Re: [Qemu-devel] [RFC PATCH] fpu: add compile time check for old glibc/libm and fma

[Alex Bennée]

Richard Henderson <address@hidden> writes:

> On 12/21/18 11:30 AM, Emilio G. Cota wrote:
>> +    ua.s = 0x0020000000000001;
>> +    ub.s = 0x3ca0000000000000;
>> +    uc.s = 0x0020000000000000;
>> +    ur.h = fma(ua.h, ub.h, uc.h);
>> +    if (ur.s != 0x0020000000000001) {
>
> Forgot your ull's, but otherwise ok.
>
> In email to Alex, I did wonder if we should check for fma hardware (at least 
> on
> x86).  Without a hardware insn, the libm routine is probably no faster than
> softmmu.

My only worry is we could end up doing a bunch of these processor
id/capability type checks. Is a correct but slow libm FMA going to be
much slower than our FMA? Will users even notice?

--
Alex Bennée