From dd8c386432c9bb983dbd36fde2d1b8449c2c53d9 Mon Sep 17 00:00:00 2001
From: Arnold Schwaighofer <aschwaighofer@apple.com>
Date: Fri, 10 Oct 2014 01:27:02 +0000
Subject: [PATCH] SimplifyCFG: Don't convert phis into selects if we could
 remove undef behavior instead

We used to transform this:

  define void @test6(i1 %cond, i8* %ptr) {
  entry:
    br i1 %cond, label %bb1, label %bb2

  bb1:
    br label %bb2

  bb2:
    %ptr.2 = phi i8* [ %ptr, %entry ], [ null, %bb1 ]
    store i8 2, i8* %ptr.2, align 8
    ret void
  }

into this:

  define void @test6(i1 %cond, i8* %ptr) {
    %ptr.2 = select i1 %cond, i8* null, i8* %ptr
    store i8 2, i8* %ptr.2, align 8
    ret void
  }

because the simplifycfg transformation into selects would happen to happen
before the simplifycfg transformation that removes unreachable control flow
(We have 'unreachable control flow' due to the store to null which is undefined
behavior).

The existing transformation that removes unreachable control flow in simplifycfg
is:

  /// If BB has an incoming value that will always trigger undefined behavior
  /// (eg. null pointer dereference), remove the branch leading here.
  static bool removeUndefIntroducingPredecessor(BasicBlock *BB)

Now we generate:

  define void @test6(i1 %cond, i8* %ptr) {
    store i8 2, i8* %ptr.2, align 8
    ret void
  }

I did not see any impact on the test-suite + externals.

rdar://18596215

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219462 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/Utils/SimplifyCFG.cpp          | 13 +++++
 .../SimplifyCFG/UnreachableEliminate.ll       | 47 +++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 0dde18522f4..bc9574221b3 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1010,6 +1010,8 @@ static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2,
   return true;
 }
 
+static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I);
+
 /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and
 /// BB2, hoist any common code in the two blocks up into the branch block.  The
 /// caller of this function guarantees that BI's block dominates BB1 and BB2.
@@ -1094,6 +1096,12 @@ HoistTerminator:
       if (BB1V == BB2V)
         continue;
 
+      // Check for passingValueIsAlwaysUndefined here because we would rather
+      // eliminate undefined control flow then converting it to a select.
+      if (passingValueIsAlwaysUndefined(BB1V, PN) ||
+          passingValueIsAlwaysUndefined(BB2V, PN))
+       return Changed;
+
       if (isa<ConstantExpr>(BB1V) && !isSafeToSpeculativelyExecute(BB1V, DL))
         return Changed;
       if (isa<ConstantExpr>(BB2V) && !isSafeToSpeculativelyExecute(BB2V, DL))
@@ -1508,6 +1516,11 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
     if (ThenV == OrigV)
       continue;
 
+    // Don't convert to selects if we could remove undefined behavior instead.
+    if (passingValueIsAlwaysUndefined(OrigV, PN) ||
+        passingValueIsAlwaysUndefined(ThenV, PN))
+      return false;
+
     HaveRewritablePHIs = true;
     ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
     ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
diff --git a/test/Transforms/SimplifyCFG/UnreachableEliminate.ll b/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
index 5fff37bc71f..21428c62f53 100644
--- a/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
+++ b/test/Transforms/SimplifyCFG/UnreachableEliminate.ll
@@ -71,3 +71,50 @@ U:
 T:
         ret i32 2
 }
+
+
+;; We can either convert the following control-flow to a select or remove the
+;; unreachable control flow because of the undef store of null. Make sure we do
+;; the latter.
+
+define void @test5(i1 %cond, i8* %ptr) {
+
+; CHECK-LABEL: test5
+; CHECK: entry:
+; CHECK-NOT: select
+; CHECK:  store i8 2, i8* %ptr
+; CHECK:  ret
+
+entry:
+  br i1 %cond, label %bb1, label %bb3
+
+bb3:
+ br label %bb2
+
+bb1:
+ br label %bb2
+
+bb2:
+  %ptr.2 = phi i8* [ %ptr, %bb3 ], [ null, %bb1 ]
+  store i8 2, i8* %ptr.2, align 8
+  ret void
+}
+
+; CHECK-LABEL: test6
+; CHECK: entry:
+; CHECK-NOT: select
+; CHECK:  store i8 2, i8* %ptr
+; CHECK:  ret
+
+define void @test6(i1 %cond, i8* %ptr) {
+entry:
+  br i1 %cond, label %bb1, label %bb2
+
+bb1:
+  br label %bb2
+
+bb2:
+  %ptr.2 = phi i8* [ %ptr, %entry ], [ null, %bb1 ]
+  store i8 2, i8* %ptr.2, align 8
+  ret void
+}