Re: gcc -ansi versus SSE4.2 detection

On 06/05/2015 09:27 PM, Tom Lane wrote:
> [ this is a bit roundabout, bear with me ]
>
> I noticed that, contrary to project policy, a //-style comment snuck into
> pg_regress.c a month or two back. I had had the idea that buildfarm
> member pademelon would complain about such comments, given its stone-age
> C compiler, but evidently not. After some experimentation it seems that
> "gcc -ansi" can be used to throw errors for // comments, so I'm planning
> to enable that flag on dromedary. However I found out that adding -ansi
> also caused configure to stop selecting "-msse4.2", which seemed odd,
> since that switch has no bearing on C language conformance. And it fell
> back to the slicing-by-8 CRC implementation too.

Hmm, that's odd. -ansi has no effect on the CRC implementation on my system.

> Investigation showed that that's because -ansi causes the compiler to
> stop defining __SSE4_2__, which configure supposes must get defined if
> we are targeting an SSE 4.2 processor. This seems a bit dumb to me:
> if we have determined that "-msse4.2" works, isn't that sufficient
> evidence that we can use the intrinsics? Or if not, isn't there a less
> fragile way to find out the target?
>
> Also, it seems like the logic in configure.in is broken in any case:
> if we are able to compile the intrinsics, should it not pick the
> runtime-determination option for CRC? It isn't doing that.

It's quite subtle. The point of the __SSE4.2__ test is to determine if
we are targeting a system that has SSE4.2 instructions. If it's defined,
then we can assume that SSE4.2 instructions are always available. For
example, if you pass CFLAGS=-msse4.2, gcc can freely use SSE4.2
instructions when optimizing, and the produced binary will not work on a
system without SSE4.2 support. In that case, __SSE4.2__ is defined, and
we don't need the run-time check or the fallback implementation, because
we can also freely assume that SSE 4.2 support is available.

If __SSE4.2__ is not defined, but the compiler accepts -msse4.2, that
means that the compiler will normally not use SSE4.2 instructions, and
the produced binary must work without them. We will use the -msse4.2
flag when compiling pg_crc32c_sse42.c, and at runtime, we check that SSE
4.2 instructions are available before using it.

- Heikki