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Re: Strenghten assumption about dynamic type changes (placement new)
- From: Jan Hubicka <hubicka at ucw dot cz>
- To: Richard Biener <richard dot guenther at gmail dot com>
- Cc: Jan Hubicka <hubicka at ucw dot cz>, Jason Merrill <jason at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 24 Jul 2014 11:23:04 +0200
- Subject: Re: Strenghten assumption about dynamic type changes (placement new)
- Authentication-results: sourceware.org; auth=none
- References: <20140719154408 dot GA8665 at kam dot mff dot cuni dot cz> <CAFiYyc0hDA+9G+wBVkf8Gif8vsDdpvyf4r3X8P=7G-P=QaWwqQ at mail dot gmail dot com> <20140722135434 dot GA12329 at kam dot mff dot cuni dot cz> <CAFiYyc37YkpYFGX2WJ14BUyEQ5k6ggvLrntsemtMEmat+6RgXA at mail dot gmail dot com> <20140722151720 dot GA25121 at kam dot mff dot cuni dot cz> <CAFiYyc0o5mRFF3V_nFBX5+TSDNKT9=BfqoUj6xN=gaNiY+UJuA at mail dot gmail dot com> <20140723144222 dot GA23992 at kam dot mff dot cuni dot cz> <3aa8219d-8bde-4a69-afbd-97bf15396bf2 at email dot android dot com> <20140723162951 dot GA28234 at kam dot mff dot cuni dot cz> <CAFiYyc1FSAEsr7=OJMXVnHSPU9JaPgPEksEHNizknwWs2xvOAw at mail dot gmail dot com>
> > *shortptr = exp
> > <longer dependency chain with shortptr>
> > var = *shortptr
> > *intptr = exp
> > <longer dependency chain with intptr>
> > var = *intptr
>
> Yes (that is, you can't hoist the *intptr = exp store above the var = *shortptr
> load with TBAA only). You can probably still hoist the <longer dependency
> chain with intptr>, it's not clear from your example.
Well, this is placement new version of this, where of course the movement is not desirable.
Obvioulsy the chains can not overlap:
#include <new>
#include <stdio.h>
struct A {short a; short b; A(){a=1;}};
struct B {int a; B(){a=2;}};
struct A a;
struct A *pa = &a;
struct B *pb = reinterpret_cast<struct B *>(&a);
int
main()
{
int sum;
struct A *ppa = pa;
struct B *ppb = pb;
if (!pa || !pb)
return 1;
ppa->~A();
new (ppa) A();
asm ("#asm1":"=m"(ppa->a):"m"(ppa->a));
sum = ppa->a*11;
new (ppb) B();
asm ("#asm2":"=m"(ppb->a):"m"(ppb->a));
sum += ppb->a*11;
printf ("%i\n",sum);
return 0;
}
Of course it makes us i.e. in
t(short *a, short *b, int *c)
{
int i;
for (i=0;i<1000000;i++)
c[i]=a[i]+b[i];
}
generate the fallback case when vectorizing where c is overlapping with a or b, while clang doesn't.
Honza
>
> That said, being able to optimize union accesses with TBAA at all
> is still nice (esp. for GCC). Now, the C frontend still forces alias-set zero
> for this case because of the RTL alias oracle disfunctionality which doesn't
> treat a must-alias as an alias if it can TBAA disambiguate.
>
> Richard.
>
> > Honza