> On 28 Aug 2025, at 16:38, Jakub Jelinek <ja...@redhat.com> wrote:
>
> On Thu, Aug 28, 2025 at 04:50:15PM +0200, Jakub Jelinek wrote:
>> On Thu, Aug 28, 2025 at 04:24:46PM +0200, Richard Biener wrote:
>>> That said, if we ever want to improve on this (and solve the redundant
>>> store elimination issue), we need an actual GIMPLE/RTL statement
>>> doing what std::start_lifetime_as <T> does (but not generate code).
>>>
>>> Meaning, I'd vote for a p = __builtin_start_lifetime_as (p, (T *)0);
>>
>> I thought earlier about just providing size.
>> null pointer with the new type would provide even more information,
>> iff we only support constant size types.
>> And we can then handle it in GIMPLE as if it is (or could be)
>> void *__builtin_start_lifetime_as (void *p)
>> {
>> union { old_type old; T new; } u;
>> __builtin_memcpy (u.old, p, sizeof (T));
>> __builtin_memcpy (p, u.new, sizeof (T));
>> return p;
>> }
>> (but of course for the const case or even for data races we actually
>> don't want either of those copies).
>> Now, I bet any kind of user function could do something like that if
>> it isn't inlined/exposed to the compiler, so not sure if we need to
>> handle it in too many places through GIMPLE, perhaps say that it
>> does or could access directly just the [p, p + sz) bytes (and indirectly
>> anything reachable).
>> But how to lower it into RTL is unclear, having it as CALL_EXPR will
>> mean it has to follow the call ABIs and all extra overhead.
>> UNSPEC_VOLATILE might be nice, but I guess we don't have a precedent
>> of backend independent constants for those.
>> Or expand it (just for RTL?) as inline asm,
>> asm volatile ("" : "=g" (ptr), "m" (*(T *)ptr) : "0" (ptr), "m" (*(const T
>> *)ptr));
>> ?
>
> Or implement it using inline asm in the headers at least for now, until
> we find a reason to add a builtin for that.
> I believe for inline asm we really don't care about the alias set of "m"
> arguments, the accesses are or can be done in any way the asm likes to.
Is the std::observable() ( for which I have an implementation to be posted that
is implemented with a builtin lowered to nothing expand() ) possibly
useful also in this context?
Iain
>
> Jakub
