[llvm-dev] RFC: Consider changing the semantics of 'fast' flag implying all fast-math-flags
Ristow, Warren via llvm-dev
llvm-dev at lists.llvm.org
Thu Nov 17 14:24:11 PST 2016
Thanks for all that. I think we’re more in agreement here than it may have appeared initially.
> So let’s just fix it!
Sounds good!
I have some other things on my plate at the moment, so I doubt I’ll get to working on this until after Thanksgiving (I don’t won’t my lack of activity to be interpreted as a loss of interest on my part to get this done).
Before work can be done to fix it, the details of precisely what changes we want to make in the fast-math-flags IR needs to be decided. There has been some discussion in this thread on that point (‘aggr’, ‘reassoc’ + ‘libm’, something else?), but no clear spec. I’d be happy to propose something concrete, and I’d fully expect that it would evolve a bit after feedback. I’m also happy for others to propose specifics. In any case, I won’t work on taking this further until sometime after Thanksgiving.
-Warren
From: mehdi.amini at apple.com [mailto:mehdi.amini at apple.com]
Sent: Thursday, November 17, 2016 2:03 PM
To: Ristow, Warren <warren.ristow at sony.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] RFC: Consider changing the semantics of 'fast' flag implying all fast-math-flags
On Nov 17, 2016, at 1:44 PM, Mehdi Amini via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
On Nov 17, 2016, at 1:24 PM, Ristow, Warren <warren.ristow at sony.com<mailto:warren.ristow at sony.com>> wrote:
On the plus side, I'm glad to see the conclusions of the last couple of posts.
From Mehdi:
Hope this clarify where I see the direction going, and even if you don’t agree with my
reasoning, the conclusion should be satisfactory on your side :)
I'd say that summarizes my thoughts on this well.
And from Nicolai:
Right. I'm not fundamentally opposed to having these flags, ...
I do agree with much of what you both say, but definitely not all of it. The philosophy of not providing what a customer requests and instead guiding them to a better alternative is something I agree with -- we don't just give them a pony. And I agree *strongly* that just because a program gets the answer a user wants with GCC (using fast-math) and we get an answer they view as "wrong", doesn't mean it's a bug of ours and that we need to change to get the same answer as GCC. That's not what our goal of GCC compatibility means to me.
But we do have a switch '-fno-reciprocal-math' that we accept, and even process/implement to some extent. But that implementation has a bug. Fixing that bug so that when a user says '-ffast-math -fno-reciprocal-math', we enable the fast-math transformations but explicitly disable the reciprocal transformations is, in my view, the right thing to do. Simply, that is a bug that we ought to fix -- unless we agree to abandon support of '-fno-reciprocal-math', which I think isn't under consideration at this stage. And FTR, I'd oppose that, not surprisingly. :)
I'm not at all trying to justify the "pony" use-case from this customer, but if we provide '-fno-reciprocal-math', I think we ought to fix bugs we find in our implementation of it. Fixing that bug doesn't guarantee we'll then get the same answers as GCC does on every program when compiled with '-ffast-math -fno-reciprocal-math', but IMO that isn't required for us to describe our behavior as "GCC compatibility" in this respect.
Fast-math is "unusual", in that the user is explicitly opening the door to allowing us to do non-compliant transformations. As compared with GCC, our implementation can have a subset or a superset of these non-compliant transformations, and we can still call that "GCC compatibility". As an analogous "not unusual" feature, both we and GCC do type based alias analysis. It's a perfectly standard-compliant thing to do optimizations based on conclusions from the tbaa. We both support the switch '-f[no-]strict-aliasing' to control this (and we both enable it by default). Referring to this as "GCC compatibility" is perfectly legitimate, in my view. But if a user program has an aliasing bug in it, and our tbaa directs us to aggressively optimize it, whereas GCC's doesn't (and so the user gets the answer they wanted with GCC, but not with us), this does not mean we have a bug, or that saying we're GCC compatible in terms of '-f[no-]strict-aliasing' is a "lie". We can do a superset or subset of the optimizations that GCC does in terms of alias analysis, and we can quite reasonably describe us a GCC compatible in terms of us providing this capability. A user insisting we have a bug in this tbaa situation is analogous to your "pony" request about "float test_div(float a, float b) { return a/b; }". And (unrelated to Clang/LLVM) I've had this sort of objection from users in tbaa situations in the past, where I've had to defend my point that just because GCC didn't optimize it as aggressively as the compiler I was providing, it wasn't a bug in our compiler. So I'm all for not giving everyone a pony.
But irrespective of how silly a test-case it may be to do:
{
float x = a / c;
float y = b / c;
if (y == 1.0f) {
// do some processing for when 'b' and 'c' are equal
} else {
// do other processing
}
use(x, y);
}
I cannot in good conscience tell the customer that it's OK for us to do:
float tmp = 1.0f / c;
float x = a * tmp;
float y = b * tmp;
when they specified '-ffast-math -fno-reciprocal-math'. They can rightfully come back and say "what do you mean by '-fno-reciprocal-math'?" I have to call that a compiler-bug.
I agree with all you wrote above :)
But I’d add that a legitimate fix could be for the clang driver to issue an error (or a warning) saying “-fno-reciprocal-math” isn’t compatible with -ffast-math, disabling -fxxxxxx” (with xxxxx being one or the other ;))
I don’t want to add confusion, I feel I’m doing a bad job here somehow: I’m not saying we *should* do this (rejecting in the driver). So let’s just fix it!
—
Mehdi
-----Original Message-----
From: Nicolai Hähnle [mailto:nhaehnle at gmail.com]
Sent: Thursday, November 17, 2016 12:36 PM
To: Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>>; Ristow, Warren <warren.ristow at sony.com<mailto:warren.ristow at sony.com>>; mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] RFC: Consider changing the semantics of 'fast' flag implying all fast-math-flags
On 17.11.2016 19:54, Kaylor, Andrew wrote:
All that said, I think we (the company I work for, Sony) will have to
implement support for these switches. It comes down to GCC has these
switches (e.g., -fno-reciprocal-math and -fno-associative-math), and they do suppress the transformations for our customers.
They switch to Clang/LLVM, they use the same switches, and it doesn't
"work". So as a practical matter, I think we will support them.
Whether the LLVM community in general feels that that's required, is
another question. Until for your recent comments here, and Nicolai's
comments above, I would have thought the answer was clearly yes. But maybe that's not the case.
I think this is a very good point. You (Sony) are not the only ones
who are concerned with GCC-command line compatibility. It definitely
should hold some weight. Given that this is something we could do
with just a little more effort, I’m not sure mere simplicity is enough
reason not to do it.
Right. I'm not fundamentally opposed to having these flags, as long as we can agree that the *only* reason for having them is slightly better GCC compatibility. The "slightly better" is important, too, because promising real compatibility with any kind of fast math-type setting would simply be a lie.
So (to answer Mehdi's question in a different part of the thread), I'd consider keeping arcp around a wart, but an acceptable one. I'm fine
with: IR 'fast' becomes IR 'reassociation' (or similar; algebraically correct transforms that may change rounding), and reciprocal math becomes "this thing that should logically be enabled by 'reassociation', but instead requires 'arcp' for GCC-'compatibility' reasons".
And to be clear, 'reassociation' should _not_ by itself allow transforms like X * (Y + 1) --> X * Y + X which can change the NaN-ness of the result when Infs are among the arguments. That's what 'reassociation' + 'ninf' is for.
Also, on a slight tangent...
I'd be really curious to know if there is anybody who really needs
arcp without fp-contract=fast or vice versa, or who needs both of
these but not the X*log2(0.5*Y) transform you mentioned, and so
on.[1]
I just wanted to mention that fp-contract relates to things like FMA
and shouldn’t be confused with fast-math.
It's conceptually the same type of thing though, isn't it? At least fp-contract=fast, which means "use FMA even when it changes floating point results (due to different rounding)". This is kind of like the 'fast' flag, which means "do all sorts of transformations even when they change floating point results (due to different rounding)". I don't know whether clang -ffast-math enables fp-contract=fast, but I'd say that in a clean, from-scratch design, fp-contract=fast shouldn't be a separate flag.
Cheers,
Nicolai
-Andy
*From:*Ristow, Warren [mailto:warren.ristow at sony.com]
*Sent:* Thursday, November 17, 2016 12:51 AM
*To:* mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>
*Cc:* Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>>;
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>; Nicolai Hähnle <nhaehnle at gmail.com<mailto:nhaehnle at gmail.com>>
*Subject:* RE: [llvm-dev] RFC: Consider changing the semantics of 'fast'
flag implying all fast-math-flags
Those are all good points. Your reassociation point in the context of
inlining is particularly interesting.
FWIW, we also have a case where a customer wants '-fno-associative-math'
to suppress reassociation under '-ffastmath'. It would take me a
while to find the specifics of the issue, but it was (if my memory is
right) more of a real use-case. (That is to say, the code that was "failing"
due to reassociation didn't have an obvious fix like the reciprocal
situation, here, other than to turn off fast-math.) In fact, the
request to suppress reassociation was the motivation for creating
PR27372 in the first place (which eventually fed into this thread). I
have to say that on the reassociation point, my concern is that to
really suppress that, we will have to suppress so much, that there
will hardly be any point in using -ffast-math.
I'd say your comments here are very similar to what Nicolai said in
another subthread of this discussion:
I'd be really curious to know if there is anybody who really needs
arcp
without fp-contract=fast or vice versa, or who needs both of these
but
not the X*log2(0.5*Y) transform you mentioned, and so on.[1]
...
[1] One case I _can_ think of (and which may have been a reason for
the
proliferation of flags in the first place) is somebody who enables
fast
math, but then doesn't want their results to change when they update
the
compiler and get a new set of optimizations. But IMO that's a use
case
that should be explicitly rejected.
I think those are all really good points, and an argument can be made
that when -ffast-math gives you results you don't want, then you just
have to turn it off. Essentially, the user can't "have his cake and
eat it too".
All that said, I think we (the company I work for, Sony) will have to
implement support for these switches. It comes down to GCC has these
switches (e.g., -fno-reciprocal-math and -fno-associative-math), and
they do suppress the transformations for our customers. They switch
to Clang/LLVM, they use the same switches, and it doesn't "work". So
as a practical matter, I think we will support them. Whether the LLVM
community in general feels that that's required, is another question.
Until for your recent comments here, and Nicolai's comments above, I
would have thought the answer was clearly yes. But maybe that's not
the case.
In summary, irrespective of any (subjective?) assessment of how
legitimate a particular use-case is, do we want switches like:
-ffast-math -fno-reciprocal-math
-ffast-math -fno-associative-math
to work?
For me, the answer is yes, because I have multiple customers that tell
me they really want to leave -ffast-math on, but they want to be able
to disable these sub-categories. I've been approaching this under the
assumption that the answer is yes for the Clang/LLVM community in general.
Thanks,
-Warren
*From:*mehdi.amini at apple.com<mailto:mehdi.amini at apple.com> <mailto:mehdi.amini at apple.com>
[mailto:mehdi.amini at apple.com]
*Sent:* Wednesday, November 16, 2016 10:46 PM
*To:* Ristow, Warren <warren.ristow at sony.com<mailto:warren.ristow at sony.com>
<mailto:warren.ristow at sony.com>>
*Cc:* Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>
<mailto:andrew.kaylor at intel.com>>; llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
<mailto:llvm-dev at lists.llvm.org>; Nicolai Hähnle <nhaehnle at gmail.com<mailto:nhaehnle at gmail.com>
<mailto:nhaehnle at gmail.com>>
*Subject:* Re: [llvm-dev] RFC: Consider changing the semantics of 'fast'
flag implying all fast-math-flags
On Nov 16, 2016, at 10:04 PM, Ristow, Warren <warren.ristow at sony.com<mailto:warren.ristow at sony.com>
<mailto:warren.ristow at sony.com>> wrote:
Can you elaborate what kind of runtime failure is the reciprocal
transformation triggering?
Yes. It was along the lines of:
{
float x = a / c;
float y = b / c;
if (y == 1.0f) {
// do some processing for when 'b' and 'c' are equal
} else {
// do other processing
}
use(x, y);
}
Of course they understood they could easily change this code once
they understood the issue.
But the fact that it "failed" for non-edge-case values of 'c', they
were worried. As an example of the non-edge-case aspect, when 'c'
is 41.0f (and so of course 'b' is 41.0f), intuitively they felt that
this “would work precisely”, even with fast-math. Once they
understood more, they agreed this was reasonable with fast-math, but
they had the underlying concern that if they encountered one case
where 'num' and 'den' were equal (and non-edge-case), yet 'num /
den' wasn't precisely 1.0f, then even if they fixed this situation
where they encountered it, it might be lurking elsewhere in their
code, and so they wanted to disable that transformation.
Thanks for elaborating.
I’d be reluctant to call this situation a real use-case though.
Is the the distinction on reciprocal really make sense here? This user
can have the same “surprising" anywhere in their code-base with
reassociation as well:
void foo (float a, float b) {
float x = a - b;
if (x == 0)
… // only if a == b
}
That would sound totally reasonable, unless foo is inlined and
reassociation would lead to a non-zero value for x even when a and b
passed in to foo "if it wasn’t inlined" would be identical!
(Reminds me somehow of a client that was bitten by nnan: their
assumption was that as long as they didn’t introduce NaN in the
program everything was fine. However with fast-math some
transformations were introducing NaN where there wasn’t before and
propagating to other computation that were transformed under the
assumption that no NaN would show up, it also turns out that making
the code safe against NaN and efficient at the same time is hard,
especially when the code itself it compiled with fast-math).
—
Mehdi
*From:* mehdi.amini at apple.com<mailto:mehdi.amini at apple.com> <mailto:mehdi.amini at apple.com>
[mailto:mehdi.amini at apple.com]
*Sent:* Wednesday, November 16, 2016 7:11 PM
*To:* Ristow, Warren <warren.ristow at sony.com<mailto:warren.ristow at sony.com>
<mailto:warren.ristow at sony.com>>
*Cc:* Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>
<mailto:andrew.kaylor at intel.com>>; llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
<mailto:llvm-dev at lists.llvm.org>; Nicolai Hähnle <nhaehnle at gmail.com<mailto:nhaehnle at gmail.com>
<mailto:nhaehnle at gmail.com>>
*Subject:* Re: [llvm-dev] RFC: Consider changing the semantics of
'fast' flag implying all fast-math-flags
On Nov 16, 2016, at 6:22 PM, Ristow, Warren
<warren.ristow at sony.com<mailto:warren.ristow at sony.com> <mailto:warren.ristow at sony.com>> wrote:
... except that Warren’s proposal that started this
discussion seems to imply that he
has a use case that requires reciprocals to be turned off separately.
Just to close this loose end, yes I have a use case.
Specifically, we have a customer that turns on '‑ffast‑math',
but was getting a runtime failure due to the
reciprocal-transformation being done.
Can you elaborate what kind of runtime failure is the reciprocal
transformation triggering?
—
Mehdi
They don't want turn off fast‑math because they like the
performance improvement, and can live with the imprecision in
most cases. So they wanted to suppress just the
reciprocal-transformation. I intended to tell them the solution
was simple: use '‑ffast‑math ‑fno‑reciprocal‑math'. But on
trying it myself, I ran into the issue here.
Thanks,
-Warren
*From:* Kaylor, Andrew [mailto:andrew.kaylor at intel.com]
*Sent:* Wednesday, November 16, 2016 4:13 PM
*To:* Mehdi Amini <mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>
<mailto:mehdi.amini at apple.com>>; Ristow, Warren
<warren.ristow at sony.com<mailto:warren.ristow at sony.com>
<mailto:warren.ristow at sony.com>>; llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
<mailto:llvm-dev at lists.llvm.org>; Nicolai Hähnle
<nhaehnle at gmail.com<mailto:nhaehnle at gmail.com> <mailto:nhaehnle at gmail.com>>
*Subject:* RE: [llvm-dev] RFC: Consider changing the semantics
of 'fast' flag implying all fast-math-flags
I don’t really like the idea of updating checks of
UnsafeAlgebra() to depend on all of the other flags. It seems
like it would be preferable to look at each optimization and
figure out which flags it actually requires. I suspect that in
many cases the “new” flag (i.e. allowing reassociation, etc.)
will be what is actually needed anyway.
I would be inclined to agree with Niolai’s suggestion of
combining all the flags related to value safety, except that
Warren’s proposal that started this discussion seems to imply
that he has a use case that requires reciprocals to be turned
off separately.
-Andy
*From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On
Behalf Of *Mehdi Amini via llvm-dev
*Sent:* Wednesday, November 16, 2016 8:55 AM
*To:* Ristow, Warren <warren.ristow at sony.com<mailto:warren.ristow at sony.com>
<mailto:warren.ristow at sony.com>>
*Cc:* llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org> <mailto:llvm-dev at lists.llvm.org>
*Subject:* Re: [llvm-dev] RFC: Consider changing the semantics
of 'fast' flag implying all fast-math-flags
On Nov 15, 2016, at 11:59 PM, Ristow, Warren
<warren.ristow at sony.com<mailto:warren.ristow at sony.com> <mailto:warren.ristow at sony.com>> wrote:
Hi,
Thanks for the quick feedback. I see your points, but I
have a few questions/comments. I'll start at the end of the
previous post:
...
I think these are valuable problems to solve, but you should tackle them piece by piece:
1) the clang part of overriding the individual FMF and emitting the right IR is the first thing to fix.
2) the backend is still using the global UnsafeFPMath and it should be killed.
I addressed this point (2) for the reciprocal aspect in the
patch, but of course that wasn't useful without doing
something about (1).
Regarding (1), over
at https://reviews.llvm.org/D26708#596610, David made the
same point that it should be done in Clang. I can
understand that, but I wonder whether having the concept of
the 'fast' flag in the IR that implies all the other FMF
makes sense? I'm not seeing a good reason for it, but since
this is very new to me, I can easily imagine I'm missing the
big picture.
For example, in the LLVM IR
(http://llvm.org/docs/LangRef.html#fast-math-flags) the
fast-math flags 'nnan', 'ninf', 'nsz', 'arcp' and 'fast’ are
defined. Except for 'fast', each of these has a fairly
specific definition of what they mean. For example, for 'arcp':
arcp => "Allow optimizations to use the reciprocal of an
argument rather
than perform division."
'fast' is unusual, in that it describes a fairly generic set
of aggressive floating-point optimizations:
fast => "Allow algebraically equivalent transformations
that may dramatically
change results in floating point (e.g.
reassociate). This flag implies
all the others."
Very loosely, 'fast' means "all the aggressive
FP-transformations that are not controlled by one of the
other 4, plus it implies all the other 4". If for
terminology, we call those additional aggressive
optimizations 'aggr', then we have:
'fast' == 'aggr' + 'nnan' + 'ninf' + 'nsz' + 'arcp'
So as I see it, if we want to disable only one of the other
ones (like 'arcp', in my case), there isn't any way to
express that with these IR flags defined this way. In
short, we cannot turn on all the flags besides 'arcp'. To
do that, what we want is that somehow for the Clang switches:
'-ffast-math -fno-reciprocal-math'
to ultimately result in LLVM IR that has the following flags
on in appropriate FP ops:
'aggr' + 'nnan' + 'ninf' + ‘nsz'
Make sense, I missed that we can’t *subtract* from fast at the
IR level.
I wouldn’t be opposed to have something along the line of
“aggr”, but there is a tradeoff: some transformation may be
harder to guard with this model.
Maybe that could be a starting point: changing the
“UnsafeAlgebra” bit in the FMF to be “aggr” you mention and
replace all the query to FastMathFlags::UnsafeAlgebra() to
return true if all the bits are set in the Flags. This alone
should be nothing more than a mechanical change I believe.
The important part is then auditing all the users of
UnsafeAlgebra() in the middle end and check if they can be
“downgraded” to aggr safely: i.e. if they don’t need aggr *and*
another flag.
—
Mehdi
But I don't see a way to express 'aggr' in the IR. We could
do this, if we change the definition of the IR 'fast' flag
to remove that sentence about implying all the others:
fast => "Allow algebraically equivalent transformations
that may dramatically
change results in floating point (e.g. reassociate).
(If we do something like that, we may want to change the
name from 'fast' to something else (like 'aggr'), to avoid
tying it too closely to the concept of the '-ffast-math'
switch.)
As an aside, I don't know if the "reassociate" example is
the only other transformation that's allowed by 'fast' (I
presume it isn't), but I think reassociation would be better
expressed by a separate flag, which could then be controlled
independently via '-f[no]-associative-math' switch. Not
having that flag exist separately in the FMF is the origin
of PR27372. But creating that flag and using it in the
appropriate places would still run into these problems of
'fast' implying all the others, which would make it
impossible to disable reassociation while leaving all the
other FMF transformations enabled.
To ask a concrete question using the current definition of
'fast' (which includes enabling reassociation, as the LLVM
IR documentation of FMF says), how can we express in the IR
that reciprocal-transformations are not allowed, but
reassociation is allowed?
So the bottom line is that I do see there are issues in
Clang that are relevant. But as long as 'fast' means
"'aggr' plus all the other FMF transformations", I don't see
how we can effectively disable a subset of those other FMF
transformations (while leaving 'aggr' transformations, such
as reassociation, enabled). With that in mind, my patch
took one step in having 'fast' no longer imply all the others.
Thanks,
-Warren
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