[llvm-commits] [llvm] r60374 - in /llvm/trunk: lib/Transforms/Scalar/LoopStrengthReduce.cpp test/CodeGen/X86/2008-12-01-loop-iv-used-outside-loop.ll

[Chris Lattner]

On Dec 1, 2008, at 2:00 PM, Dale Johannesen wrote:
> URL: http://llvm.org/viewvc/llvm-project?rev=60374&view=rev
> Log:
> Consider only references to an IV within the loop when
> figuring out the base of the IV.  This produces better
> code in the example.  (Addresses use (IV) instead of
> (BASE,IV) - a significant improvement on low-register
> machines like x86).

Dale, this is an outstanding patch, thank you for tackling this!  For  
anyone following on the sidelines, this helps fix a performance  
problem when compiling an inner loop of the GCC lexer.

One question:

>
>   // Otherwise, remove all of the CSE's we found from each of the  
> base values.
>   for (unsigned i = 0; i != NumUses; ++i) {
> +    // For this purpose, consider only uses that are inside the loop.
> +    if (!L->contains(Uses[i].Inst->getParent()))
> +      continue;
> +

>
> @@ -1450,6 +1464,12 @@
>
> +      // If this reference is not in the loop and we have a Common  
> base,
> +      // that has been added into the induction variable and must be
> +      // subtracted off here.
> +      if (HaveCommonExprs && !L->contains(User.Inst->getParent()))
> +        RewriteExpr = SE->getMinusSCEV(RewriteExpr, CommonExprs);
> +

These two hunks add a very subtle (and undocumented/commented) issue:  
users outside the loop are missing common exprs from their base.   
Would it be possible to change the first hunk to something like:

     if (!L->contains(Uses[i].Inst->getParent())) {
       Uses[i].Base = SE->getSubExpr(Uses[i].Base, CommonExprs);
       continue;
     }

I think this eliminates the need for the second hunk.  Could this work?

-Chris