Fold certain additions through selects (and their compares) so as to eliminate subtractions. This code is often produced by the SMAX expansion in SCEV.
This implements test/Transforms/InstCombine/2007-12-18-AddSelCmpSub.ll
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45158 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 7989ebf..b059c49 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -2130,6 +2130,67 @@
return new PtrToIntInst(I2, CI->getType());
}
}
+
+ // add (select (icmp 0 (sub m A)) X Y) A ->
+ // add (select (icmp A m) X Y) A
+ //
+ // add (select X 0 (sub n A)) A ->
+ // select X A n ->
+ {
+ SelectInst *SI = dyn_cast<SelectInst>(LHS);
+ Value *Other = RHS;
+ if (!SI) {
+ SI = dyn_cast<SelectInst>(RHS);
+ Other = LHS;
+ }
+ if (SI) {
+ Value *TV = SI->getTrueValue();
+ Value *FV = SI->getFalseValue();
+ Value *A;
+
+ // Can we fold the add into the argument of the compare?
+ Value *Cond = SI->getCondition();
+ if (ICmpInst *IC = dyn_cast<ICmpInst>(Cond)) {
+ Value *ICOp0 = IC->getOperand(0);
+ Value *ICOp1 = IC->getOperand(1);
+ ConstantInt *C3, *C4;
+
+ // Check both arguments of the compare for a matching subtract.
+ if (match(ICOp0, m_ConstantInt(C3)) && C3->getValue() == 0 &&
+ match(ICOp1, m_Sub(m_ConstantInt(C4), m_Value(A))) &&
+ A == Other) {
+ // We managed to fold the add into the RHS of the select condition.
+ Cond = new ICmpInst(IC->getPredicate(), A, C4, "asis", SI);
+ } else if (match(ICOp1, m_ConstantInt(C3)) && C3->getValue() == 0 &&
+ match(ICOp0, m_Sub(m_ConstantInt(C4), m_Value(A))) &&
+ A == Other) {
+ // We managed to fold the add into the LHS of the select condition.
+ Cond = new ICmpInst(IC->getPredicate(), C4, A, "asis", SI);
+ }
+ }
+
+ // Can we fold the add into the argument of the select?
+ // We check both true and false select arguments for a matching subtract.
+ ConstantInt *C1, *C2;
+ if (match(FV, m_ConstantInt(C1)) && C1->getValue() == 0 &&
+ match(TV, m_Sub(m_ConstantInt(C2), m_Value(A))) &&
+ A == Other) {
+ // We managed to fold the add into the true select value,
+ // picking up a simplified condition, if available.
+ return new SelectInst(Cond, C2, A, "adselsub");
+ } else if (match(TV, m_ConstantInt(C1)) && C1->getValue() == 0 &&
+ match(FV, m_Sub(m_ConstantInt(C2), m_Value(A))) &&
+ A == Other) {
+ // We managed to fold the add into the false select value,
+ // picking up a simplified condition, if available.
+ return new SelectInst(Cond, A, C2, "adselsub");
+ } else if (Cond != SI->getCondition()) {
+ // We only managed to fold the add into the select condition.
+ SI->setOperand(0, Cond);
+ Changed = true;
+ }
+ }
+ }
return Changed ? &I : 0;
}