On Tue, Apr 12, 2022 at 09:14:55PM -0400, Michael Meissner wrote:
> This is V4 of the patch. Compared to V3 of the patch, GCC will just
> ignore -m{,no-}power8-fusion and -m{,no-}power8-fusion-sign.
But incorrectly :-(
> The splitting of signed halfword and word loads into unsigned load and
> sign extension is now suppressed with -Os, but it is done normally if we
> are not optimizing for space.
I have no idea what that means. Other than that I asked to remove that.
> This code makes the -mpower8-fusion option a nop. It is accepted without
> warning, but it does nothing. Power8 fusion is only enabled if we are tuning
> for a power8.
It should *delete* the option, and have
;; This option existed in the past, but now is always off.
mno-power8-fusion
Target RejectNegative Undocumented Ignore
> The undocumented -mpower8-fusion-sign option is also made into a nop.
That one should be deleted.
> + /* The Power8 fusion option was removed. We ignore using it in #pragma and
> + attribute target. Users may have used the options to suppress errors if
> + they declare an inline function to be specifically power8 and the
> function
> + was included by power9 or power10 which turned off the power8 fusion
> + support. */
> + { "power8-fusion", 0, false, true },
What does the comment mean?
> + /* Don't print options that exist for backwards compatibility, but are
> + ignored now like -mpower8-fusion. */
> + if (!mask)
> + continue;
No. Such options should not be in the mask at all.
> +/* Power8 has special fusion operations that are enabled if we are tuning for
> + power8. This used to be settable with an option (-mpower8-fusion), but
> that
> + option has been removed. */
> +#define TARGET_P8_FUSION (rs6000_tune == PROCESSOR_POWER8)
The plan was to not have p8 fusion at all. GCC never implemented any of
the more useful p8 fusion things anyway, and those were only marginally
beneficial anyway.
> +/* Power8 fusion does not fuse loads with sign extends. If we are not
> + optimizing for space, split loads with sign extension to loads with zero
> + extension and an explicit sign extend operation, so that the zero
> extending
> + load can be fused. */
> +#define TARGET_P8_FUSION_SIGN (TARGET_P8_FUSION
> \
> + && !optimize_function_for_size_p (cfun))
As I said before, don't do this. Just remove the whole thing.
> +; The -mpower8-fusion and -mpower8-fusion-sign options existed in the past,
> but
> +; they are ignored now.
Don't put them together. It is much easier for everything if they are
separate, boring, and exactly like everything else.
> mpower8-fusion
> -Target Mask(P8_FUSION) Var(rs6000_isa_flags)
> -Fuse certain integer operations together for better performance on power8.
> +Target Undocumented Ignore
It should be deleted, instead, and be replaced with
;; This option existed in the past, but now is always off.
mno-power8-fusion
Target RejectNegative Undocumented Ignore
mpower8-fusion
Target RejectNegative Undocumented WarnRemoved
i.e. just like all other removed flags. If someone explicitly tries to
enable it he/she *should* get a warning.
> mpower8-fusion-sign
> -Target Undocumented Mask(P8_FUSION_SIGN) Var(rs6000_isa_flags)
> -Allow sign extension in fusion operations.
> +Target Undocumented Ignore
And this one should be completely removed, since no one ever used it.
Segher
