[SROA] Reuse existing lifetime markers if possible

[jdoerfert]

Summary

If the underlying alloca did not change, we do not necessarily need new
lifetime markers. This patch adds a check and reuses the old ones if
possible.

[arsenm]

LGTM

llvm/lib/Transforms/Scalar/SROA.cpp

bool visitMemTransferInst(MemTransferInst &II) {
     return !II.isVolatile();
   }
 
   bool visitIntrinsicInst(IntrinsicInst &II) {
     assert(II.isLifetimeStartOrEnd());
     LLVM_DEBUG(dbgs() << "    original: " << II << "\n");
     assert(II.getArgOperand(1) == OldPtr);
 
+    bool EntireRange = (NewBeginOffset == NewAllocaBeginOffset &&
+                        NewEndOffset == NewAllocaEndOffset);
+
+    // If the new lifetime marker would not differ from the old, just keep it.
+    if (&OldAI == &NewAI && EntireRange)
+      return true;
+
     // Record this instruction for deletion.
     Pass.DeadInsts.insert(&II);
 
     // Lifetime intrinsics are only promotable if they cover the whole alloca.
     // Therefore, we drop lifetime intrinsics which don't cover the whole
     // alloca.
     // (In theory, intrinsics which partially cover an alloca could be
     // promoted, but PromoteMemToReg doesn't handle that case.)
     // FIXME: Check whether the alloca is promotable before dropping the
     // lifetime intrinsics?
-    if (NewBeginOffset != NewAllocaBeginOffset ||
-        NewEndOffset != NewAllocaEndOffset)
+    if (!EntireRange)
       return true;
 
     ConstantInt *Size =
         ConstantInt::get(cast<IntegerType>(II.getArgOperand(0)->getType()),
                          NewEndOffset - NewBeginOffset);
     // Lifetime intrinsics always expect an i8* so directly get such a pointer
     // for the new alloca slice.
     Type *PointerTy = IRB.getInt8PtrTy(OldPtr->getType()->getPointerAddressSpace());

llvm/test/Transforms/SROA/reuse_lifetime_markers.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -sroa -S | FileCheck %s
+;
+; Make sure we reuse the lifetime marker and do not create a new one that looks the same but without the call site attributes.
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #0
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg) #0
+
+define hidden void @old_markers() {
+;
+; CHECK-LABEL: define {{[^@]+}}@old_markers(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VV_SROA_4:%.*]] = alloca [3 x i32*]
+; CHECK-NEXT:    [[VV_SROA_4_0__SROA_CAST61:%.*]] = bitcast [3 x i32*]* [[VV_SROA_4]] to i8*
+; CHECK-NEXT:    [[VV_SROA_4_0__SROA_CAST94:%.*]] = bitcast [3 x i32*]* [[VV_SROA_4]] to i8*
+; CHECK-NEXT:    call void @llvm.lifetime.start.p0i8(i64 24, i8* nonnull align 8 dereferenceable(24) [[VV_SROA_4_0__SROA_CAST94]])
+; CHECK-NEXT:    br i1 undef, label [[DO_BODY:%.*]], label [[IF_END31:%.*]]
+; CHECK:       do.body:
+; CHECK-NEXT:    ret void
+; CHECK:       if.end31:
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nonnull align 8 dereferenceable(24) [[VV_SROA_4_0__SROA_CAST61]], i8* noalias nonnull align 8 undef, i64 24, i1 false)
+; CHECK-NEXT:    unreachable
+;
+entry:
+  %vv.sroa.4 = alloca [3 x i32*]
+  %vv.sroa.4.0..sroa_cast61 = bitcast [3 x i32*]* %vv.sroa.4 to i8*
+  %vv.sroa.4.0..sroa_cast94 = bitcast [3 x i32*]* %vv.sroa.4 to i8*
+  call void @llvm.lifetime.start.p0i8(i64 24, i8* nonnull align 8 dereferenceable(24) %vv.sroa.4.0..sroa_cast94)
+  br i1 undef, label %do.body, label %if.end31
+
+do.body:                                          ; preds = %entry
+  ret void
+
+if.end31:                                         ; preds = %entry
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nonnull align 8 dereferenceable(24) %vv.sroa.4.0..sroa_cast61, i8* noalias nonnull align 8 undef, i64 24, i1 false)
+  unreachable
+}
+
+define hidden void @new_markers() {
+;
+; CHECK-LABEL: define {{[^@]+}}@new_markers(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VV_SROA_4:%.*]] = alloca [3 x i32*]
+; CHECK-NEXT:    [[VV_SROA_4_0__SROA_CAST61:%.*]] = bitcast [3 x i32*]* [[VV_SROA_4]] to i8*
+; CHECK-NEXT:    br i1 undef, label [[DO_BODY:%.*]], label [[IF_END31:%.*]]
+; CHECK:       do.body:
+; CHECK-NEXT:    ret void
+; CHECK:       if.end31:
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nonnull align 8 dereferenceable(24) [[VV_SROA_4_0__SROA_CAST61]], i8* noalias nonnull align 8 undef, i64 24, i1 false)
+; CHECK-NEXT:    unreachable
+;
+entry:
+  %vv.sroa.4 = alloca [3 x i32*]
+  %vv.sroa.4.0..sroa_cast61 = bitcast [3 x i32*]* %vv.sroa.4 to i8*
+  %vv.sroa.4.0..sroa_cast94 = bitcast [3 x i32*]* %vv.sroa.4 to i8*
+  call void @llvm.lifetime.start.p0i8(i64 8, i8* nonnull align 8 dereferenceable(24) %vv.sroa.4.0..sroa_cast94)
+  br i1 undef, label %do.body, label %if.end31
+
+do.body:                                          ; preds = %entry
+  ret void
+
+if.end31:                                         ; preds = %entry
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nonnull align 8 dereferenceable(24) %vv.sroa.4.0..sroa_cast61, i8* noalias nonnull align 8 undef, i64 24, i1 false)
+  unreachable
+}