Re: Reduce timing overhead of EXPLAIN ANALYZE using rdtsc? - Mailing list pgsql-hackers

On Tue, Feb 24, 2026 at 5:28 AM Andres Freund <andres@anarazel.de> wrote:
> On 2026-02-23 16:24:57 +0100, David Geier wrote:
> > The code wasn't compiling properly on Windows because __x86_64__ is not
> > defined in Visual C++. I've changed the code to use
> >
> >   #if defined(__x86_64__) || defined(_M_X64)
>
> Independently of this patchset I wonder if it'd be worth introducing a
> PG_ARCH_X64 or such, to avoid this kind of thing.

+1

I've already borrowed USE_SSE2 for this meaning in commit b9278871f,
but that's conflating two different things and I'd actually prefer the
above, plus one that includes 32-bit as well.

+static bool
+is_rdtscp_available()
+{
+ uint32 r[4] = {0, 0, 0, 0};
+
+#if defined(HAVE__GET_CPUID)
+ if (!__get_cpuid(0x80000001, &r[0], &r[1], &r[2], &r[3]))
+ return false;
+#elif defined(HAVE__CPUID)
+ __cpuid(r, 0x80000001);
+#else
+#error cpuid instruction not available
+#endif
+
+ return (r[3] & (1 << 27)) != 0;
+}

I'm hoping to centralize CPU-specific checks like the above:

https://www.postgresql.org/message-id/CANWCAZbKQ2im1r4ztcyfqNh_6gaJzyewabExYrkACNvMNyqxog%40mail.gmail.com

is_rdtscp_available() is an easy thing to delegate to my patch, but I
agree it would be easier if that was abstracted a bit more so that a
different leaf can be passed each time. The latter could also be used
to simplify the frequency and hypervisor stuff as well.

--
John Naylor
Amazon Web Services