https://bugs.llvm.org/show_bug.cgi?id=36633
Bug ID: 36633
Summary: Assert in SCEV: LHS is not available at Loop Entry
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: nemanja.i....@gmail.com
CC: llvm-bugs@lists.llvm.org
Created attachment 20015
--> https://bugs.llvm.org/attachment.cgi?id=20015&action=edit
Failing test case reduced by bugpoint
Compiling the attached test case with either:
$ clang -O2 test.ll -S --target=x86_64-unknown-linux-gnu
or
$ clang -O2 test.ll -S --target=powerpc64le-unknown-linux-gnu
(and presumably other targets that I haven't checked)
Causes the following assert:
ScalarEvolution.cpp:9077: bool
llvm::ScalarEvolution::isLoopEntryGuardedByCond(const llvm::Loop*,
llvm::CmpInst::Predicate, const llvm::SCEV*, const llvm::SCEV*): Assertion
`isAv
ailableAtLoopEntry(LHS, L) && "LHS is not available at Loop Entry"' failed.
The following patch fixes it and seems like a rather logical fix to someone
like me with no real understanding of how SCEV works. Mind you I only say it
seems to make sense because of the names of the function and the SCEV type
(i.e. if the SCEV is of type "unknown" it should imply that
isKnownOnEveryIteration() should return false).
However, the patch breaks lit test cases which leads me to believe that it is
overly conservative:
Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp (revision 326779)
+++ lib/Analysis/ScalarEvolution.cpp (working copy)
@@ -8771,6 +8771,9 @@ bool ScalarEvolution::isKnownOnEveryIteration(ICmp
const SCEVAddRecExpr *LHS,
const SCEV *RHS) {
const Loop *L = LHS->getLoop();
+ if (static_cast<SCEVTypes>(LHS->getStart()->getSCEVType()) == scUnknown ||
+ static_cast<SCEVTypes>(RHS->getSCEVType()) == scUnknown)
+ return false;
return isLoopEntryGuardedByCond(L, Pred, LHS->getStart(), RHS) &&
isLoopBackedgeGuardedByCond(L, Pred, LHS->getPostIncExpr(*this),
RHS);
}
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