Re: [PATCH] Fix PR46728 (move pow/powi folds to tree phases)

["William J. Schmidt" <wschmidt at linux dot vnet dot ibm dot com>]

Richi, thank you for the detailed review!

I'll plan to move the power-series expansion into the existing IL walk
during pass_cse_sincos.  As part of this, I'll move
tree_expand_builtin_powi and its subfunctions from builtins.c into
tree-ssa-math-opts.c.  I'll submit this as a separate patch.

I will also stop attempting to make code generation match completely at
-O0.  If there are tests in the test suite that fail only at -O0 due to
these changes, I'll modify the tests to require -O1 or higher.

I understand that you'd prefer that I leave the existing
canonicalization folds in place, and only un-canonicalize them during my
new pass (now, during cse_sincos).  Actually, that was my first approach
to this issue.  The problem that I ran into is that the various folds
are not performed just by the front end, but can pop up later, after my
pass is done.  In particular, pass_fold_builtins will undo my changes,
turning expressions involving roots back into expressions involving
pow/powi.  It wasn't clear to me whether the folds could kick in
elsewhere as well, so I took the approach of shutting them down.  I see
now that this does lose some optimizations such as
pow(sqrt(cbrx(x)),6.0), as you pointed out.

Should I attempt to leave the folds in place, and screen out the
particular cases that are causing trouble in pass_fold_builtins?  Or is
it too fragile to try to catch all places where folds occur?  If there's
a flag that indicates parsing is complete, I suppose I could disable
individual folds once we're into the optimizer.  I'd appreciate your
guidance.

Thanks,
Bill