http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43721
Andrew Stubbs <ams at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |ams at gcc dot gnu.org
--- Comment #6 from Andrew Stubbs <ams at gcc dot gnu.org> 2010-10-20 16:53:13
UTC ---
Here's the problem: expand_divmod always prefers the straight "div" library
call over the "divmod" library call, no exceptions.
Yes, "divmodsi4" in a .md is indeed preferred over "divsi4", so the
optimization works fine on targets that have those, but ARM doesn't, so those
rules are irrelevant.
ARM does not provide a separate library function for "mod", so expand_divmod
does use "divmod" for mod operations, but never for div operations.
The reason for the apparent opposite rules appears to be that the divmodsi4
rule can be coded to auto-detect the most optimal kind of underlying operation
to use, whereas the library call is fixed, once chosen.
I see two possible ways to fix this:
1. Teach CSE (or somewhere) that div and divmod library calls have some
overlap.
2. Always prefer divmod, and find some other way to convert it to div, where
appropriate.
I don't see any way to generate the RTL with the right call, so I'm pretty sure
it does have to be fixed up after the fact.
Or, I could be way off. :)
