[llvm-dev] fp-contract=fast and pragmas

Keane, Erich via llvm-dev
Wed, 30 Jun 2021 17:36:27 -0000

I personally am not/have nothing to do with it, but I don’t doubt Intel is doing
something like that.

The difference is that the flag alters the ‘default’ semantics.  Here, the
#pragma ALSO attempts to alter the ‘default’ semantics explicitly, but is
ignored.

From: Steve (Numerics) Canon <scanon at apple.com>
Sent: Wednesday, June 30, 2021 10:34 AM
To: Keane, Erich <erich.keane at intel.com>
Cc: Kaylor, Andrew <andrew.kaylor at intel.com>; John McCall <rjmccall
at apple.com>; llvm-dev at lists.llvm.org; cfe-dev at lists.llvm.org; Yaxun
Liu <yaxun.liu at amd.com>; Blower, Melanie I <melanie.blower at
intel.com>; Sanjay Patel <spatel at rotateright.com>; Renato Golin
<rengolin at gmail.com>; Hal Finkel <hal.finkel.llvm at gmail.com>;
guille at berkeley.edu; ueno.masakazu at jp.fujitsu.com; Matthew.Arsenault at
amd.com
Subject: Re: fp-contract=fast and pragmas

Aren’t you all (Intel) the ones promoting -f(no-)protect-parens, which is a
command line flag that overrides source code semantics in _exactly_ the same
manner?


– Steve



On Jun 30, 2021, at 1:30 PM, Keane, Erich <erich.keane at
intel.com<mailto:erich.keane at intel.com>> wrote:

It seems awkward to me that we have a command-line switch that overrides source
code to this extent.  Typically our command-line arguments cause us to change
‘defaults’, rarely do they cause us to ignore the source code.  IMO, there is a
bit of a natural ‘order’ to where how an option like this should be specified,
that is, code overrides command line overrides default.

At bare minimum, having a pragma like this that is supported, but just ignored
in this case needs to have some level of diagnostic.  Silently ignoring a
developer’s preference is the worst thing we can do.

From: Steve (Numerics) Canon <scanon at apple.com<mailto:scanon at
apple.com>>
Sent: Wednesday, June 30, 2021 10:20 AM
To: Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at
intel.com>>
Cc: John McCall <rjmccall at apple.com<mailto:rjmccall at
apple.com>>; llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>; cfe-dev at lists.llvm.org<mailto:cfe-dev at
lists.llvm.org>; Yaxun Liu <yaxun.liu at amd.com<mailto:yaxun.liu at
amd.com>>; Keane, Erich <erich.keane at intel.com<mailto:erich.keane
at intel.com>>; Blower, Melanie I <melanie.blower at
intel.com<mailto:melanie.blower at intel.com>>; Sanjay Patel <spatel
at rotateright.com<mailto:spatel at rotateright.com>>; Renato Golin
<rengolin at gmail.com<mailto:rengolin at gmail.com>>; Hal Finkel
<hal.finkel.llvm at gmail.com<mailto:hal.finkel.llvm at gmail.com>>;
guille at berkeley.edu<mailto:guille at berkeley.edu>; ueno.masakazu at
jp.fujitsu.com<mailto:ueno.masakazu at jp.fujitsu.com>; Matthew.Arsenault
at amd.com<mailto:Matthew.Arsenault at amd.com>
Subject: Re: fp-contract=fast and pragmas

It sounds to me like this test is simply incompatible with fp-contract=fast and
should not be used in that mode.

– Steve



On Jun 30, 2021, at 11:14 AM, Kaylor, Andrew <andrew.kaylor at
intel.com<mailto:andrew.kaylor at intel.com>> wrote:

Hi John,

Let me be clarify that ICC-compatibility isn’t my goal here. We can do that
out-of-tree for Intel compilers based on LLVM.

My motivation is a problem I’m working on with the LLVM test suite. The
Polybench benchmarks in the test are currently attempting to use ‘#pragma STDC
FP_CONTRACT OFF’ to create a value-safe kernel whose results can be compared
against an otherwise identical kernel that is compiled with whatever options the
test suite is configured to use. This strategy fails if the test suite is
configured to compile with ‘-ffp-contract=fast’. That’s the problem I’m trying
to solve by having clang respect the pragma.

See https://reviews.llvm.org/D25346, https://reviews.llvm.org/D102861, and
https://reviews.llvm.org/D104935.

-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20210630/31783088/attachment.html>