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

[eli.friedman]

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 .

[eli.friedman]

Ping. (Not sure who else would be appropriate to review this.)

[chandlerc]

This is the correct fix. Sorry for the delay reviewing.

lib/Transforms/Scalar/SROA.cpp

bool visitIntrinsicInst(IntrinsicInst &II) {
     Value *New;
     if (II.getIntrinsicID() == Intrinsic::lifetime_start)
       New = IRB.CreateLifetimeStart(Ptr, Size);
     else
       New = IRB.CreateLifetimeEnd(Ptr, Size);
 
     (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) {
     DEBUG(dbgs() << "    original: " << PN << "\n");
     assert(BeginOffset >= NewAllocaBeginOffset && "PHIs are unsplittable");
     assert(EndOffset <= NewAllocaEndOffset && "PHIs are unsplittable");
 
     // We would like to compute a new pointer in only one place, but have it be

test/Transforms/SROA/basictest.ll

entry:
   store i64 %3, i64* %2, align 8
   %4 = bitcast %struct.STest* %outData to i8*
   call void @llvm.memcpy.p0i8.p0i8.i64(i8* %4, i8* %0, i64 16, i32 4, i1 false)
   call void @llvm.lifetime.end(i64 16, i8* %0)
   ret void
 }
 
 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
+}