From 39390d8317003ffc0627a30ae913bba3b3616efb Mon Sep 17 00:00:00 2001
From: Roman Lebedev <lebedev.ri@gmail.com>
Date: Fri, 31 May 2019 09:47:16 +0000
Subject: [PATCH] [InstCombine] 'C-(C2-X) --> X+(C-C2)' constant-fold

It looks this fold was already partially happening, indirectly
via some other folds, but with one-use limitation.
No other fold here has that restriction.

https://rise4fun.com/Alive/ftR

llvm-svn: 362217
---
 llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp      | 7 ++++++-
 .../test/Transforms/InstCombine/addsub-constant-folding.ll | 4 ++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index d422b07d49fe..e6b32ba13a4a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1609,8 +1609,13 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
       if (Instruction *R = foldOpIntoPhi(I, PN))
         return R;
 
-    // C-(X+C2) --> (C-C2)-X
     Constant *C2;
+
+    // C-(C2-X) --> X+(C-C2)
+    if (match(Op1, m_Sub(m_Constant(C2), m_Value(X))))
+      return BinaryOperator::CreateAdd(X, ConstantExpr::getSub(C, C2));
+
+    // C-(X+C2) --> (C-C2)-X
     if (match(Op1, m_Add(m_Value(X), m_Constant(C2))))
       return BinaryOperator::CreateSub(ConstantExpr::getSub(C, C2), X);
   }
diff --git a/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll b/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
index 66c1e6e91684..2d9918737d2a 100644
--- a/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
+++ b/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
@@ -485,7 +485,7 @@ define i32 @const_sub_const_sub_extrause(i32 %arg) {
 ; CHECK-LABEL: @const_sub_const_sub_extrause(
 ; CHECK-NEXT:    [[T0:%.*]] = sub i32 8, [[ARG:%.*]]
 ; CHECK-NEXT:    call void @use(i32 [[T0]])
-; CHECK-NEXT:    [[T1:%.*]] = sub i32 2, [[T0]]
+; CHECK-NEXT:    [[T1:%.*]] = add i32 [[ARG]], -6
 ; CHECK-NEXT:    ret i32 [[T1]]
 ;
   %t0 = sub i32 8, %arg
@@ -508,7 +508,7 @@ define <4 x i32> @vec_const_sub_const_sub_extrause(<4 x i32> %arg) {
 ; CHECK-LABEL: @vec_const_sub_const_sub_extrause(
 ; CHECK-NEXT:    [[T0:%.*]] = sub <4 x i32> <i32 8, i32 8, i32 8, i32 8>, [[ARG:%.*]]
 ; CHECK-NEXT:    call void @vec_use(<4 x i32> [[T0]])
-; CHECK-NEXT:    [[T1:%.*]] = sub <4 x i32> <i32 2, i32 2, i32 2, i32 2>, [[T0]]
+; CHECK-NEXT:    [[T1:%.*]] = add <4 x i32> [[ARG]], <i32 -6, i32 -6, i32 -6, i32 -6>
 ; CHECK-NEXT:    ret <4 x i32> [[T1]]
 ;
   %t0 = sub <4 x i32> <i32 8, i32 8, i32 8, i32 8>, %arg