[SROA] Fix crash with lifetime intrinsic partially covering alloca.

Summary:
PromoteMemToReg looks specifically for the pattern
bitcast+lifetime.start (or a bitcast-equivalent GEP); any offset
will lead to an assertion failure.

Fixes https://llvm.org/bugs/show_bug.cgi?id=27999 .

Differential Revision: https://reviews.llvm.org/D22737

llvm-svn: 277969

llvm/lib/Transforms/Scalar/SROA.cpp

@@ -2890,7 +2890,13 @@ class llvm::sroa::AllocaSliceRewriter
 
     (void)New;
     DEBUG(dbgs() << "          to: " << *New << "\n");
-    return true;
+
+    // Lifetime intrinsics are only promotable if they cover the whole alloca.
+    // (In theory, intrinsics which partially cover an alloca could be
+    // promoted, but PromoteMemToReg doesn't handle that case.)
+    bool IsWholeAlloca = NewBeginOffset == NewAllocaBeginOffset &&
+                         NewEndOffset == NewAllocaEndOffset;
+    return IsWholeAlloca;
   }
 
   bool visitPHINode(PHINode &PN) {

llvm/test/Transforms/SROA/basictest.ll

@@ -1669,3 +1669,18 @@ entry:
 }
 
 declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind
+
+define void @PR27999() unnamed_addr {
+; CHECK-LABEL: @PR27999(
+; CHECK: alloca [2 x i64], align 8
+; CHECK: call void @llvm.lifetime.start(i64 16,
+; CHECK: call void @llvm.lifetime.end(i64 8,
+entry-block:
+  %0 = alloca [2 x i64], align 8
+  %1 = bitcast [2 x i64]* %0 to i8*
+  call void @llvm.lifetime.start(i64 16, i8* %1)
+  %2 = getelementptr inbounds [2 x i64], [2 x i64]* %0, i32 0, i32 1
+  %3 = bitcast i64* %2 to i8*
+  call void @llvm.lifetime.end(i64 8, i8* %3)
+  ret void
+}