[cfe-dev] [llvm-dev] Should isnan be optimized out in fast-math mode?

Chris Tetreault via cfe-dev cfe-dev at lists.llvm.org
Mon Sep 13 09:45:57 PDT 2021

Honestly, we can do this until the end of time. I think we both agree, that for either scheme, there exists workarounds. The question is which workarounds are more palatable, which is a matter of opinion. I think we’ve come to an impasse, so let me just state that my opinion on the question “Should isnan be optimized out in fast-math mode?” is “Yes”, which is what you asked to get in your original message. I think that the implementation of fast-math will be cleaner if we don’t special case a bunch of random constructs in order to do what the user meant instead of what they said. I think fast-math is a notorious footgun, and any attempts to mitigate this will only reduce the effectiveness of the tool, while not really improving the user experience.

As a user, if I read that:

if (isnan(x)) {

… is guaranteed to work, and I read that fast-math enables the compiler to reason about constructs like `x + 0` being equal to `x`, then I’m going to be very confused when:

if (isnan(x + 0)) {

… does not also work. I’m going to open a bug and complain, and the slide down the slippery slope will continue. You and I understand the difference, and the technical reason why `isnan(x)` is supported but `isnan(x + 0)` isn’t, but Joe Coder just trying to figure out why he’s got NaN in his matrices despite his careful NaN handling code. Joe is not a compiler expert, and on the face of it, it seems like a silly limitation. This will never end until fast-math is gutted.

   Chris Tetreault

From: Serge Pavlov <sepavloff at gmail.com>
Sent: Friday, September 10, 2021 9:21 PM
To: Chris Tetreault <ctetreau at quicinc.com>
Cc: Richard Smith <richard at metafoo.co.uk>; llvm-dev at lists.llvm.org; cfe-dev at lists.llvm.org
Subject: Re: [llvm-dev] [cfe-dev] Should isnan be optimized out in fast-math mode?

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On Sat, Sep 11, 2021 at 2:39 AM Chris Tetreault <ctetreau at quicinc.com<mailto:ctetreau at quicinc.com>> wrote:
The problem is that math code is often templated, so `template <typename T>  MyMatrixT<T> safeMul(const MyMatrixT<T> & lhs …` is going to be in a header.

No problem, the user can write:
#ifdef __FAST_MATH__
#undef isnan
#define isnan(x) false
and put it somewhere in the headers.

On Sat, Sep 11, 2021 at 2:39 AM Chris Tetreault <ctetreau at quicinc.com<mailto:ctetreau at quicinc.com>> wrote:
Regardless, my position isn’t “there is no NaN”. My position is “you cannot count on operations on NaN working”.

Exactly. Attempts to express the condition of -ffast-math as restrictions on types are not fruitful. I think it is the reason why GCC documentation does not use simple and clear "there is no NaN" but prefers more complicated wording about arithmetic.

On Sat, Sep 11, 2021 at 2:39 AM Chris Tetreault <ctetreau at quicinc.com<mailto:ctetreau at quicinc.com>> wrote:
I think working around these sorts of issues is something that C and C++ developers are used to. These sorts of “inconsistent” between compilers behaviors is something we accept because we know it comes with improved performance. In this case, the fix is easy, so I don’t think this corner case is worth supporting. Especially when the fix is also just one line:
#define myIsNan(x) (reinterpret_cast<uint32_t>(x) == THE_BIT_PATTERN_OF_MY_SENTINEL_NAN)

It won't work in this way. If `x == 5.0`, then `reinterpret_cast<uint32_t>(x) == 5`. What you need there is a bitcast. Standard C does not have such. To emulate it a reinterpret_cast of memory can be used: `*reinterpret_cast<int *>(&x)`. Another way is to use a union. Both these solutions require operations with memory, which is not good for performance, especially on GPU and ML cores. Of course, a smart compiler can eliminate memory operation, but it does not have to do it always, as it is only optimization. Moving a value between float and integer pipelines also may incur a performance penalty. At the same time this check often may be done with a single instruction.

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