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[LVI] Exploit nsw/nuw when computing constant ranges 

As the title says. Modelled after similar code in SCEV.

This is useful when analysing induction variables in loops which have been canonicalized by other passes. I wrote the tests as non-loops specifically to avoid the generality introduced in http://reviews.llvm.org/D17174. While that can handle many induction variables without *needing* to exploit nsw, there's no reason not to use it if we've already proven it.

Differential Revision: http://reviews.llvm.org/D17177

llvm-svn: 260705
This commit is contained in:
Philip Reames 2016-02-12 19:05:16 +00:00
parent f6892eda24
commit 2b9100dfbd
2 changed files with 76 additions and 0 deletions
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21
llvm/lib/Analysis/LazyValueInfo.cpp
View File

@ -961,6 +961,27 @@ bool LazyValueInfoCache::solveBlockValueConstantRange(LVILatticeVal &BBLV,
if (isa<BinaryOperator>(BBI)) {
if (ConstantInt *RHS = dyn_cast<ConstantInt>(BBI->getOperand(1))) {
RHSRange = ConstantRange(RHS->getValue());
// Try to use information about wrap flags to refine the range LHS can
// legally have. This is a slightly weird way to implement forward
// propagation over overflowing instructions, but it seems to be the only
// clean one we have. NOTE: Because we may have speculated the
// instruction, we can't constrain other uses of LHS even if they would
// seem to be equivelent control dependent with this op.
if (auto *OBO = dyn_cast<OverflowingBinaryOperator>(BBI)) {
unsigned WrapKind = 0;
if (OBO->hasNoSignedWrap())
WrapKind |= OverflowingBinaryOperator::NoSignedWrap;
if (OBO->hasNoUnsignedWrap())
WrapKind |= OverflowingBinaryOperator::NoUnsignedWrap;
if (WrapKind) {
auto OpCode = static_cast<Instruction::BinaryOps>(BBI->getOpcode());
auto NoWrapCR =
ConstantRange::makeNoWrapRegion(OpCode, RHS->getValue(), WrapKind);
LHSRange = LHSRange.intersectWith(NoWrapCR);
}
}
} else {
BBLV.markOverdefined();
return true;

55
llvm/test/Transforms/CorrelatedValuePropagation/basic.ll
View File

@ -199,3 +199,58 @@ out:
next:
ret void
}
; Can we use nsw in LVI to prove lack of overflow?
define i1 @add_nsw(i32 %s) {
; CHECK-LABEL: @add_nsw(
entry:
%cmp = icmp sgt i32 %s, 0
br i1 %cmp, label %positive, label %out
positive:
%add = add nsw i32 %s, 1
%res = icmp sgt i32 %add, 0
br label %next
next:
; CHECK: next:
; CHECK: ret i1 true
ret i1 %res
out:
ret i1 false
}
define i1 @add_nsw2(i32 %s) {
; CHECK-LABEL: @add_nsw2(
entry:
%cmp = icmp sge i32 %s, 0
br i1 %cmp, label %positive, label %out
positive:
%add = add nsw i32 %s, 1
%res = icmp ne i32 %add, 0
br label %next
next:
; CHECK: next:
; CHECK: ret i1 true
ret i1 %res
out:
ret i1 false
}
define i1 @add_nuw(i32 %s) {
; CHECK-LABEL: @add_nuw(
entry:
%cmp = icmp ult i32 %s, 400
br i1 %cmp, label %positive, label %out
positive:
%add = add nsw i32 %s, 1
%res = icmp ne i32 %add, -100
br label %next
next:
; CHECK: next:
; CHECK: ret i1 true
ret i1 %res
out:
ret i1 false
}