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<div class="moz-cite-prefix">On 03/15/2017 12:10 PM, Adam Nemet via
llvm-dev wrote:<br>
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cite="mid:FAB9D19D-F8F0-4364-BE54-5203FFB1295E@apple.com"
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Relevant to this discussion is <a moz-do-not-send="true"
href="http://bugs.llvm.org/show_bug.cgi?id=25721" class="">http://bugs.llvm.org/show_bug.cgi?id=25721</a> (-ffp-contract=fast
does not work with LTO). I am working on adding function
attributes for fp-contract=fast which should fix this.</blockquote>
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Great!<br>
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<div class="">Also now that we have backend optimization remarks,
I am planning to report missed optimization when we can’t fuse
FMAs due “fast” not being on. This will show up in the
opt-viewer. Then the user can opt in either with the
command-line switch or the new function attribute.</div>
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<br>
That seems useful.<br>
<br>
Thanks again,<br>
Hal<br>
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<div class="">Adam<br class="">
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<div class="">On Mar 15, 2017, at 6:27 AM, Renato Golin
via cfe-dev <<a moz-do-not-send="true"
href="mailto:cfe-dev@lists.llvm.org" class="">cfe-dev@lists.llvm.org</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<div class="">
<div class="">Folks,<br class="">
<br class="">
I've been asking around people about the state of FP
contract, which<br class="">
seems to be "on" but it's not really behaving like it,
at least not as<br class="">
I would expect:<br class="">
<br class="">
int foo(float a, float b, float c) { return a*b+c; }<br
class="">
<br class="">
$ clang -target aarch64-linux-gnu -O2 -S fma.c
-ffp-contract=on -o -<br class="">
(...)<br class="">
fmul s0, s0, s1<br class="">
fadd s0, s0, s2<br class="">
(...)<br class="">
<br class="">
$ clang -target aarch64-linux-gnu -O2 -S fma.c
-ffp-contract=fast -o -<br class="">
(...)<br class="">
fmadd s0, s0, s1, s2<br class="">
(...)<br class="">
<br class="">
I'm not sure this works in Fortran either, but
defaulting to "on" when<br class="">
(I believe) the language should allow contraction and
not doing it is<br class="">
not a good default.<br class="">
<br class="">
i haven't worked out what would be necessary to make
it work on a<br class="">
case-by-case basis (what kinds of fusions does C
allow?) to make sure<br class="">
we don't do all or nothing, but if we don't want to
start that<br class="">
conversation now, then I'd recommend we just turn it
all the way to 11<br class="">
(like GCC) and let people turn it off if they really
mean it.<br class="">
<br class="">
The rationale is that:<br class="">
<br class="">
* Contracted operations increase precision (less
rounding steps)<br class="">
* It performs equal or faster on all architectures I
know (true everywhere?)<br class="">
* Users already expect that (certainly, GCC users do)<br
class="">
* Makes us look good on benchmarks :)<br class="">
<br class="">
A recent SPEC2k6 comparison Linaro did for AArch64,
enabling<br class="">
-ffp-contract=fast took the edge of GCC in a number of
cases and in<br class="">
some of them made them comparable in performance. So,
any reasons not<br class="">
to?<br class="">
<br class="">
If we go with it, we need to first finish the job that
Sebastian was<br class="">
dong on the test-suite, then just turn it on by
default. A second<br class="">
stage would be to add tests/benchmarks that explicitly
test FP<br class="">
precision, so that we have some extra guarantee that
we're doing the<br class="">
right thing.<br class="">
<br class="">
Opinions?<br class="">
<br class="">
cheers,<br class="">
--renato<br class="">
_______________________________________________<br
class="">
cfe-dev mailing list<br class="">
<a moz-do-not-send="true"
href="mailto:cfe-dev@lists.llvm.org" class="">cfe-dev@lists.llvm.org</a><br
class="">
<a class="moz-txt-link-freetext" href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev">http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a><br
class="">
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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