[llvm-dev] LLVM v3.9.0 and math built-ins

[Mehdi Amini via llvm-dev]

> On Sep 16, 2016, at 1:30 AM, Martin J. O'Riordan via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> A little while ago I asked a question on CFE-Dev about a change in the behaviour of programs using the ISO C math functions, although that question should have been put to LLVM-Dev.  But I got excellent clarification of the problem anyway.  However, since then I have been trying to adapt our out-of-tree implementation to get the previous behaviour.  The problem is that something like:
>  
> #include <math.h>
>  
> extern double foo(double);
>  
> int useMathName() {
>   if ((exp(1.0) < 2.71) || (exp(1.0) > 2.72))
>     return -1;
>   return 0;
> }
>  
> int useOtherName() {
>   if ((foo(1.0) < 2.71) || (foo(1.0) > 2.72))
>     return -1;
>   return 0;
> }
>  
> With v3.8.0 the compiler elided both the calls to ‘exp’ and the tests, so the function ‘useMathName’ reduced to simply:
>  
> return 0;
>  
> But this was not correct as it ignored the possibility that the math functions could have the side-effect of changing ‘errno’, and this was fixed in v3.9.0, and the calls are no longer elided, though using the “as-if” rule, the tests are still eliminated and ‘useMathName’ becomes:
>  
> (void)exp(1.0);
> (void)exp(1.0);


It is not clear to me: isn’t the spec saying that errno shall be set on underflow and overflow. The constant folding should be able to detect this and fold if it does not happen?

Also, because we don’t really model errno, we don’t constant fold these call anymore :(


> return 0;
>  
> So I changed our implementation so that ‘-fno-math-errno’ is the default for the tool-chain, and the ‘-fmath-errno’ option is not passed on to the CC1 phase.  I expected that this would allow the compiler to elide the calls, but it is not doing so.  I don’t want to use ‘-ffast-math’ as this has lots of FP issues, and I don’t want to make it the default.
>  
> Any idea why the math functions are not elided?  I am using ‘-O3’ and implying ‘-fno-math-errno’.  I have verified that our Toolchain implements ‘IsMathErrnoDefault’ and returns ‘false’, and that the option ‘-fmath-errno’ is not being passed to the CC1 stage.  Since our math function implementation does not in fact change ‘errno’, it is very desirable that this elision occurs in an FP safe way.

You can check that clang when emitting the IR is adding the attribute "readnone” on the declaration of exp(). If it does not then it assumes errno.

As a starting point, have you tried to pass -fno-math-errno?

— 
Mehdi

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