[llvm-commits] patch: folding (zext (and x, cst))

[Nick Lewycky]

On 15 June 2011 10:37, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Wed, Jun 15, 2011 at 10:19 AM, Nick Lewycky <nlewycky at google.com>
> wrote:
> >> > How about some calls to CombineTo?
> >>
> >> You need a CombineTo call to handle the chain on the load.
> >
> > Okay, but CombineTo is undocumented. It appears to be CombineTo(Old, New)
> to
> > replace Old with New, so I tried calling it with CombineTo(LN0, ExtLoad)
> but
> > that triggered the assert on the first line of CombineTo. Could you
> explain
> > a little more about what I need to do?
>
> Err, try ReplaceAllUsesOfValueWith.
>

It's almost ReplaceLoadWithPromotedLoad, but I need to use the Trunc value
for more things. I ended up not doing this and going back to CombineTo.

>> > Patch attached, please review! Carefully! ;)
> >>
> >> Your use of ExtendUsesToFormExtLoad (and the !N0.hasOneUse() case in
> >> general) looks suspicious.
> >
> > Cargo-culted from nearby. Look at the way "fold (zext (load x)) -> (zext
> > (truncate (zextload x)))" is handled. I think what's going on is that the
> > (load x) may have other uses, so the ExtendUsesToFormExtLoad function
> checks
> > whether inserting the necessary truncates would be free or the users
> could
> > be extended, etc.
>
> Your current patch ends up duplicating the load; if you really want to
> handle the case where the loaded value has multiple uses, look at what
> the existing uses of ExtendUsesToFormExtLoad do.
>

Okay, I pulled this thread and ended up refactoring. I've only tested the
resulting patch on make check + the llvm nightly test suite, but I'm much
closer to confident in its correctness. Updated patch attached.

Thanks for the review so far!

Nick
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