On 09/24/2014 03:37 PM, Andres Freund wrote:
> >+/*
> >+ * pg_fetch_add_until_u32 - saturated addition to variable
> >+ *
> >+ * Returns the the value of ptr after the arithmetic operation.
> >+ *
> >+ * Full barrier semantics.
> >+ */
> >+STATIC_IF_INLINE uint32
> >+pg_atomic_fetch_add_until_u32(volatile pg_atomic_uint32 *ptr, int32 add_,
> >+                                                        uint32 until)
> >+{
> >+      CHECK_POINTER_ALIGNMENT(ptr, 4);
> >+      return pg_atomic_fetch_add_until_u32_impl(ptr, add_, until);
> >+}
> >+
>
>This was a surprise to me, I don't recall discussion of an "fetch-add-until"
>operation, and hadn't actually ever heard of it before.
It was included from the first version on, and I'd mentioned it a couple
times.


There doesn't seem to be any hardware implementations of that in the patch. Is there any architecture that has an instruction or compiler intrinsic for that?

- Heikki


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