On 1/4/2021 4:11 AM, 9il wrote:
[...]
The reason those switches are provided is because the write/read is a
performance hog.
D provides a couple functions in druntime which guarantee rounding intermediate
values to float/double precision. Those can be used as required. This is better
than a compiler switch because having compiler switches that influence floating
point results is poor design.
> Since C99 the default x87 behavior is precise.
Not entirely:
float f(float a, float b) {
float d = (a + b) - b;
return d;
}
f:
sub esp, 4
fld DWORD PTR [esp+12]
fld st(0)
fadd DWORD PTR [esp+8]
[no write/read to memory here, so no round to float]
fsubrp st(1), st
fstp DWORD PTR [esp]
fld DWORD PTR [esp]
add esp, 4
ret
In any case, let's try your example https://cpp.godbolt.org/z/7sa8dP with dmd
for 32 bits:
push EAX
push EAX
fld float ptr 010h[ESP]
fadd float ptr 0Ch[ESP]
fstp float ptr [ESP] // there's the write
fld float ptr [ESP] // there's the read!
fsub float ptr 0Ch[ESP]
fstp float ptr 4[ESP] // the write
fld float ptr 4[ESP] // the read
add ESP,8
ret 8
It's semantically equivalent to the godbolt asm you posted.
