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Re: [arm][patch] fix arm_neon_ok check on !arm_arch7

From: Ramana Radhakrishnan <ramana dot radhakrishnan at arm dot com>
To: Andrew Stubbs <andrew dot stubbs at gmail dot com>, Kyrylo Tkachov <Kyrylo dot Tkachov at arm dot com>
Cc: James Greenhalgh <James dot Greenhalgh at arm dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
Date: Mon, 12 Jan 2015 13:50:24 +0000
Subject: Re: [arm][patch] fix arm_neon_ok check on !arm_arch7
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Sorry about the slow response- have been on holiday and still catchingup on email.


On 12/01/15 13:16, Andrew Stubbs wrote:

Ping.

On 23/12/14 16:46, Andrew Stubbs wrote:

On 03/12/14 15:03, Andrew Stubbs wrote:

The tools have always allowed us to drop down the arch to
march=armv5te along with using -mfpu=neon. We are now changing command
line behaviour, so an inform in terms of diagnostics to the user would
be useful as it states that we don't really have mfpu=neon generating
neon code any more because of this particular case. If we are to do
this then the original patch is probably not enough as it then doesn't
handle the case of TARGET_VFP3 / TARGET_VFP5 / TARGET_NEON_FP16 /
TARGET_FP16 / TARGET_FPU_ARMV8 etc. etc. etc.


I'll take a look at those shortly.


Or, not so shortly.


Sigh.

It seems that, on ARM, the arch/CPU setting is basically orthogonal to
the FPU setting, and the compiler doesn't even try to match the one to
the other. The assembler does the same. In fact, the testcases that
James refers to, that have hard-coded -march options, really do emit
armv4 code with Neon, say, although most probably don't have
vectorizable code. They only work because they're most likely executed
on Neon hardware.

Yes - though I'm surprised as I run an armv5te soft float only test runonce a while on my Sheevaplug and don't see these issues. Maybe others do.


This means that there's no obvious patch to fix the issue, in the
compiler. It's easy to reject Neon for pre-v7 CPUs, but that has
consequences, as we've seen. We'd have to have a table of fall-back FPUs
or something, and that doesn't seem straight-forward (and anyway, I'm
not sure what values to enter into that table).

So, I've attacked the problem from the other end, and updated the
compiler check.

OK to commit?

In principle ok, but I'd like a comment in there explaining why we'vedone this. Can you also post under what configurations these have beentested ?



Ramana


Andrew

Follow-Ups:
- Re: [arm][patch] fix arm_neon_ok check on !arm_arch7
  - From: Andrew Stubbs

References:
- Re: [arm][patch] fix arm_neon_ok check on !arm_arch7
  - From: Andrew Stubbs

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