Re: [patch] fold-const.c: Don't transform X & C into (X >> C') & 1in fold_binary.

[Kazu Hirata <kazu at cs dot umass dot edu>]

Hi Richard,

> > Roger agreed that it was a good idea to disable this transformation in
> > fold, but I would like to know what others think as well.  Jeff?
> > Richard?
> 
> Certainly no objections here ;)  As I suspect you know...

Great! :-)

> > IIRC, do_jump contains code to reverse the effect of
> > fold_single_bit_test.  If this patch is approved, I am planning to
> > adjust that code as a follow-up patch.
> 
> ...I only added that do_jump code because the conversion in fold
> was causing a MIPS performance regression.  I would have been much
> happier removing the fold transformation if I thought I could have
> got away with it, but others seemed to believe that it was useful
> in some cases.

I am guessing that the reason why people like (a >> 2) & 1 is that it
can be directly used for a store flag type of operation, but as I have
mentioned in the original post, do_store_flag can take (a & 4) != 0
and emit (a >> 2) & 1.

> If your argument holds true, I'd be glad to see both the fold code
> and the do_jump code removed.  (At least, assuming I've understood
> correctly, and that this:
> 
> > AFAICT, it does not matter whether we do this transformation or not as
> > far as conditional branches are concerned.  Combine canonicalizes a
> > single bit test into zero_extract regardless of whether we have
> >
> >   a & 4
> >
> > or
> >
> >   (a >> 2) & 1
> 
> means that, even without the jump code, we will continue to avoid the
> shift on MIPS if the user writes "(X >> 2) & 1" themselves.)

Oh, I should have been a bit more careful here.  I meant that if we
are setting cc0 or a MODE_CC register, combine canonicalizes a single
bit test into zero_extract (regardless of whether we have a & 4 or
(a >> 2) & 1.  If the MIPS port has no such insn, I suspect with my
patch, (a >> 2) & 1 would stay as is.  If you like, I'd be happy to
teach fold_binary to transform (a >> 2) & 1 != 0 back to a & 4 != 0
for you

Kazu Hirata