[llvm] r338200 - [InstCombine] try to fold 'sub' to 'not'

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Sat Jul 28 09:48:44 2018
New Revision: 338200

URL: http://llvm.org/viewvc/llvm-project?rev=338200&view=rev
Log:
[InstCombine] try to fold 'sub' to 'not'

https://rise4fun.com/Alive/jDd

Patterns with add/sub combos can be improved using
'not' ops. This is better for analysis and may lead
to follow-on transforms because 'xor' and 'add' are 
commutative/associative. It can also help codegen.  

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/trunk/test/Transforms/InstCombine/sub-not.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=338200&r1=338199&r2=338200&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Sat Jul 28 09:48:44 2018
@@ -926,7 +926,13 @@ Instruction *InstCombiner::foldAddWithCo
   if (Instruction *NV = foldBinOpIntoSelectOrPhi(Add))
     return NV;
 
-  Value *X;
+  Value *X, *Y;
+
+  // add (sub X, Y), -1 --> add (not Y), X
+  if (match(Op0, m_OneUse(m_Sub(m_Value(X), m_Value(Y)))) &&
+      match(Op1, m_AllOnes()))
+    return BinaryOperator::CreateAdd(Builder.CreateNot(Y), X);
+
   // zext(bool) + C -> bool ? C + 1 : C
   if (match(Op0, m_ZExt(m_Value(X))) &&
       X->getType()->getScalarSizeInBits() == 1)

Modified: llvm/trunk/test/Transforms/InstCombine/sub-not.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub-not.ll?rev=338200&r1=338199&r2=338200&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/sub-not.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/sub-not.ll Sat Jul 28 09:48:44 2018
@@ -40,8 +40,8 @@ define <2 x i8> @sub_not_vec(<2 x i8> %x
 
 define i8 @dec_sub(i8 %x, i8 %y) {
 ; CHECK-LABEL: @dec_sub(
-; CHECK-NEXT:    [[S:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = add i8 [[S]], -1
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i8 [[Y:%.*]], -1
+; CHECK-NEXT:    [[R:%.*]] = add i8 [[TMP1]], [[X:%.*]]
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %s = sub i8 %x, %y
@@ -64,8 +64,8 @@ define i8 @dec_sub_extra_use(i8 %x, i8 %
 
 define <2 x i8> @dec_sub_vec(<2 x i8> %x, <2 x i8> %y) {
 ; CHECK-LABEL: @dec_sub_vec(
-; CHECK-NEXT:    [[S:%.*]] = sub <2 x i8> [[X:%.*]], [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = add <2 x i8> [[S]], <i8 -1, i8 undef>
+; CHECK-NEXT:    [[TMP1:%.*]] = xor <2 x i8> [[Y:%.*]], <i8 -1, i8 -1>
+; CHECK-NEXT:    [[R:%.*]] = add <2 x i8> [[TMP1]], [[X:%.*]]
 ; CHECK-NEXT:    ret <2 x i8> [[R]]
 ;
   %s = sub <2 x i8> %x, %y