Re-commit: Mark values as trivially dead when their only use is a sta…

…rt or end lifetime intrinsic.

Summary:
If the only use of a value is a start or end lifetime intrinsic then mark the intrinsic as trivially dead. This should allow for that value to then be removed as well.

Currently, this only works for allocas, globals, and arguments.

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79355

llvm/lib/Transforms/Utils/Local.cpp

@@ -403,9 +403,22 @@ bool llvm::wouldInstructionBeTriviallyDead(Instruction *I,
         II->getIntrinsicID() == Intrinsic::launder_invariant_group)
       return true;
 
-    // Lifetime intrinsics are dead when their right-hand is undef.
-    if (II->isLifetimeStartOrEnd())
-      return isa<UndefValue>(II->getArgOperand(1));
+    if (II->isLifetimeStartOrEnd()) {
+      auto *Arg = II->getArgOperand(1);
+      // Lifetime intrinsics are dead when their right-hand is undef.
+      if (isa<UndefValue>(Arg))
+        return true;
+      // If the right-hand is an alloc, global, or argument and the only uses
+      // are lifetime intrinsics then the intrinsics are dead.
+      if (isa<AllocaInst>(Arg) || isa<GlobalValue>(Arg) || isa<Argument>(Arg))
+        return llvm::all_of(Arg->uses(), [](Use &Use) {
+          if (IntrinsicInst *IntrinsicUse =
+                  dyn_cast<IntrinsicInst>(Use.getUser()))
+            return IntrinsicUse->isLifetimeStartOrEnd();
+          return false;
+        });
+      return false;
+    }
 
     // Assumptions are dead if their condition is trivially true.  Guards on
     // true are operationally no-ops.  In the future we can consider more

llvm/test/Analysis/BasicAA/modref.ll

@@ -80,7 +80,6 @@ define void @test3a(i8* %P, i8 %X) {
 
   %P2 = getelementptr i8, i8* %P, i32 2
   store i8 %Y, i8* %P2
-; CHECK-NEXT: call void @llvm.lifetime.end
   call void @llvm.lifetime.end.p0i8(i64 10, i8* %P)
   ret void
 ; CHECK-NEXT: ret void

llvm/test/Transforms/Attributor/memory_locations.ll

@@ -386,11 +386,11 @@ define void @callerD2() {
   store i8 0, i8* %unknown
   ret void
 }
-; CHECK: Function Attrs: argmemonly nounwind willreturn
+
+; CHECK: Function Attrs: nofree {{(norecurse )?}}nosync nounwind readnone willreturn
 define void @callerE(i8* %arg) {
 ; CHECK-LABEL: define {{[^@]+}}@callerE
-; CHECK-SAME: (i8* nocapture [[ARG:%.*]])
-; CHECK-NEXT:    call void @llvm.lifetime.start.p0i8(i64 4, i8* nocapture [[ARG]])
+; CHECK-SAME: (i8* nocapture nofree readnone [[ARG:%.*]])
 ; CHECK-NEXT:    ret void
 ;
   call void @llvm.lifetime.start.p0i8(i64 4, i8* %arg)

llvm/test/Transforms/DCE/basic.ll

@@ -11,5 +11,63 @@ define void @test() {
   ret void
 }
 
+declare void @llvm.lifetime.start.p0i8(i64, i8* nocapture) nounwind
+declare void @llvm.lifetime.end.p0i8(i64, i8* nocapture) nounwind
+
+; CHECK-LABEL: @test_lifetime_alloca
+define i32 @test_lifetime_alloca() {
+; Check that lifetime intrinsics are removed along with the pointer.
+; CHECK-NEXT: @llvm.dbg.value
+; CHECK-NEXT: ret i32 0
+; CHECK-NOT: llvm.lifetime.start
+; CHECK-NOT: llvm.lifetime.end
+  %i = alloca i8, align 4
+  call void @llvm.lifetime.start.p0i8(i64 -1, i8* %i)
+  call void @llvm.lifetime.end.p0i8(i64 -1, i8* %i)
+  ret i32 0
+}
+
+; CHECK-LABEL: @test_lifetime_arg
+define i32 @test_lifetime_arg(i8*) {
+; Check that lifetime intrinsics are removed along with the pointer.
+; CHECK-NEXT: llvm.dbg.value
+; CHECK-NEXT: ret i32 0
+; CHECK-NOT: llvm.lifetime.start
+; CHECK-NOT: llvm.lifetime.end
+  call void @llvm.lifetime.start.p0i8(i64 -1, i8* %0)
+  call void @llvm.lifetime.end.p0i8(i64 -1, i8* %0)
+  ret i32 0
+}
+
+@glob = global i8 1
+
+; CHECK-LABEL: @test_lifetime_global
+define i32 @test_lifetime_global() {
+; Check that lifetime intrinsics are removed along with the pointer.
+; CHECK-NEXT: llvm.dbg.value
+; CHECK-NEXT: ret i32 0
+; CHECK-NOT: llvm.lifetime.start
+; CHECK-NOT: llvm.lifetime.end
+  call void @llvm.lifetime.start.p0i8(i64 -1, i8* @glob)
+  call void @llvm.lifetime.end.p0i8(i64 -1, i8* @glob)
+  ret i32 0
+}
+
+; CHECK-LABEL: @test_lifetime_bitcast
+define i32 @test_lifetime_bitcast(i32*) {
+; Check that lifetime intrinsics are NOT removed when the pointer is a bitcast.
+; It's not uncommon for two bitcasts to be made: one for lifetime, one for use.
+; TODO: Support the above case.
+; CHECK-NEXT: bitcast
+; CHECK-NEXT: llvm.dbg.value
+; CHECK-NEXT: llvm.lifetime.start
+; CHECK-NEXT: llvm.lifetime.end
+; CHECK-NEXT: ret i32 0
+  %2 = bitcast i32* %0 to i8*
+  call void @llvm.lifetime.start.p0i8(i64 -1, i8* %2)
+  call void @llvm.lifetime.end.p0i8(i64 -1, i8* %2)
+  ret i32 0
+}
+
 ; CHECK: [[add]] = !DILocalVariable
 ; CHECK: [[sub]] = !DILocalVariable

llvm/test/Transforms/DeadStoreElimination/lifetime.ll

@@ -12,7 +12,7 @@ define void @test1() {
 
   store i8 0, i8* %A  ;; Written to by memset
   call void @llvm.lifetime.end.p0i8(i64 1, i8* %A)
-; CHECK: lifetime.end
+; CHECK-NOT: lifetime.end
 
   call void @llvm.memset.p0i8.i8(i8* %A, i8 0, i8 -1, i1 false)
 ; CHECK-NOT: memset
@@ -26,11 +26,9 @@ define void @test2(i32* %P) {
   %Q = getelementptr i32, i32* %P, i32 1
   %R = bitcast i32* %Q to i8*
   call void @llvm.lifetime.start.p0i8(i64 4, i8* %R)
-; CHECK: lifetime.start
   store i32 0, i32* %Q  ;; This store is dead.
 ; CHECK-NOT: store
   call void @llvm.lifetime.end.p0i8(i64 4, i8* %R)
-; CHECK: lifetime.end
   ret void
 }
 

llvm/test/Transforms/InstCombine/vararg.ll

@@ -1,4 +1,4 @@
-; RUN: opt < %s -instcombine -instcombine-infinite-loop-threshold=2 -S | FileCheck %s
+; RUN: opt < %s -instcombine -instcombine-infinite-loop-threshold=3 -S | FileCheck %s
 
 %struct.__va_list = type { i8*, i8*, i8*, i32, i32 }