[llvm] [LSR] Do not consider uses in lifetime intrinsics (PR #149492)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

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.


---
Full diff: https://github.com/llvm/llvm-project/pull/149492.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp (+5) 
- (added) llvm/test/Transforms/LoopStrengthReduce/X86/lifetime-use.ll (+59) 


``````````diff
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
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/149492