From 1640e687280e653947811f4305f021be8f9472e5 Mon Sep 17 00:00:00 2001
From: Brian Gesiak <modocache@gmail.com>
Date: Sun, 19 Mar 2017 04:40:42 +0000
Subject: [PATCH] [Analysis] bitreverse(undef) returns undef

Summary:
The reverse of an artbitrary bitpattern is also an arbitrary
bitpattern.

Reviewers: trentxintong, arsenm, majnemer

Reviewed By: majnemer

Subscribers: majnemer, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D31118

llvm-svn: 298201
---
 llvm/lib/Analysis/ConstantFolding.cpp              |  3 ++-
 .../test/Transforms/InstCombine/bitreverse-fold.ll | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 6d8d4a1c3e90..85ca622a25dc 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -1768,7 +1768,8 @@ Constant *ConstantFoldScalarCall(StringRef Name, unsigned IntrinsicID, Type *Ty,
     }
 
     if (isa<UndefValue>(Operands[0])) {
-      if (IntrinsicID == Intrinsic::bswap)
+      if (IntrinsicID == Intrinsic::bswap ||
+          IntrinsicID == Intrinsic::bitreverse)
         return Operands[0];
       return nullptr;
     }
diff --git a/llvm/test/Transforms/InstCombine/bitreverse-fold.ll b/llvm/test/Transforms/InstCombine/bitreverse-fold.ll
index ecdfbc8cb5f9..b798ad33b3f0 100644
--- a/llvm/test/Transforms/InstCombine/bitreverse-fold.ll
+++ b/llvm/test/Transforms/InstCombine/bitreverse-fold.ll
@@ -37,6 +37,13 @@ define i32 @reverse_neg1_i32() {
   ret i32 %x
 }
 
+; CHECK-LABEL: @reverse_undef_i32(
+; CHECK-NEXT: ret i32 undef
+define i32 @reverse_undef_i32() {
+  %x = call i32 @llvm.bitreverse.i32(i32 undef)
+  ret i32 %x
+}
+
 ; CHECK-LABEL: @reverse_false_i1(
 ; CHECK-NEXT: ret i1 false
 define i1 @reverse_false_i1() {
@@ -51,6 +58,13 @@ define i1 @reverse_true_i1() {
   ret i1 %x
 }
 
+; CHECK-LABEL: @reverse_undef_i1(
+; CHECK-NEXT: ret i1 undef
+define i1 @reverse_undef_i1() {
+  %x = call i1 @llvm.bitreverse.i1(i1 undef)
+  ret i1 %x
+}
+
 ; CHECK-LABEL: @reverse_false_v2i1(
 ; CHECK-NEXT: ret <2 x i1> zeroinitializer
 define <2 x i1> @reverse_false_v2i1() {