Why don't we switch to C like floating pointed arithmetic instead of automatic expansion to reals?

[deadalnix via Digitalmars-d]

On Friday, 5 August 2016 at 08:17:00 UTC, Ilya Yaroshenko wrote:
> 1. Could you please provide an assembler example with clang or 
> recent gcc?

I have better: compile your favorite project with 
-Wdouble-promotion and enjoy the rain of warnings.

But try it yourself:

float foo(float a, float b) {
   return 3.0 * a / b;
}

GCC 5.3 gives me

foo(float, float):
         cvtss2sd        xmm0, xmm0
         cvtss2sd        xmm1, xmm1
         mulsd   xmm0, QWORD PTR .LC0[rip]
         divsd   xmm0, xmm1
         cvtsd2ss        xmm0, xmm0
         ret
.LC0:
         .long   0
         .long   1074266112

Which clearly uses double precision.

And clang 3.8:

LCPI0_0:
         .quad   4613937818241073152     # double 3
foo(float, float):                               # @foo(float, 
float)
         cvtss2sd        xmm0, xmm0
         mulsd   xmm0, qword ptr [rip + .LCPI0_0]
         cvtss2sd        xmm1, xmm1
         divsd   xmm0, xmm1
         cvtsd2ss        xmm0, xmm0
         ret

which uses double as well.

> 2. C compilers not promote double to 80-bit reals anyway.

VC++ does it on 32 bits build, but initialize the x87 unit to 
double precision (on 80 bits floats - yes that's a x87 setting).

VC++ will keep using float for x64 builds.

Intel compiler use compiler flags to promote or not.

In case you were wondering, this is not limited to X86/64 as GCC 
gives me on ARM:

foo(float, float):
         fmov    d2, 3.0e+0
         fcvt    d0, s0
         fmul    d0, d0, d2
         fcvt    d1, s1
         fdiv    d0, d0, d1
         fcvt    s0, d0
         ret

Which also promotes to double.