Printing shortest decimal form of floating point number with Mir

[Walter Bright]

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.