On 31/07/25 11:55 am, Michael Meissner wrote:
> I have rewritten this page, removing the 'ordered' terminology in the
> coomit messages and comments that was a problem.
>
> I have also rewritten the patch to hopefully make it easier to
> understand.
>
> I have built it on both big endian and little endian servers with no
> regressions.
>
>
> Proposed commit message follows:
>
> In bug PR target/118541 on power9, power10, and power11 systems, for the
> function:
>
> extern double __ieee754_acos (double);
>
> double
> __acospi (double x)
> {
> double ret = __ieee754_acos (x) / 3.14;
> return __builtin_isgreater (ret, 1.0) ? 1.0 : ret;
> }
>
> GCC currently generates the following code:
>
> Power9 Power10 and Power11
> ====== ===================
> bl __ieee754_acos bl __ieee754_acos@notoc
> nop plfd 0,.LC0@pcrel
> addis 9,2,.LC2@toc@ha xxspltidp 12,1065353216
> addi 1,1,32 addi 1,1,32
> lfd 0,.LC2@toc@l(9) ld 0,16(1)
> addis 9,2,.LC0@toc@ha fdiv 0,1,0
> ld 0,16(1) mtlr 0
> lfd 12,.LC0@toc@l(9) xscmpgtdp 1,0,12
> fdiv 0,1,0 xxsel 1,0,12,1
> mtlr 0 blr
> xscmpgtdp 1,0,12
> xxsel 1,0,12,1
> blr
>
> This is because ifcvt.cc optimizes the conditional floating point move to use
> the
> XSCMPGTDP instruction.
>
> However, the XSCMPGTDP instruction will generate an interrupt if one of the
> arguments is a signalling NaN and signalling NaNs can generate an interrupt.
> The IEEE comparison functions (isgreater, etc.) require that the comparison
> not
> raise an interrupt.
>
> The following patch changes the PowerPC back end so that ifcvt.cc will not
> change
> the if/then test and move into a conditional move if the comparison is one of
> the comparisons that do not raise an error with signalling NaNs and -Ofast is
> not used. If a normal comparison is used or -Ofast is used, GCC will continue
> to generate XSCMPGTDP and XXSEL.
The isgreater() builtin is not supposed to set floating point exceptions,
irrespective of which compilation flags are used. So, even if -Ofast is used or
-ffinite-math-only is used, isgreater() should not set floating point
exceptions.
Using XSCMPGTDP will end up throwing exceptions if at least one of the operand
is
a qNaN or an sNaN.
Surya
