From 764b0fd5a37168313cd7f8d82753b10392ff2b2b Mon Sep 17 00:00:00 2001
From: Philip Reames <listmail@philipreames.com>
Date: Wed, 21 Aug 2019 15:51:57 +0000
Subject: [PATCH] [instcombine] icmp eq/ne (sub C, Y), C -> icmp eq/ne Y, 0

Noticed while looking at pr43028.

llvm-svn: 369541
---
 .../Transforms/InstCombine/InstCombineCompares.cpp |  5 +++
 llvm/test/Transforms/InstCombine/icmp-sub.ll       | 40 ++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 5e0a3c3..952f295 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2411,6 +2411,11 @@ Instruction *InstCombiner::foldICmpSubConstant(ICmpInst &Cmp,
   const APInt *C2;
   APInt SubResult;
 
+  // icmp eq/ne (sub C, Y), C -> icmp eq/ne Y, 0
+  if (match(X, m_APInt(C2)) && *C2 == C && Cmp.isEquality())
+    return new ICmpInst(Cmp.getPredicate(), Y,
+                        ConstantInt::get(Y->getType(), 0));
+
   // (icmp P (sub nuw|nsw C2, Y), C) -> (icmp swap(P) Y, C2-C)
   if (match(X, m_APInt(C2)) &&
       ((Cmp.isUnsigned() && Sub->hasNoUnsignedWrap()) ||
diff --git a/llvm/test/Transforms/InstCombine/icmp-sub.ll b/llvm/test/Transforms/InstCombine/icmp-sub.ll
index c66581b..3e7cb4e 100644
--- a/llvm/test/Transforms/InstCombine/icmp-sub.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-sub.ll
@@ -84,3 +84,43 @@ define i1 @test_negative_combined_sub_signed_overflow(i8 %x) {
   %z = icmp slt i8 %y, -1
   ret i1 %z
 }
+
+define i1 @test_sub_0_Y_eq_0(i8 %y) {
+; CHECK-LABEL: @test_sub_0_Y_eq_0(
+; CHECK-NEXT:    [[Z:%.*]] = icmp eq i8 [[Y:%.*]], 0
+; CHECK-NEXT:    ret i1 [[Z]]
+;
+  %s = sub i8 0, %y
+  %z = icmp eq i8 %s, 0
+  ret i1 %z
+}
+
+define i1 @test_sub_0_Y_ne_0(i8 %y) {
+; CHECK-LABEL: @test_sub_0_Y_ne_0(
+; CHECK-NEXT:    [[Z:%.*]] = icmp ne i8 [[Y:%.*]], 0
+; CHECK-NEXT:    ret i1 [[Z]]
+;
+  %s = sub i8 0, %y
+  %z = icmp ne i8 %s, 0
+  ret i1 %z
+}
+
+define i1 @test_sub_4_Y_ne_4(i8 %y) {
+; CHECK-LABEL: @test_sub_4_Y_ne_4(
+; CHECK-NEXT:    [[Z:%.*]] = icmp ne i8 [[Y:%.*]], 0
+; CHECK-NEXT:    ret i1 [[Z]]
+;
+  %s = sub i8 4, %y
+  %z = icmp ne i8 %s, 4
+  ret i1 %z
+}
+
+define i1 @test_sub_127_Y_eq_127(i8 %y) {
+; CHECK-LABEL: @test_sub_127_Y_eq_127(
+; CHECK-NEXT:    [[Z:%.*]] = icmp eq i8 [[Y:%.*]], 0
+; CHECK-NEXT:    ret i1 [[Z]]
+;
+  %s = sub i8 127, %y
+  %z = icmp eq i8 %s, 127
+  ret i1 %z
+}
-- 
2.7.4