelementptr i8, ptr [[SCEVGEP6]], i64 8
+; CHECK-NEXT: [[TMP4:%.*]] = load i32, ptr [[SCEVGEP7]], align 4
+; CHECK-NEXT: [[SCEVGEP4:%.*]] = getelementptr i8, ptr [[SCEVGEP3]], i64 [[LSR_IV]]
+; CHECK-NEXT: [[SCEVGEP5:%.*]] = getelementptr i8, ptr [[SCEVGEP4]], i64 8
+; CHECK-NEXT: [[TMP5:%.*]] = load i32, ptr [[SCEVGEP5]], align 4
+; CHECK-NEXT: [[SCEVGEP1:%.*]] = getelementptr i8, ptr [[SCEVGEP]], i64 [[LSR_IV]]
+; CHECK-NEXT: [[SCEVGEP2:%.*]] = getelementptr i8, ptr [[SCEVGEP1]], i64 8
+; CHECK-NEXT: [[TMP6:%.*]] = load i32, ptr [[SCEVGEP2]], align 4
+; CHECK-NEXT: [[LSR_IV_NEXT]] = add nsw i64 [[LSR_IV]], 4
+; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[LSR_IV_NEXT]], 0
+; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label %[[EXIT:.*]], label %[[LOOP]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: call void @llvm.lifetime.end.p0(i64 64, ptr [[ALLOCA]])
+; CHECK-NEXT: ret void
+;
+entry:
+ %alloca = alloca [4 x [4 x i32]], align 16
+ call void @llvm.lifetime.start.p0(i64 64, ptr %alloca)
+ br label %loop
+
+loop:
+ %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %loop ]
+ %gep1 = getelementptr [4 x [12 x [4 x [4 x i32]]]], ptr %p, i64 0, i64 0, i64 0, i64 %indvars.iv, i64 0
+ %0 = load i32, ptr %gep1, align 4
+ %gep2 = getelementptr [6 x [4 x [4 x i32]]], ptr %p, i64 0, i64 0, i64 0, i64 %indvars.iv
+ %1 = load i32, ptr %gep2, align 4
+ %gep3 = getelementptr [4 x [4 x i32]], ptr %alloca, i64 0, i64 3, i64 %indvars.iv
+ %2 = load i32, ptr %gep3, align 4
+ %gep4 = getelementptr [6 x [4 x [4 x i32]]], ptr %p, i64 0, i64 %idx, i64 3, i64 %indvars.iv
+ %3 = load i32, ptr %gep4, align 4
+ %indvars.iv.next = add i64 %indvars.iv, 1
+ %exitcond.not = icmp eq i64 %indvars.iv, 1
+ br i1 %exitcond.not, label %exit, label %loop
+
+exit:
+ call void @llvm.lifetime.end.p0(i64 64, ptr %alloca)
+ ret void
+}
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We have other intrinsics like this that convey information, but get dropped late - assumes spring to mind. Do we have some utility for an ignorable use somewhere? |
We have isAssumeLikeIntrinsic(). |
nikic
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lifetime.start and lifetime.end are primarily intended for use on allocas, to enable stack coloring and other liveness optimizations. This is necessary because all (static) allocas are hoisted into the entry block, so lifetime markers are the only way to convey the actual lifetimes. However, lifetime.start and lifetime.end are currently *allowed* to be used on non-alloca pointers. We don't actually do this in practice, but just the mere fact that this is possible breaks the core purpose of the lifetime markers, which is stack coloring of allocas. Stack coloring can only work correctly if all lifetime markers for an alloca are analyzable. * If a lifetime marker may operate on multiple allocas via a select/phi, we don't know which lifetime actually starts/ends and handle it incorrectly (#104776). * Stack coloring operates on the assumption that all lifetime markers are visible, and not, for example, hidden behind a function call or escaped pointer. It's not possible to change this, as part of the purpose of lifetime markers is that they work even in the presence of escaped pointers, where simple use analysis is insufficient. I don't think there is any way to have coherent semantics for lifetime markers on allocas, while also permitting them on arbitrary pointer values. This PR restricts lifetimes to operate on allocas only. As a followup, I will also drop the size argument, which is superfluous if we always operate on an alloca. (This change also renders various code handling lifetime markers on non-alloca dead. I plan to clean up that kind of code after dropping the size argument as well.) In practice, I've only found a few places that currently produce lifetimes on non-allocas: * CoroEarly replaces the promise alloca with the result of an intrinsic, which will later be replaced back with an alloca. I think this is the only place where there is some legitimate loss of functionality, but I don't think this is particularly important (I don't think we'd expect the promise in a coroutine to admit useful lifetime optimization.) * SafeStack moves unsafe allocas onto a separate frame. We can safely drop lifetimes here, as SafeStack performs its own stack coloring. * Similar for AddressSanitizer, it also moves allocas into separate memory. * LSR sometimes replaces the lifetime argument with a GEP chain of the alloca (where the offsets ultimately cancel out). This is just unnecessary. (Fixed separately in #149492.) * InferAddrSpaces sometimes makes lifetimes operate on an addrspacecast of an alloca. I don't think this is necessary.
llvm-sync bot
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Jul 21, 2025
lifetime.start and lifetime.end are primarily intended for use on allocas, to enable stack coloring and other liveness optimizations. This is necessary because all (static) allocas are hoisted into the entry block, so lifetime markers are the only way to convey the actual lifetimes. However, lifetime.start and lifetime.end are currently *allowed* to be used on non-alloca pointers. We don't actually do this in practice, but just the mere fact that this is possible breaks the core purpose of the lifetime markers, which is stack coloring of allocas. Stack coloring can only work correctly if all lifetime markers for an alloca are analyzable. * If a lifetime marker may operate on multiple allocas via a select/phi, we don't know which lifetime actually starts/ends and handle it incorrectly (llvm/llvm-project#104776). * Stack coloring operates on the assumption that all lifetime markers are visible, and not, for example, hidden behind a function call or escaped pointer. It's not possible to change this, as part of the purpose of lifetime markers is that they work even in the presence of escaped pointers, where simple use analysis is insufficient. I don't think there is any way to have coherent semantics for lifetime markers on allocas, while also permitting them on arbitrary pointer values. This PR restricts lifetimes to operate on allocas only. As a followup, I will also drop the size argument, which is superfluous if we always operate on an alloca. (This change also renders various code handling lifetime markers on non-alloca dead. I plan to clean up that kind of code after dropping the size argument as well.) In practice, I've only found a few places that currently produce lifetimes on non-allocas: * CoroEarly replaces the promise alloca with the result of an intrinsic, which will later be replaced back with an alloca. I think this is the only place where there is some legitimate loss of functionality, but I don't think this is particularly important (I don't think we'd expect the promise in a coroutine to admit useful lifetime optimization.) * SafeStack moves unsafe allocas onto a separate frame. We can safely drop lifetimes here, as SafeStack performs its own stack coloring. * Similar for AddressSanitizer, it also moves allocas into separate memory. * LSR sometimes replaces the lifetime argument with a GEP chain of the alloca (where the offsets ultimately cancel out). This is just unnecessary. (Fixed separately in llvm/llvm-project#149492.) * InferAddrSpaces sometimes makes lifetimes operate on an addrspacecast of an alloca. I don't think this is necessary.
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We should ignore uses of pointers in lifetime intrinsics, as these are not actually materialized in the final code, so don't affect register pressure or anything else LSR needs to model.
Handling these only results in peculiar rewrites where additional intermediate GEPs are introduced.