LICM: Don't sink stores out of loops that may throw.

Summary:
This hasn't been caught before because it requires noalias or similarly
strong alias analysis to actually reproduce.

Fixes http://llvm.org/PR27952 .

Reviewers: hfinkel, sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20944

llvm-svn: 271858

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -928,6 +928,16 @@ bool llvm::promoteLoopAccessesToScalars(
 
   const DataLayout &MDL = Preheader->getModule()->getDataLayout();
 
+  if (SafetyInfo->MayThrow) {
+    // If a loop can throw, we have to insert a store along each unwind edge.
+    // That said, we can't actually make the unwind edge explicit. Therefore,
+    // we have to prove that the store is dead along the unwind edge.
+    //
+    // Currently, this code just special-cases alloca instructions.
+    if (!isa<AllocaInst>(GetUnderlyingObject(SomePtr, MDL)))
+      return false;
+  }
+
   // Check that all of the pointers in the alias set have the same type.  We
   // cannot (yet) promote a memory location that is loaded and stored in
   // different sizes.  While we are at it, collect alignment and AA info.

llvm/test/Transforms/LICM/scalar_promote-unwind.ll

@@ -0,0 +1,71 @@
+; RUN: opt < %s -basicaa -licm -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Make sure we don't hoist the store out of the loop; %a would
+; have the wrong value if f() unwinds
+
+define void @test1(i32* nocapture noalias %a, i1 zeroext %y) uwtable {
+entry:
+  br label %for.body
+
+for.body:
+  %i.03 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %0 = load i32, i32* %a, align 4
+  %add = add nsw i32 %0, 1
+  store i32 %add, i32* %a, align 4
+  br i1 %y, label %if.then, label %for.inc
+
+; CHECK: define void @test1
+; CHECK: load i32, i32*
+; CHECK-NEXT: add
+; CHECK-NEXT: store i32
+
+if.then:
+  tail call void @f()
+  br label %for.inc
+
+for.inc:
+  %inc = add nuw nsw i32 %i.03, 1
+  %exitcond = icmp eq i32 %inc, 10000
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret void
+}
+
+; We can hoist the store out of the loop here; if f() unwinds,
+; the lifetime of %a ends.
+
+define void @test2(i1 zeroext %y) uwtable {
+entry:
+  %a = alloca i32
+  br label %for.body
+
+for.body:
+  %i.03 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %0 = load i32, i32* %a, align 4
+  %add = add nsw i32 %0, 1
+  store i32 %add, i32* %a, align 4
+  br i1 %y, label %if.then, label %for.inc
+
+if.then:
+  tail call void @f()
+  br label %for.inc
+
+for.inc:
+  %inc = add nuw nsw i32 %i.03, 1
+  %exitcond = icmp eq i32 %inc, 10000
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret void
+
+; CHECK: define void @test2
+; CHECK: store i32
+; CHECK-NEXT: ret void
+  ret void
+}
+
+declare void @f() uwtable