Floating point rounding

[ag0aep6g via Digitalmars-d-learn]

On 03/02/2017 10:49 PM, Guillaume Chatelet wrote:
> Thx for the investigation!
> Here is the code for FloatingPointControl
> https://github.com/dlang/phobos/blob/master/std/math.d#L4809
>
> Other code (enableExceptions / disableExceptions) seems to have two code
> path depending on "version(X86_Any)", rounding doesn't.
>
> Maybe that's the bug?

I don't think that's it. But I think I've figured out what's wrong:

dmd generates SSE instructions for floating point math. 
FloatingPointControl only minds the control register for the FPU. But 
SSE instructions are not affected by that. SSE has a separate control 
register: MXCSR.

Too see that dmd generates SSE instructions:

     echo 'void main() { float x; x += 0.1f; }' > test.d && \
     dmd -c test.d && \
     objdump -d -Mintel -j.text._Dmain test.o

Note the instructions movss and addss, and the xmm registers.

To affect those instructions, FloatinPointControl.setsetControlState [1] 
must use ldmxcsr in addition to or instead of fldcw.

This should be a pretty obvious bug. Rounding (and floating point 
exceptions?) must not be covered in the phobos tests at all.


[1] 
https://github.com/dlang/phobos/blob/750593738bcbe9577c3f5951b5b639392871f90e/std/math.d#L4906-L4933