Re: enable sqrt insns for cdce3.c

[Hans-Peter Nilsson <hp@bitrange.com>]

On Mon, 22 Apr 2024, Alexandre Oliva wrote:
> [Revamped version of this patch, combined with others, to follow]
> 
> On Mar 10, 2021, Hans-Peter Nilsson <hp@bitrange.com> wrote:

Time flies...

> > On Wed, 10 Mar 2021, Alexandre Oliva wrote:

> Is mmix a sqrt_insn effective target?  proc
> check_effective_target_sqrt_insn in
> gcc/testsuite/lib/target-supports.exp suggests it shouldn't pass, so I'm
> surprised it would still try to run the test despite the added
> /* { dg-require-effective-target sqrt_insn } */ directive.

The effective-target sqrt_insn predicate says "supports hardware 
square root instructions" and doesn't make a difference between 
sqrtdf2 (double) and sqrtsf3 (float).  I'm extrapolating that 
the "divine meaning" of the comment is that such an instruction 
must be present for all supported floating-point modes for the 
predicate to yield true (when the predicate is correctly 
implemented).

(We could also fix the predicate description to actually say 
"for all floating-point modes" and/or split the predicate into 
mode-specific variants, etc. ;-)

MMIX has sqrtdf2 but not sqrtsf2, and the latter is what's used 
in cdce3.c.

> cdce3 is supposed to shrink-wrap the sqrtf(x) call into something like
> (x >= 0 ? .SQRT(x) : sqrtf(x)), where .SQRT stands for a square root
> instruction.

...for 32-bit single floats.

> Since we don't know why it still runs for you, I'm keeping the mmix
> explicit skip in the new version of the patch.

Thanks, that does seem like TRT.

brgds, H-P