[PATCH] D58877: [InstCombine] fold add(add(A, ~B), 1) -> sub(A, B)

[Amaury SECHET via Phabricator via llvm-commits]

deadalnix created this revision.
deadalnix added reviewers: lebedev.ri, RKSimon.
Herald added a project: LLVM.

As per title.


Repository:
  rL LLVM

https://reviews.llvm.org/D58877

Files:
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  test/Transforms/InstCombine/add.ll


Index: test/Transforms/InstCombine/add.ll
===================================================================
--- test/Transforms/InstCombine/add.ll
+++ test/Transforms/InstCombine/add.ll
@@ -950,9 +950,7 @@
 ; E = (A + ~B) + 1 = A - B
 define i32 @add_to_sub(i32 %A, i32 %B) {
 ; CHECK-LABEL: @add_to_sub(
-; CHECK-NEXT:    [[C:%.*]] = xor i32 [[B:%.*]], -1
-; CHECK-NEXT:    [[D:%.*]] = add i32 [[C]], [[A:%.*]]
-; CHECK-NEXT:    [[E:%.*]] = add i32 [[D]], 1
+; CHECK-NEXT:    [[E:%.*]] = sub i32 [[A:%.*]], [[B:%.*]]
 ; CHECK-NEXT:    ret i32 [[E]]
 ;
   %C = xor i32 %B, -1
@@ -964,9 +962,7 @@
 ; E = (~B + A) + 1 = A - B
 define i32 @add_to_sub2(i32 %A, i32 %B) {
 ; CHECK-LABEL: @add_to_sub2(
-; CHECK-NEXT:    [[C:%.*]] = xor i32 [[B:%.*]], -1
-; CHECK-NEXT:    [[D:%.*]] = add i32 [[C]], [[A:%.*]]
-; CHECK-NEXT:    [[E:%.*]] = add i32 [[D]], 1
+; CHECK-NEXT:    [[E:%.*]] = sub i32 [[A:%.*]], [[B:%.*]]
 ; CHECK-NEXT:    ret i32 [[E]]
 ;
   %C = xor i32 %B, -1
Index: lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -892,6 +892,10 @@
   if (!match(Op1, m_APInt(C)))
     return nullptr;
 
+  // (X + ~Y) + 1 --> X - Y
+  if (C->isOneValue() && match(Op0, m_c_Add(m_Value(X), m_Not(m_Value(Y)))))
+    return BinaryOperator::CreateSub(X, Y);
+
   if (C->isSignMask()) {
     // If wrapping is not allowed, then the addition must set the sign bit:
     // X + (signmask) --> X | signmask


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