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<p class="MsoNormal">>In my mind that’s mainly a way to avoid a “Big Bang” effect by staging the changes, allowing a much
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<p class="MsoNormal">>“NFC” changes as possible, then updating individual user of UnsafeAlgebra() to use the right set of
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<p class="MsoNormal">>flags, and killing this API when possible..<span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Yeah, I thought that was your motivation. Maybe I misunderstood your comment about auditing current users of UnsafeAlgebra(). I initially read it as implying
that there might be some oddity in the current implementation that some code is relying on that we’d need to look for. Since the flag is distinct, the fact that it “implies” the others seems like a potential hazard so a problem like that wouldn’t surprise
me.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">If you were just suggesting that the changes to individual callers can be deferred, then I can live with that.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">-Andy<o:p></o:p></span></p>
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<p class="MsoNormal"><a name="_____replyseparator"></a><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> mehdi.amini@apple.com [mailto:mehdi.amini@apple.com]
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<b>Sent:</b> Wednesday, November 16, 2016 4:20 PM<br>
<b>To:</b> Kaylor, Andrew <andrew.kaylor@intel.com><br>
<b>Cc:</b> Ristow, Warren <warren.ristow@sony.com>; llvm-dev@lists.llvm.org; Nicolai Hähnle <nhaehnle@gmail.com><br>
<b>Subject:</b> Re: [llvm-dev] RFC: Consider changing the semantics of 'fast' flag implying all fast-math-flags<o:p></o:p></span></p>
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<p class="MsoNormal">On Nov 16, 2016, at 4:13 PM, Kaylor, Andrew <<a href="mailto:andrew.kaylor@intel.com">andrew.kaylor@intel.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I don’t really like the idea of updating checks of UnsafeAlgebra() to depend on all of the other flags. </span><o:p></o:p></p>
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<p class="MsoNormal">In my mind that’s mainly a way to avoid a “Big Bang” effect by staging the changes, allowing a much “NFC” changes as possible, then updating individual user of UnsafeAlgebra() to use the right set of flags, and killing this API when possible..<o:p></o:p></p>
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<p class="MsoNormal">— <o:p></o:p></p>
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<p class="MsoNormal">Mehdi<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">-Andy</span><o:p></o:p></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span class="apple-converted-space"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> </span></span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">llvm-dev
[<a href="mailto:llvm-dev-bounces@lists.llvm.org">mailto:llvm-dev-bounces@lists.llvm.org</a>]<span class="apple-converted-space"> </span><b>On Behalf Of<span class="apple-converted-space"> </span></b>Mehdi Amini via llvm-dev<br>
<b>Sent:</b><span class="apple-converted-space"> </span>Wednesday, November 16, 2016 8:55 AM<br>
<b>To:</b><span class="apple-converted-space"> </span>Ristow, Warren <<a href="mailto:warren.ristow@sony.com">warren.ristow@sony.com</a>><br>
<b>Cc:</b><span class="apple-converted-space"> </span><a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b><span class="apple-converted-space"> </span>Re: [llvm-dev] RFC: Consider changing the semantics of 'fast' flag implying all fast-math-flags</span><o:p></o:p></p>
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<p class="MsoNormal">On Nov 15, 2016, at 11:59 PM, Ristow, Warren <<a href="mailto:warren.ristow@sony.com"><span style="color:purple">warren.ristow@sony.com</span></a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">Hi,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">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:</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">> I think these are valuable problems to solve, but you should tackle them piece by piece:</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">> 1) the clang part of overriding the individual FMF and emitting the right IR is the first thing to fix.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">> 2) the backend is still using the global UnsafeFPMath and it should be killed.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">I addressed this point (2) for the reciprocal aspect in the patch, but of course that wasn't useful without doing something about (1).</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">Regarding (1), over at<span class="apple-converted-space"> </span><a href="https://reviews.llvm.org/D26708#596610"><span style="color:purple">https://reviews.llvm.org/D26708#596610</span></a>,
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.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">For example, in the LLVM IR (<a href="http://llvm.org/docs/LangRef.html#fast-math-flags"><span style="color:purple">http://llvm.org/docs/LangRef.html#fast-math-flags</span></a>)
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':</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> arcp => "Allow optimizations to use the reciprocal of an argument rather</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> than perform division."</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">'fast' is unusual, in that it describes a fairly generic set of aggressive floating-point optimizations:</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> fast => "Allow algebraically equivalent transformations that may dramatically</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> change results in floating point (e.g. reassociate). This flag implies</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> all the others."</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">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:</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> 'fast' == 'aggr' + 'nnan' + 'ninf' + 'nsz' + 'arcp'</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">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:</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> '-ffast-math -fno-reciprocal-math'</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">to ultimately result in LLVM IR that has the following flags on in appropriate FP ops:</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> 'aggr' + 'nnan' + 'ninf' + ‘nsz'</span><o:p></o:p></p>
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<p class="MsoNormal">Make sense, I missed that we can’t *subtract* from fast at the IR level.<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">Mehdi<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">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
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> fast => "Allow algebraically equivalent transformations that may dramatically</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D"> change results in floating point (e.g. reassociate).</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">(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'
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">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?</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">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.</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">Thanks,</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New";color:#1F497D">-Warren</span><o:p></o:p></p>
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