Re: GCC 4.5.0 Status Report (2009-05-05)

[Richard Guenther <richard.guenther@gmail.com>]

On Wed, May 6, 2009 at 5:46 PM, Richard Earnshaw <rearnsha@arm.com> wrote:
> On Wed, 2009-05-06 at 17:44 +0200, Michael Matz wrote:
>> Hi,
>>
>> On Wed, 6 May 2009, Ramana Radhakrishnan wrote:
>>
>> > >>> The slush that I requested last week has been lifted.  However, I
>> > >>> have asked for relative calm until the cond-optab branch has been
>> > >>> merged to mainline, which will hopefully occur on Friday, May 8th.
>> > >>
>> > >> As of this morning (UK), native bootstrap on ARM is still broken.
>> > >
>> > > Is this PR 39978?
>> >
>> > I see what's been reported at
>> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40031#c2 on gcc55 which appears
>> > to be the same crash as what Julian's been seeing with
>> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39929#c12.
>>
>> As written by Andrew Pinski already, you'll need a different method of
>> emitting the load of the PIC register.  The ARM backend tries to emit that
>> load as soon as it sees the PIC register being used (which is a slightly
>> odd method as it magically emits instructions at a place unrelated to the
>> instruction that currently are supposed to be emitted).
>>
>> With expand from SSA this happens earlier than before, in fact so early
>> that you don't have any basic block in RTL yet.  All are still in gimple
>> form.  That's why the emission of RTL insns after the function entry is
>> not going to work.  That's only possible if everything is transformed
>> already.
>>
>> The easiest solution would be to just make a note that you need the PIC
>> register and then, when expanding the prologue emit the necessary
>> instructions.  IMO that makes sense as PIC register setup usually is
>> something the prologue does, like all the other register setups necessary.
>>
>
> That won't work because the PIC register on ARM is a pseudo, so
> generating it during prologue generation is too late.  It needs to exist
> before data flow analysis starts on the RTL.

So you can generate it unconditionally and let DCE remove it if it turns
out not necessary?

Richard.