enhance x-(-A) -> x+A to preserve NUW/NSW.
Use the presence of NSW/NUW to fold "icmp (x+cst), x" to a constant in
cases where it would otherwise be undefined behavior.
Surprisingly (to me at least), this triggers hundreds of the times in
a few benchmarks: lencode, ldecode, and 466.h264ref seem to *really*
like this.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91812 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 07f7b26..9033877 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -258,7 +258,7 @@
Instruction *FoldICmpDivCst(ICmpInst &ICI, BinaryOperator *DivI,
ConstantInt *DivRHS);
Instruction *FoldICmpAddOpCst(ICmpInst &ICI, Value *X, ConstantInt *CI,
- ICmpInst::Predicate Pred);
+ ICmpInst::Predicate Pred, Value *TheAdd);
Instruction *FoldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
ICmpInst::Predicate Cond, Instruction &I);
Instruction *FoldShiftByConstant(Value *Op0, ConstantInt *Op1,
@@ -2737,9 +2737,13 @@
if (Op0 == Op1) // sub X, X -> 0
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
- // If this is a 'B = x-(-A)', change to B = x+A.
- if (Value *V = dyn_castNegVal(Op1))
- return BinaryOperator::CreateAdd(Op0, V);
+ // If this is a 'B = x-(-A)', change to B = x+A. This preserves NSW/NUW.
+ if (Value *V = dyn_castNegVal(Op1)) {
+ BinaryOperator *Res = BinaryOperator::CreateAdd(Op0, V);
+ Res->setHasNoSignedWrap(I.hasNoSignedWrap());
+ Res->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
+ return Res;
+ }
if (isa<UndefValue>(Op0))
return ReplaceInstUsesWith(I, Op0); // undef - X -> undef
@@ -6604,13 +6608,13 @@
{
Value *X; ConstantInt *Cst;
- // icmp (X+Cst), X
+ // icmp X+Cst, X
if (match(Op0, m_Add(m_Value(X), m_ConstantInt(Cst))) && Op1 == X)
- return FoldICmpAddOpCst(I, X, Cst, I.getPredicate());
-
+ return FoldICmpAddOpCst(I, X, Cst, I.getPredicate(), Op0);
+
// icmp X, X+Cst
if (match(Op1, m_Add(m_Value(X), m_ConstantInt(Cst))) && Op0 == X)
- return FoldICmpAddOpCst(I, X, Cst, I.getSwappedPredicate());
+ return FoldICmpAddOpCst(I, X, Cst, I.getSwappedPredicate(), Op1);
}
return Changed ? &I : 0;
}
@@ -6618,7 +6622,8 @@
/// FoldICmpAddOpCst - Fold "icmp pred (X+CI), X".
Instruction *InstCombiner::FoldICmpAddOpCst(ICmpInst &ICI,
Value *X, ConstantInt *CI,
- ICmpInst::Predicate Pred) {
+ ICmpInst::Predicate Pred,
+ Value *TheAdd) {
// If we have X+0, exit early (simplifying logic below) and let it get folded
// elsewhere. icmp X+0, X -> icmp X, X
if (CI->isZero()) {
@@ -6633,6 +6638,13 @@
// (X+4) != X -> true.
if (Pred == ICmpInst::ICMP_NE)
return ReplaceInstUsesWith(ICI, ConstantInt::getTrue(X->getContext()));
+
+ // If this is an instruction (as opposed to constantexpr) get NUW/NSW info.
+ bool isNUW = false, isNSW = false;
+ if (BinaryOperator *Add = dyn_cast<BinaryOperator>(TheAdd)) {
+ isNUW = Add->hasNoUnsignedWrap();
+ isNSW = Add->hasNoSignedWrap();
+ }
// From this point on, we know that (X+C <= X) --> (X+C < X) because C != 0,
// so the values can never be equal. Similiarly for all other "or equals"
@@ -6642,6 +6654,10 @@
// (X+2) <u X --> X >u (MAXUINT-2) --> X > 253
// (X+MAXUINT) <u X --> X >u (MAXUINT-MAXUINT) --> X != 0
if (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_ULE) {
+ // If this is an NUW add, then this is always false.
+ if (isNUW)
+ return ReplaceInstUsesWith(ICI, ConstantInt::getFalse(X->getContext()));
+
Value *R = ConstantExpr::getSub(ConstantInt::get(CI->getType(), -1ULL), CI);
return new ICmpInst(ICmpInst::ICMP_UGT, X, R);
}
@@ -6649,8 +6665,12 @@
// (X+1) >u X --> X <u (0-1) --> X != 255
// (X+2) >u X --> X <u (0-2) --> X <u 254
// (X+MAXUINT) >u X --> X <u (0-MAXUINT) --> X <u 1 --> X == 0
- if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_UGE)
+ if (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_UGE) {
+ // If this is an NUW add, then this is always true.
+ if (isNUW)
+ return ReplaceInstUsesWith(ICI, ConstantInt::getTrue(X->getContext()));
return new ICmpInst(ICmpInst::ICMP_ULT, X, ConstantExpr::getNeg(CI));
+ }
unsigned BitWidth = CI->getType()->getPrimitiveSizeInBits();
ConstantInt *SMax = ConstantInt::get(X->getContext(),
@@ -6662,8 +6682,16 @@
// (X+MINSINT) <s X --> X >s (MAXSINT-MINSINT) --> X >s -1
// (X+ -2) <s X --> X >s (MAXSINT- -2) --> X >s 126
// (X+ -1) <s X --> X >s (MAXSINT- -1) --> X != 127
- if (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SLE)
+ if (Pred == ICmpInst::ICMP_SLT || Pred == ICmpInst::ICMP_SLE) {
+ // If this is an NSW add, then we have two cases: if the constant is
+ // positive, then this is always false, if negative, this is always true.
+ if (isNSW) {
+ bool isTrue = CI->getValue().isNegative();
+ return ReplaceInstUsesWith(ICI, ConstantInt::get(ICI.getType(), isTrue));
+ }
+
return new ICmpInst(ICmpInst::ICMP_SGT, X, ConstantExpr::getSub(SMax, CI));
+ }
// (X+ 1) >s X --> X <s (MAXSINT-(1-1)) --> X != 127
// (X+ 2) >s X --> X <s (MAXSINT-(2-1)) --> X <s 126
@@ -6671,6 +6699,14 @@
// (X+MINSINT) >s X --> X <s (MAXSINT-(MINSINT-1)) --> X <s -2
// (X+ -2) >s X --> X <s (MAXSINT-(-2-1)) --> X <s -126
// (X+ -1) >s X --> X <s (MAXSINT-(-1-1)) --> X == -128
+
+ // If this is an NSW add, then we have two cases: if the constant is
+ // positive, then this is always true, if negative, this is always false.
+ if (isNSW) {
+ bool isTrue = !CI->getValue().isNegative();
+ return ReplaceInstUsesWith(ICI, ConstantInt::get(ICI.getType(), isTrue));
+ }
+
assert(Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SGE);
Constant *C = ConstantInt::get(X->getContext(), CI->getValue()-1);
return new ICmpInst(ICmpInst::ICMP_SLT, X, ConstantExpr::getSub(SMax, C));