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If the LHS of the comparison is a loop-invariant we also want to move it

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to the RHS. This simple change allows to compute loop iteration count 
for loops with condition similar to the one in the testcase (which seems 
to be quite common).

llvm-svn: 46959
This commit is contained in:
Wojciech Matyjewicz 2008-02-11 18:37:34 +00:00
parent 6f30a0798f
commit adae053b53
2 changed files with 17 additions and 2 deletions
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4
llvm/lib/Analysis/ScalarEvolution.cpp
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@ -1819,8 +1819,8 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
// At this point, we would like to compute how many iterations of the
// loop the predicate will return true for these inputs.
if (isa<SCEVConstant>(LHS) && !isa<SCEVConstant>(RHS)) {
// If there is a constant, force it into the RHS.
if (LHS->isLoopInvariant(L) && !RHS->isLoopInvariant(L)) {
// If there is a loop-invariant, force it into the RHS.
std::swap(LHS, RHS);
Cond = ICmpInst::getSwappedPredicate(Cond);
}

15
llvm/test/Analysis/ScalarEvolution/2008-02-11-ReversedCondition.ll Normal file
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@ -0,0 +1,15 @@
; RUN: llvm-as < %s | opt -scalar-evolution -analyze | not grep Unpredictable
define void @foo(i32 %n) {
entry:
br label %header
header:
%i = phi i32 [ 0, %entry ], [ %i.inc, %next ]
%cond = icmp sgt i32 %n, %i
br i1 %cond, label %next, label %return
next:
%i.inc = add i32 %i, 1
br label %header
return:
ret void
}