On Tue, May 10, 2022 at 07:27:30AM -0500, Segher Boessenkool wrote:
> > IMHO, it's something we want to fix as well, based on the reasons:
> > 1) bif names have the corresponding mnemonics, users would expect 1-1
> > mapping here.
> > 2) clang emits xs{min,max}dp all the time, with cpu type power7/8/9/10.
> > 3) according to uarch info, xs{min,max}cdp use the same units and have
> > the same latency,
> > no benefits to replace with xs{min,max}cdp.
>
> I never understood any of this. Mike? Why do we do those "c" things
> at all, ever?
In the power7, we only had x{s,v}{min,max}{sp,dp}. But those aren't useful for
optimizing normal (a > b) ? a : b without using -ffast-math. Power9 added the
'c' and 'j' versions of the insns. GCC never generates the 'j' version.
Basically for ?: we generate:
* Code = power8, no -ffast-math: Generate compare, move;
* Code = power8, -ffast-math: Generate xsmaxdp/xsmindp;
* Code = power9, no -ffast-mth: Generate xsmaxcdp/xsmincdp; (and)
* Code = power9, -ffast-math: Generate xsmaxcdp/xsmincdp.
For the __builtin_fmax and __builtin_fmin functions:
* Code = power8, no -ffast-math: Generate call to fmax/fmin;
* Code = power8, -ffast-math: Generate xsmaxdp/xsmindp;
* Code = power9, no -ffast-mth: Generate call to fmax/fmin; (and)
* Code = power9, -ffast-math: Generate xsmaxcdp/xsmincdp.
For IEEE 128-bit floating point, we only have xs{min,max}cqp. We do not have
the version without 'c' nor do we have the 'j' version.
--
Michael Meissner, IBM
PO Box 98, Ayer, Massachusetts, USA, 01432
email: meiss...@linux.ibm.com
