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

Steve (Numerics) Canon via llvm-dev
Wed, 30 Jun 2021 17:34:22 -0000

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>
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>
> Sent: Wednesday, June 30, 2021 10:20 AM
> To: Kaylor, Andrew <andrew.kaylor at intel.com>
> Cc: 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>; Keane, Erich
<erich.keane at intel.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
>
> 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/D25346>, https://reviews.llvm.org/D102861
<https://reviews.llvm.org/D102861>, and https://reviews.llvm.org/D104935
<https://reviews.llvm.org/D104935>.

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