nicholas at mxc.ca
Fri Feb 27 00:50:02 CST 2009
David Greene wrote:
> We've upgraded to llvm 2.4 and we're hitting an assert in SCEV:
> llvm/include/llvm/Analysis/ScalarEvolutionExpressions.h:669: RetVal
> RetVal>::visitCouldNotCompute(llvm::SCEVCouldNotCompute*) [with SC =
> llvm::SCEVExpander, RetVal = llvm::Value*]: Assertion `0 && "Invalid use of
> SCEVCouldNotCompute!"' failed.
> This happens in SCEVExpander::visitAddRecExpr where we drop down
> to this code:
> // If this is a chain of recurrences, turn it into a closed form, using the
> // folders, then expandCodeFor the closed form. This allows the folders to
> // simplify the expression without having to build a bunch of special code
> // into this folder.
> SCEVHandle IH = SE.getUnknown(I); // Get I as a "symbolic" SCEV.
> SCEVHandle V = S->evaluateAtIteration(IH, SE);
> //cerr << "Evaluated: " << *this << "\n to: " << *V << "\n";
> return expand(V);
> SCEVAddRecExpr::evaluateAtIteration trips up because a binomial coefficient
> requires 65 bits to calculate. There's a big FIXME there:
> // FIXME: Temporary hack to avoid generating integers that are too wide.
> // Although, it's not completely clear how to determine how much
> // widening is safe; for example, on X86, we can't really widen
> // beyond 64 because we need to be able to do multiplication
> // that's CalculationBits wide, but on X86-64, we can safely widen up to
> // 128 bits.
> if (CalculationBits > 64)
> return new SCEVCouldNotCompute();
> As an experiment I applied revision 62958 which gets rid of the fixme.
> Codegen then asserts that it doesn't know how to do i128 arithmetic (on
> x86-64). I tried applying revision 57709 to no avail.
> I would like to just avoid getting into this FIXME situation altogether.
> Given that this particular test works with llvm 2.3, I suspect that SCEV got
> a little smarter, just smart enough to get itself into trouble on this test.
> Unfortunately, I'm not familiar enough with SCEV and SCEVExpander specifically
> to know how to bypass binomial coefficient expansion. What should happen in
> SCEVExpander::visitAddRecExpr to avoid it?
Take a look at llvm.org/PR2857. Did the patch there make it into LLVM
2.4 or no?
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