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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.

llvm-svn: 91812
This commit is contained in:
Chris Lattner 2009-12-21 04:04:05 +00:00
parent 900ce231f9
commit ffbd02829c
2 changed files with 60 additions and 11 deletions
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58
llvm/lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -258,7 +258,7 @@ namespace {
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 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
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 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
{
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 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
/// 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 @@ Instruction *InstCombiner::FoldICmpAddOpCst(ICmpInst &ICI,
// (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 @@ Instruction *InstCombiner::FoldICmpAddOpCst(ICmpInst &ICI,
// (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 @@ Instruction *InstCombiner::FoldICmpAddOpCst(ICmpInst &ICI,
// (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 @@ Instruction *InstCombiner::FoldICmpAddOpCst(ICmpInst &ICI,
// (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 @@ Instruction *InstCombiner::FoldICmpAddOpCst(ICmpInst &ICI,
// (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));

13
llvm/test/Transforms/InstCombine/icmp.ll
View File

@ -69,6 +69,7 @@ entry:
%a = add i32 %x, -1
%b = icmp ult i32 %a, %x
ret i1 %b
; CHECK: @test7
; CHECK: %b = icmp ne i32 %x, 0
; CHECK: ret i1 %b
}
@ -78,6 +79,7 @@ entry:
%a = add i32 %x, -1
%b = icmp eq i32 %a, %x
ret i1 %b
; CHECK: @test8
; CHECK: ret i1 false
}
@ -86,6 +88,7 @@ entry:
%a = add i32 %x, -2
%b = icmp ugt i32 %x, %a
ret i1 %b
; CHECK: @test9
; CHECK: icmp ugt i32 %x, 1
; CHECK: ret i1 %b
}
@ -96,6 +99,16 @@ entry:
%b = icmp slt i32 %a, %x
ret i1 %b
; CHECK: @test10
; CHECK: %b = icmp ne i32 %x, -2147483648
; CHECK: ret i1 %b
}
define i1 @test11(i32 %x) {
%a = add nsw i32 %x, 8
%b = icmp slt i32 %x, %a
ret i1 %b
; CHECK: @test11
; CHECK: ret i1 true
}