Turn X-(X-Y) into Y. According to my auto-simplifier this is the most common
simplification present in fully optimized code (I think instcombine fails to
transform some of these when "X-Y" has more than one use). Fires here and
there all over the test-suite, for example it eliminates 8 subtractions in
the final IR for 445.gobmk, 2 subs in 447.dealII, 2 in paq8p etc.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123442 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index d5cf626..1d4ff0f 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -593,11 +593,25 @@
match(Op0, m_Add(m_Specific(Op1), m_Value(X))))
return X;
- /// i1 sub -> xor.
+ // i1 sub -> xor.
if (MaxRecurse && Op0->getType()->isIntegerTy(1))
if (Value *V = SimplifyXorInst(Op0, Op1, TD, DT, MaxRecurse-1))
return V;
+ // X - (X - Y) -> Y. More generally Z - (X - Y) -> (Z - X) + Y if everything
+ // simplifies.
+ Value *Y = 0, *Z = Op0;
+ if (MaxRecurse && match(Op1, m_Sub(m_Value(X), m_Value(Y)))) // Z - (X - Y)
+ // See if "V === Z - X" simplifies.
+ if (Value *V = SimplifyBinOp(Instruction::Sub, Z, X, TD, DT, MaxRecurse-1))
+ // It does! Now see if "W === V + Y" simplifies.
+ if (Value *W = SimplifyBinOp(Instruction::Add, V, Y, TD, DT,
+ MaxRecurse-1)) {
+ // It does, we successfully reassociated!
+ ++NumReassoc;
+ return W;
+ }
+
// Mul distributes over Sub. Try some generic simplifications based on this.
if (Value *V = FactorizeBinOp(Instruction::Sub, Op0, Op1, Instruction::Mul,
TD, DT, MaxRecurse))