From 0ade2abdb01f4a16b1f08d1a78d664b9e9d5f3b5 Mon Sep 17 00:00:00 2001
From: Sanjay Patel <spatel@rotateright.com>
Date: Wed, 22 Jan 2020 09:29:19 -0500
Subject: [PATCH] [InstCombine] fneg(X + C) --> -C - X

This is 1 of the potential folds uncovered by extending D72521.

We don't seem to do this in the backend either (unless I'm not
seeing some target-specific transform).

icc and gcc (appears to be target-specific) do this transform.

Differential Revision: https://reviews.llvm.org/D73057
---
 .../Transforms/InstCombine/InstCombineAddSub.cpp   |  6 +++
 llvm/test/Transforms/InstCombine/fneg.ll           | 50 ++++++++++++++++------
 2 files changed, 44 insertions(+), 12 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 6eaae8f..d707dbf 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2132,6 +2132,12 @@ static Instruction *foldFNegIntoConstant(Instruction &I) {
   if (match(&I, m_FNeg(m_OneUse(m_FDiv(m_Constant(C), m_Value(X))))))
     return BinaryOperator::CreateFDivFMF(ConstantExpr::getFNeg(C), X, &I);
 
+  // With NSZ [ counter-example with -0.0: -(-0.0 + 0.0) != 0.0 + -0.0 ]:
+  // -(X + C) --> -X + -C --> -C - X
+  if (I.hasNoSignedZeros() &&
+      match(&I, m_FNeg(m_OneUse(m_FAdd(m_Value(X), m_Constant(C))))))
+    return BinaryOperator::CreateFSubFMF(ConstantExpr::getFNeg(C), X, &I);
+
   return nullptr;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/fneg.ll b/llvm/test/Transforms/InstCombine/fneg.ll
index fde5577..01949e8 100644
--- a/llvm/test/Transforms/InstCombine/fneg.ll
+++ b/llvm/test/Transforms/InstCombine/fneg.ll
@@ -378,6 +378,8 @@ define float @fneg_fneg_sel_extra_use3(float %x, float %y, i1 %cond) {
   ret float %sel
 }
 
+; Negative test
+
 define float @fneg_fadd_constant(float %x) {
 ; CHECK-LABEL: @fneg_fadd_constant(
 ; CHECK-NEXT:    [[A:%.*]] = fadd float [[X:%.*]], 4.200000e+01
@@ -389,6 +391,8 @@ define float @fneg_fadd_constant(float %x) {
   ret float %r
 }
 
+; Negative test
+
 define float @fake_nsz_fadd_constant(float %x) {
 ; CHECK-LABEL: @fake_nsz_fadd_constant(
 ; CHECK-NEXT:    [[A:%.*]] = fadd float [[X:%.*]], 4.200000e+01
@@ -400,12 +404,11 @@ define float @fake_nsz_fadd_constant(float %x) {
   ret float %r
 }
 
-; TODO: -(X + C) --> -C - X
+; -(X + C) --> -C - X
 
 define float @fneg_nsz_fadd_constant(float %x) {
 ; CHECK-LABEL: @fneg_nsz_fadd_constant(
-; CHECK-NEXT:    [[A:%.*]] = fadd float [[X:%.*]], 4.200000e+01
-; CHECK-NEXT:    [[R:%.*]] = fneg nsz float [[A]]
+; CHECK-NEXT:    [[R:%.*]] = fsub nsz float -4.200000e+01, [[X:%.*]]
 ; CHECK-NEXT:    ret float [[R]]
 ;
   %a = fadd float %x, 42.0
@@ -413,12 +416,11 @@ define float @fneg_nsz_fadd_constant(float %x) {
   ret float %r
 }
 
-; TODO: -(X + C) --> -C - X
+; -(X + C) --> -C - X
 
 define float @fake_fneg_nsz_fadd_constant(float %x) {
 ; CHECK-LABEL: @fake_fneg_nsz_fadd_constant(
-; CHECK-NEXT:    [[A:%.*]] = fadd float [[X:%.*]], 4.200000e+01
-; CHECK-NEXT:    [[R:%.*]] = fsub fast float -0.000000e+00, [[A]]
+; CHECK-NEXT:    [[R:%.*]] = fsub fast float -4.200000e+01, [[X:%.*]]
 ; CHECK-NEXT:    ret float [[R]]
 ;
   %a = fadd float %x, 42.0
@@ -426,6 +428,8 @@ define float @fake_fneg_nsz_fadd_constant(float %x) {
   ret float %r
 }
 
+; Negative test
+
 define float @fneg_nsz_fadd_constant_extra_use(float %x) {
 ; CHECK-LABEL: @fneg_nsz_fadd_constant_extra_use(
 ; CHECK-NEXT:    [[A:%.*]] = fadd float [[X:%.*]], 4.200000e+01
@@ -439,6 +443,8 @@ define float @fneg_nsz_fadd_constant_extra_use(float %x) {
   ret float %r
 }
 
+; Negative test
+
 define float @fake_fneg_nsz_fadd_constant_extra_use(float %x) {
 ; CHECK-LABEL: @fake_fneg_nsz_fadd_constant_extra_use(
 ; CHECK-NEXT:    [[A:%.*]] = fadd float [[X:%.*]], 4.200000e+01
@@ -452,12 +458,11 @@ define float @fake_fneg_nsz_fadd_constant_extra_use(float %x) {
   ret float %r
 }
 
-; TODO: -(X + C) --> -C - X
+; -(X + C) --> -C - X
 
 define <2 x float> @fneg_nsz_fadd_constant_vec(<2 x float> %x) {
 ; CHECK-LABEL: @fneg_nsz_fadd_constant_vec(
-; CHECK-NEXT:    [[A:%.*]] = fadd <2 x float> [[X:%.*]], <float 4.200000e+01, float 4.300000e+01>
-; CHECK-NEXT:    [[R:%.*]] = fneg reassoc nnan nsz <2 x float> [[A]]
+; CHECK-NEXT:    [[R:%.*]] = fsub reassoc nnan nsz <2 x float> <float -4.200000e+01, float -4.300000e+01>, [[X:%.*]]
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
   %a = fadd <2 x float> %x, <float 42.0, float 43.0>
@@ -465,15 +470,36 @@ define <2 x float> @fneg_nsz_fadd_constant_vec(<2 x float> %x) {
   ret <2 x float> %r
 }
 
-; TODO: -(X + C) --> -C - X
+; -(X + C) --> -C - X
 
 define <2 x float> @fake_fneg_nsz_fadd_constant_vec(<2 x float> %x) {
 ; CHECK-LABEL: @fake_fneg_nsz_fadd_constant_vec(
-; CHECK-NEXT:    [[A:%.*]] = fadd <2 x float> [[X:%.*]], <float 4.200000e+01, float undef>
-; CHECK-NEXT:    [[R:%.*]] = fsub nsz <2 x float> <float undef, float -0.000000e+00>, [[A]]
+; CHECK-NEXT:    [[R:%.*]] = fsub nsz <2 x float> <float -4.200000e+01, float undef>, [[X:%.*]]
 ; CHECK-NEXT:    ret <2 x float> [[R]]
 ;
   %a = fadd <2 x float> %x, <float 42.0, float undef>
   %r = fsub nsz <2 x float> <float undef, float -0.0>, %a
   ret <2 x float> %r
 }
+
+@g = external global i16, align 1
+
+define float @fneg_nsz_fadd_constant_expr(float %x) {
+; CHECK-LABEL: @fneg_nsz_fadd_constant_expr(
+; CHECK-NEXT:    [[R:%.*]] = fsub nsz float fneg (float bitcast (i32 ptrtoint (i16* @g to i32) to float)), [[X:%.*]]
+; CHECK-NEXT:    ret float [[R]]
+;
+  %a = fadd float %x, bitcast (i32 ptrtoint (i16* @g to i32) to float)
+  %r = fneg nsz float %a
+  ret float %r
+}
+
+define float @fake_fneg_nsz_fadd_constant_expr(float %x) {
+; CHECK-LABEL: @fake_fneg_nsz_fadd_constant_expr(
+; CHECK-NEXT:    [[R:%.*]] = fsub nsz float fneg (float bitcast (i32 ptrtoint (i16* @g to i32) to float)), [[X:%.*]]
+; CHECK-NEXT:    ret float [[R]]
+;
+  %a = fadd float %x, bitcast (i32 ptrtoint (i16* @g to i32) to float)
+  %r = fsub nsz float -0.0, %a
+  ret float %r
+}
-- 
2.7.4