[llvm-commits] [llvm] r142731 - in /llvm/trunk: lib/Analysis/ScalarEvolution.cpp test/Analysis/ScalarEvolution/load.ll

[Duncan Sands]

Hi Nick,

>     maybe you could use instsimplify here?  It does constant folding and more
>     besides.  It can break LCSSA form, so y so,
>     check out the loop transform users of instsimplify to seou may need to check
>     for that (ife how).
>
>
> As far as I can see, LCSSA isn't documented and I don't understand it well
> enough to know whether I'm preserving it or breaking it etc.

you don't need to know, there is a helper that tells you:
replacementPreservesLCSSAForm

  I know it involves
> creating 1-operand PHI nodes in strategic places and that's about it.
>
> Are there cases where InstructionSimplify will return a Constant that the
> constant folder won't?

Yes, because the constant folder only folds constants, while InstructionSimplify
can fold instructions to constants (eg: X - X -> 0) as well as to other pre
existing instructions (eg: (X + Y) - X -> Y).

  This code is entirely based off of a mapping to
> Constant*, so instsimplify won't help without more refactoring.

OK.

Ciao, Duncan.

>
> That refactoring would be scary because while we can represent the arbitrary
> values, we'll find ourselves effectively unrolling the loop as we build up a
> larger and larger SCEV expression. I'd like to leave unrolling to the unrolling
> pass.
>
> Nick
>
>      > @@ -4993,7 +5109,10 @@
>      >             if (const CmpInst *CI = dyn_cast<CmpInst>(I))
>      >               C = ConstantFoldCompareInstOperands(CI->getPredicate(),
>      >                                                   Operands[0],
>     Operands[1], TD);
>      > -          else
>      > +          else if (const LoadInst *LI = dyn_cast<LoadInst>(I)) {
>      > +            if (!LI->isVolatile())
>      > +              C = ConstantFoldLoadFromConstPtr(Operands[0], TD);
>      > +          } else
>      >               C = ConstantFoldInstOperands(I->getOpcode(), I->getType(),
>      >                                            Operands, TD);
>      >             if (!C) return V;
>
>     Likewise.
>
>     Ciao, Duncan.
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