From b7a48d833a3fbd3b881ff16807734046b0d01ce8 Mon Sep 17 00:00:00 2001
From: Davide Italiano <davide@freebsd.org>
Date: Wed, 23 Aug 2017 09:14:37 +0000
Subject: [PATCH] [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
---
 lib/Transforms/InstCombine/InstructionCombining.cpp |  7 +++----
 test/Transforms/InstCombine/branch.ll               | 13 ++++++++++++-
 test/Transforms/InstCombine/pr33765.ll              |  2 +-
 test/Transforms/InstCombine/select-cmp-br.ll        |  2 +-
 4 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 6690765c09b..1881f78255b 100644
--- 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 5be93cc2023..2168c9fd9a1 100644
--- 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 99ed0d13b5c..5aa99bbf7cb 100644
--- 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 59384ab7b1f..06f32828c96 100644
--- 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]]
-- 
2.50.1