Re: PATCH: PR target/40470: unabl e to find a register to spill in class ‘SSE_FIRST_REG’

[Jeff Law <law at redhat dot com>]

H.J. Lu wrote:
> On Thu, Jun 18, 2009 at 10:37 AM, Jeff Law<law@redhat.com> wrote:
>   
> > H.J. Lu wrote:
> >     
> > > On Wed, Jun 17, 2009 at 11:46 AM, Vladimir Makarov<vmakarov@redhat.com>
> > > wrote:
> > >
> > >       
> > > > Jeff Law wrote:
> > > >
> > > >         
> > > > > Vladimir Makarov wrote:
> > > > >
> > > > >           
> > > > > > IRA does not generate spill code for insn constraints.  Reload does it.
> > > > > >  To fix it in reload (spill and restore hard registers) is not a
> > > > > > trivial
> > > > > > task and potentially distracting compiler stability for many targets.
> > > > > >  So
> > > > > > I'd rather avoid it even if it could solve other passes problems like
> > > > > > 1st
> > > > > > insn scheduling for x86/x86_64.
> > > > > >
> > > > > > The originally problem occurs in the combiner (it propagates xmm0 usage
> > > > > > right after a call through the insn in question).  So imho I think the
> > > > > > best
> > > > > > way is to fix in the combiner.
> > > > > >
> > > > > >             
> > > > > Is xmm0 the return value for the call?  ie, is it the SET_DEST of the
> > > > > call?
> > > > >
> > > > >
> > > > >
> > > > >           
> > > > I looked at this problem.  It looks like
> > > >
> > > > xmm0=call
> > > > p666= xmm0
> > > > ...
> > > > insn in the question (spill failure because it needs xmm0)
> > > > ...
> > > > = p666, dead p666
> > > >
> > > > The combiner removes p666=xmm0 and uses xmm0 instead of p666 in the last
> > > > insn.
> > > >
> > > >
> > > >         
> > > xmm0 is used for function return:
> > >
> > > (call_insn 19 18 20 4 x.i:31 (set (reg:V2DI 21 xmm0)
> > >        (call (mem:QI (symbol_ref:DI ("load") [flags 0x41]
> > > <function_decl 0x7ffff310c800 load>) [0 S1 A8])
> > >            (const_int 0 [0x0]))) 908 {*call_value_0_rex64}
> > > (expr_list:REG_DEAD (reg:DI 5 di)
> > >        (nil))
> > >    (expr_list:REG_DEP_TRUE (use (reg:DI 5 di))
> > >        (nil)))
> > >
> > > xmm0 is also a fixed register for some SSE4 insns. Since
> > > we don't assign xmm0 early for those insns, how can
> > > combine do anything for them? I think the proper fix
> > > is in RA/reload.
> > >
> > >       
> > No.  The right place to fix this is in combine until such time as we have a
> > new reload pass.     Combine has had code for decades to not extend the
> > lifetime of a hard return value to avoid this exact set of problems.  It's
> > clearly not working and that's what needs to be fixed.
> >
> >     
>
> The return value in xmm0 is used in the following few insns which also
> require the fixed xmm0 for a different operand. How does combine
> know it shouldn't extend the lifetime of a hard return value when it is
> used immediately in the next few insns?
>
>   
The whole point of copying xmm0 to p666 is to free xmm0  so that it can 
be used by other insns.

Combine's removal of p666 = xmm0 is wrong as is changing the last insn 
above to use xmm0 instead of p666.



jeff