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[LVI] Improve select handling to use condition

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This patches teaches LVI to recognize clamp idioms (e.g. select(a > 5, a, 5) will always produce something greater than 5.

The tests end up being somewhat simplistic because trying to exercise the case I actually care about (a loop with a range check on a clamped secondary induction variable) ends up tripping across a couple of other imprecisions in the analysis. Ah, the joys of LVI...

Differential Revision: http://reviews.llvm.org/D16827

llvm-svn: 260627
This commit is contained in:
Philip Reames 2016-02-12 00:09:18 +00:00
parent 498b8e6d32
commit 080898a0f3
2 changed files with 68 additions and 0 deletions
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19
lib/Analysis/LazyValueInfo.cpp
View File

@ -911,6 +911,25 @@ bool LazyValueInfoCache::solveBlockValueSelect(LVILatticeVal &BBLV,
return true;
}
// Can we constrain the facts about the true and false values by using the
// condition itself? This shows up with idioms like e.g. select(a > 5, a, 5).
// TODO: We could potentially refine an overdefined true value above.
if (auto *ICI = dyn_cast<ICmpInst>(SI->getCondition())) {
LVILatticeVal TrueValTaken, FalseValTaken;
if (!getValueFromFromCondition(SI->getTrueValue(), ICI,
TrueValTaken, true))
TrueValTaken.markOverdefined();
if (!getValueFromFromCondition(SI->getFalseValue(), ICI,
FalseValTaken, false))
FalseValTaken.markOverdefined();
TrueVal = intersect(TrueVal, TrueValTaken);
FalseVal = intersect(FalseVal, FalseValTaken);
}
// TODO: handle idioms like min & max where we can use a more precise merge
// when our inputs are constant ranges.
LVILatticeVal Result; // Start Undefined.
Result.mergeIn(TrueVal, DL);
Result.mergeIn(FalseVal, DL);

49
test/Transforms/CorrelatedValuePropagation/select.ll
View File

@ -167,3 +167,52 @@ exit:
; CHECK: ret i1 true
ret i1 true
}
;; Using the condition to clamp the result
;;
define i1 @test5(i32* %p, i1 %unknown) {
; CHECK-LABEL: @test5
%pval = load i32, i32* %p
%cmp1 = icmp slt i32 %pval, 255
br i1 %cmp1, label %next, label %exit
next:
%cond = icmp sgt i32 %pval, 0
%min = select i1 %cond, i32 %pval, i32 5
;; TODO: This pointless branch shouldn't be neccessary
br label %next2
next2:
; CHECK-LABEL: next2:
; CHECK: ret i1 false
%res = icmp eq i32 %min, -1
ret i1 %res
exit:
; CHECK-LABEL: exit:
; CHECK: ret i1 true
ret i1 true
}
define i1 @test6(i32* %p, i1 %unknown) {
; CHECK-LABEL: @test6
%pval = load i32, i32* %p
%cmp1 = icmp ult i32 %pval, 255
br i1 %cmp1, label %next, label %exit
next:
%cond = icmp ne i32 %pval, 254
%sel = select i1 %cond, i32 %pval, i32 1
;; TODO: This pointless branch shouldn't be neccessary
br label %next2
next2:
; CHECK-LABEL: next2:
; CHECK: ret i1 true
%res = icmp slt i32 %sel, 254
ret i1 %res
exit:
; CHECK-LABEL: exit:
; CHECK: ret i1 true
ret i1 true
}