[InstCombine] Fold branches with irrelevant conditions to a constant.

InstCombine folds instructions with irrelevant conditions to undef.
This, as Nuno confirmed is a bug.
(see https://bugs.llvm.org/show_bug.cgi?id=33409#c1 )

Given the original motivation for the change is that of removing an
USE, we now fold to false instead (which reaches the same goal
without undesired side effects).

Fixes PR33409.

Differential Revision:  https://reviews.llvm.org/D36975

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311540 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 6690765c09b34bfdb66d542a8988c8f78d579708..1881f78255b2d615365c44cec32293d31a8eaf9c 100644 (file)
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2260,10 +2260,9 @@ Instruction *InstCombiner::visitBranchInst(BranchInst &BI) {
 
   // If the condition is irrelevant, remove the use so that other
   // transforms on the condition become more effective.
-  if (BI.isConditional() &&
-      BI.getSuccessor(0) == BI.getSuccessor(1) &&
-      !isa<UndefValue>(BI.getCondition())) {
-    BI.setCondition(UndefValue::get(BI.getCondition()->getType()));
+  if (BI.isConditional() && !isa<ConstantInt>(BI.getCondition()) &&
+      BI.getSuccessor(0) == BI.getSuccessor(1)) {
+    BI.setCondition(ConstantInt::getFalse(BI.getCondition()->getType()));
     return &BI;
   }
 

diff --git a/test/Transforms/InstCombine/branch.ll b/test/Transforms/InstCombine/branch.ll
index 5be93cc20234ae207733f457c7e47faa951c30a8..2168c9fd9a132c449829775160331f5e4ec8c5bd 100644 (file)
--- a/test/Transforms/InstCombine/branch.ll
+++ b/test/Transforms/InstCombine/branch.ll
@@ -1,10 +1,12 @@
+; Check that we fold the condition of branches of the
+; form: br <condition> dest1, dest2, where dest1 == dest2.
 ; RUN: opt -instcombine -S < %s | FileCheck %s
 
 define i32 @test(i32 %x) {
 ; CHECK-LABEL: @test
 entry:
 ; CHECK-NOT: icmp
-; CHECK: br i1 undef, 
+; CHECK: br i1 false
   %cmp = icmp ult i32 %x, 7
   br i1 %cmp, label %merge, label %merge
 merge:
@@ -13,4 +15,13 @@ merge:
   ret i32 %x
 }
 
+@global = global i8 0
 
+define i32 @pat(i32 %x) {
+; CHECK-NOT: icmp false
+; CHECK: br i1 false
+  %y = icmp eq i32 27, ptrtoint(i8* @global to i32)
+  br i1 %y, label %patatino, label %patatino
+patatino:
+  ret i32 %x
+}

diff --git a/test/Transforms/InstCombine/pr33765.ll b/test/Transforms/InstCombine/pr33765.ll
index 99ed0d13b5cf5e788d265fd604c126d263fe697f..5aa99bbf7cb9b2023a7a6b4587c382074ea84900 100644 (file)
--- a/test/Transforms/InstCombine/pr33765.ll
+++ b/test/Transforms/InstCombine/pr33765.ll
@@ -6,7 +6,7 @@
 define void @patatino(i8 %beth) {
 ; CHECK-LABEL: @patatino(
 ; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[BETH:%.*]] to i32
-; CHECK-NEXT:    br i1 undef, label [[IF_THEN9:%.*]], label [[IF_THEN9]]
+; CHECK-NEXT:    br i1 false, label [[IF_THEN9:%.*]], label [[IF_THEN9]]
 ; CHECK:       if.then9:
 ; CHECK-NEXT:    [[MUL:%.*]] = mul nuw nsw i32 [[CONV]], [[CONV]]
 ; CHECK-NEXT:    [[TINKY:%.*]] = load i16, i16* @glob, align 2

diff --git a/test/Transforms/InstCombine/select-cmp-br.ll b/test/Transforms/InstCombine/select-cmp-br.ll
index 59384ab7b1f02779544ab08ff8272bea5e9feab6..06f32828c96202429999f591e90bbf48b3c0cfb7 100644 (file)
--- a/test/Transforms/InstCombine/select-cmp-br.ll
+++ b/test/Transforms/InstCombine/select-cmp-br.ll
@@ -248,7 +248,7 @@ bb5:                                              ; preds = %entry
 define i32 @test6(i32 %arg, i1 %arg1) {
 ; CHECK-LABEL: @test6(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br i1 undef, label [[BB:%.*]], label [[BB]]
+; CHECK-NEXT:    br i1 false, label [[BB:%.*]], label [[BB]]
 ; CHECK:       bb:
 ; CHECK-NEXT:    [[TMP:%.*]] = select i1 [[ARG1:%.*]], i32 [[ARG:%.*]], i32 0
 ; CHECK-NEXT:    ret i32 [[TMP]]