[cfe-dev] [llvm-dev] [LLVMDev] Inconsistent result between GCC and Clang for __builtin_fmodf

[Stephen Canon via cfe-dev]

Reading the implementation, that’s the understatement of the century.  I’ll take a look at fixing it next weekend if no one else gets to it first.

– Steve

> On Sep 14, 2015, at 4:33 PM, David Majnemer via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> There is a rather unsettling comment above APFloat::mod which states "This is not currently correct in all cases."
> 
> I wonder if this is a symptom of that.
> 
> On Mon, Sep 14, 2015 at 12:11 PM, Zonr Chang via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Following simple program gives different answers when compiling with GCC (4.8.4) and ToT Clang:
> 
> $ cat builtin_fmodf_bugpoint.cpp
> #include <cstdio>
> 
> int main(int argc, char** argv) {
>   const float a = 1.0f;
>   const float b = 0.1f;
>   printf("%f mod %f = %f\n", a, b, __builtin_fmodf(a, b));
>   return 0;
> }
> $ g++ -o builtin_fmodf_bugpoint_gcc builtin_fmodf_bugpoint.cpp
> $ ./builtin_fmodf_bugpoint_gcc
> 1.000000 fmodf 0.100000 = 0.100000
> $ clang++ -o builtin_fmodf_bugpoint_clang builtin_fmodf_bugpoint.cpp
> 1.000000 fmodf 0.100000 = 0.000000
> 
> Clang will compile __builtin_fmod() into LLVM's "frem" [1]. Since both operands are constant, it is constant folded by IRBuilder at the time when "frem" was built. The exeuction finally goes to llvm::ConstantFoldBinaryInstruction() which uses APFloat::mod() to compute the result. As you can see from above, it gives us 0.000000 (hex: 0x0) which is different than the answer from std::fmodf() (0.100000, hex: 0x3dcccccb).
> 
> SelectionDAG also uses APFloat::mod() to fold "frem" when operands are constants. (see SelectionDAG::getNode()). However, lli interprets "frem" to a fmod() and therefore generates the same result as GCC's.
> 
> $ lli -force-interpreter builtin_fmodf_bugpoint.ll
> 1.000000 fmodf 0.100000 = 0.100000
> 
> (Please refer to attachment for the .ll)
> 
> Are these expected? Which semantic does LLVM's "frem" stick to? Any idea on how to fix it?
> 
> [1] http://llvm.org/docs/LangRef.html#i-frem <http://llvm.org/docs/LangRef.html#i-frem>
> 
> 
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