RFC/RFA: patch for PR 22156: improve SRA for bit-fields
Alexandre Oliva
aoliva@redhat.com
Fri Apr 20 18:31:00 GMT 2007
On Apr 20, 2007, "Andrew Pinski" <pinskia@gmail.com> wrote:
> On 4/20/07, Alexandre Oliva <aoliva@redhat.com> wrote:
>> I can think of a few possible ways to do it, but I don't understand
>> why this should be necessary. Could anyone give me an idea of why
>> these (D)s should cause any problem?
> Yes because they are using uninitialized variables, anything can
> happen so later passes are causing that weird stuff to happen.
But how is this different from setting a bit-field in a
formerly-uninitialized word? What you're seeing is the same, just
expressed in a slightly different way.
> Bernd's reload patch is just one example where GCC assumes
> uninitialized variables have an undefined value so it can use any
> register
And that would be correct behavior.
> and cause undefined behavior after that.
Now this would be a problem. But if it was, why wasn't it a problem
before? Just because the variable used to be in memory?
--
Alexandre Oliva http://www.lsd.ic.unicamp.br/~oliva/
FSF Latin America Board Member http://www.fsfla.org/
Red Hat Compiler Engineer aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist oliva@{lsd.ic.unicamp.br, gnu.org}
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