Strenghten assumption about dynamic type changes (placement new)
Jan Hubicka
hubicka@ucw.cz
Thu Jul 24 09:38:00 GMT 2014
> > *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
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