On Thu, Sep 10, 2026 at 04:58:45PM +0200, Josef Melcr wrote:
> this patch changes IFN_ASSUME to be pure instead of const. The const
> attribute means (not only) that the function cannot read global memory.
> While this is true for the internal call itself, it is only a dispatcher
> for the outlined assume body, which may read any memory, e.g. below:
>
> ```
> struct S
> {
> int x, y;
> };
>
> void bar (int, int);
>
> bool
> foo (struct S *s)
> {
> [[assume (bar (s->x, s->y))]];
This is not valid, assume expression is contextually converted to bool,
you can't convert void to bool. So just make it bool bar (int, int);
in the commit log.
> return s->x;
> }
> ```
>
> Now, the read does not actually happen in reality, as the assume's body
> does not make into the final binary, but the call being marked
> incorrectly as const makes our analysis less accurate.
The outlined assume body may also store memory, but as it is never actually
called (all the side-effects are as-if), I think your patch is ok.
It will make some GIMPLE DSE etc. decisions worse (e.g. any escaped memory
won't be possible to be DSEd if there is some IFN_ASSUME after it), but
hopefully when the IFN_ASSUME call is optimized out we'll be able to still
DSE it afterwards.
We could also analyze the corresponding outlined function and if it doesn't
read any memory, make a particular IFN_ASSUME call const or novops.
> PR ipa/127104
>
> gcc/ChangeLog:
>
> * internal-fn.def (ASSUME): Downgrade from const to pure.
>
> Signed-off-by: Josef Melcr <[email protected]>
Jakub
