InstCombine: Implement X - A*-B -> X + A*B.

llvm-svn: 119984

2 files changed, 9 insertions, 16 deletions

diff --git a/llvm/lib/Target/README.txt b/llvm/lib/Target/README.txt
index 1ec4cdf4ba9..4d5b17ab367 100644
--- a/llvm/lib/Target/README.txt
+++ b/llvm/lib/Target/README.txt
@@ -1736,22 +1736,6 @@ Another similar case involves truncations on 64-bit targets:
 ...
   %367 = icmp eq i32 %362, 0                      ; [#uses=1]
 
-
-//===---------------------------------------------------------------------===//
-
-Missed instcombine/dagcombine transformation:
-define i32 @a(i32 %x, i32 %y) nounwind readnone {
-entry:
-  %mul = mul i32 %y, -8
-  %sub = sub i32 %x, %mul
-  ret i32 %sub
-}
-
-Should compile to something like x+y*8, but currently compiles to an
-inefficient result.  Testcase derived from gcc.  C testcase:
-
-int a(int x, int y) { return y-x*-8; }
-
 //===---------------------------------------------------------------------===//
 
 Missed instcombine/dagcombine transformation:
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 9b72eb924ac..c04a6b2a627 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -675,6 +675,15 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
                                              C2);
         return BinaryOperator::CreateMul(Op0, CP1);
       }
+
+      // X - A*-B -> X + A*B
+      // X - -A*B -> X + A*B
+      Value *A, *B;
+      if (match(Op1I, m_Mul(m_Value(A), m_Neg(m_Value(B)))) ||
+          match(Op1I, m_Mul(m_Neg(m_Value(A)), m_Value(B)))) {
+        Value *NewMul = Builder->CreateMul(A, B);
+        return BinaryOperator::CreateAdd(Op0, NewMul);
+      }
     }
   }