[InstCombine] (A + 1) + (B ^ -1) --> A - B

Turn canonicalized subtraction back into (-1 - B) and combine it with (A + 1) into (A - B).
This is similar to the folding already done for (B ^ -1) + Const into (-1 + Const) - B.

Differential Revision: https://reviews.llvm.org/D48535

llvm-svn: 335579

2 files changed, 42 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 1101f729063..330db9eb91f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1212,6 +1212,11 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
   if (Value *V = checkForNegativeOperand(I, Builder))
     return replaceInstUsesWith(I, V);
 
+  // (A + 1) + ~B --> A - B
+  // ~B + (A + 1) --> A - B
+  if (match(&I, m_c_BinOp(m_Add(m_Value(A), m_One()), m_Not(m_Value(B)))))
+    return BinaryOperator::CreateSub(A, B);
+
   // X % C0 + (( X / C0 ) % C1) * C0 => X % (C0 * C1)
   if (Value *V = SimplifyAddWithRemainder(I)) return replaceInstUsesWith(I, V);
 
diff --git a/llvm/test/Transforms/InstCombine/add.ll b/llvm/test/Transforms/InstCombine/add.ll
index 61eb6f9317e..04a51087132 100644
--- a/llvm/test/Transforms/InstCombine/add.ll
+++ b/llvm/test/Transforms/InstCombine/add.ll
@@ -802,3 +802,40 @@ final:
   %value = add <2 x i32> <i32 123, i32 333>, %A
   ret <2 x i32> %value
 }
+
+; E = (A + 1) + ~B = A - B
+define i32 @add_not_increment(i32 %A, i32 %B) {
+; CHECK-LABEL: @add_not_increment(
+; CHECK-NEXT:    [[E:%.*]] = sub i32 %A, %B
+; CHECK-NEXT:    ret i32 [[E]]
+;
+  %C = xor i32 %B, -1
+  %D = add i32 %A, 1
+  %E = add i32 %D, %C
+  ret i32 %E
+}
+
+; E = (A + 1) + ~B = A - B
+define <2 x i32> @add_not_increment_vec(<2 x i32> %A, <2 x i32> %B) {
+; CHECK-LABEL: @add_not_increment_vec(
+; CHECK-NEXT:    [[E:%.*]] = sub <2 x i32> %A, %B
+; CHECK-NEXT:    ret <2 x i32> [[E]]
+;
+  %C = xor <2 x i32> %B, <i32 -1, i32 -1>
+  %D = add <2 x i32> %A, <i32 1, i32 1>
+  %E = add <2 x i32> %D, %C
+  ret <2 x i32> %E
+}
+
+; E = ~B + (1 + A) = A - B
+define i32 @add_not_increment_commuted(i32 %A, i32 %B) {
+; CHECK-LABEL: @add_not_increment_commuted(
+; CHECK-NEXT:    [[E:%.*]] = sub i32 %A, %B
+; CHECK-NEXT:    ret i32 [[E]]
+;
+  %C = xor i32 %B, -1
+  %D = add i32 %A, 1
+  %E = add i32 %C, %D
+  ret i32 %E
+}
+