[PATCH] D36975: [InstCombine] Fold branches with irrelevant conditions to a constant

[Davide Italiano via Phabricator via llvm-commits]

davide created this revision.

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

Given the original motivation for the change is that of removing an USE, I suggested to just fold to a constant instead (which seems to reach the same goal without side effects).

Fixes PR33409 (which also contains a more detailed discussion of the bug).


https://reviews.llvm.org/D36975

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/branch.ll
  test/Transforms/InstCombine/pr33765.ll
  test/Transforms/InstCombine/select-cmp-br.ll


Index: test/Transforms/InstCombine/select-cmp-br.ll
===================================================================
--- test/Transforms/InstCombine/select-cmp-br.ll
+++ test/Transforms/InstCombine/select-cmp-br.ll
@@ -248,7 +248,7 @@
 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]]
Index: test/Transforms/InstCombine/pr33765.ll
===================================================================
--- test/Transforms/InstCombine/pr33765.ll
+++ 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
Index: test/Transforms/InstCombine/branch.ll
===================================================================
--- test/Transforms/InstCombine/branch.ll
+++ 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:
Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2260,10 +2260,9 @@
 
   // 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<Constant>(BI.getCondition()) &&
+      BI.getSuccessor(0) == BI.getSuccessor(1)) {
+    BI.setCondition(ConstantInt::getFalse(BI.getCondition()->getType()));
     return &BI;
   }
 


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