Re: [patch] Fix thinko in powerpc default specs for -mabi

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Fri, Sep 23, 2022 at 11:49:24AM -0400, Olivier Hainque wrote:
> For a powerpc compiler configured with --with-abi=elfv2, an explicit
> -mabi option other than elfv1 fails to override the default.
> 
> For example, after
> 
>   [...]/configure --enable-languages=c --target=powerpc-elf --with-abi=elfv2
>   make all-gcc
> 
> This command:
> 
>   ./gcc/xgcc -B./gcc/ t.c -mabi=ieeelongdouble -v
> 
> issues:
> 
>   ./gcc/cc1 [...] t.c -mabi=ieeelongdouble -mabi=elfv2
> 
> elfv2 overrides the user request here, which I think
> is not as intended.

-mabi= does two separate things, unfortunately.

First, you can use it to set the base ABI: elfv1, elfv2.  But you can
also use it to set ABI variants, ABI options: -mabi={no-,}altivec,
-mabi={ieee,ibm}longdouble, -mabi=vec-{extabi,default}.  Things in that
latter category are completely orthogonal to anything else (except that
some only make sense together with some base ABIs).

Base ABI is not selectable for most, it is implied by your target
triple.  -mabi=elfv[12] only makes sense for targets that have either of
the two by default.

> This is controlled by OPTION_DEFAULT_SPECS, where we have
> 
>   {"abi", "%{!mabi=elfv*:-mabi=%(VALUE)}" },
> 
> >From https://gcc.gnu.org/pipermail/gcc-patches/2013-November/375042.html
> I don't see an explicit reason to trigger only on elfv* . It just looks
> like an oversight with a focus on elfv1 vs elfv2 at the time.
> 
> The attached patch is a proposal to correct this, simply removing the
> "elfv" prefix from the spec and adding the corresponding description
> to the block comment just above.
> 
> We have been using this for about a year now in gcc-11 based toolchains.
> This helps our dejagnu testsuite runs for VxWorks on powerpc and 
> hasn't produced any ill side effect to date.

So what exactly is this meant to do?

> 2022-09-14  Olivier Hainque  <hainque@adacore.com>
> 
>         * config/rs6000/option-defaults.h (OPTION_DEFAULT_SPECS):
>         Have any -mabi, not only -mabi=elfv*, override the --with-abi
>         configuration default.

>  /* Support for a compile-time default CPU, et cetera.  The rules are:
> +   --with-abi is ignored if -mabi is specified.
>     --with-cpu is ignored if -mcpu is specified; likewise --with-cpu-32
>       and --with-cpu-64.
>     --with-tune is ignored if -mtune or -mcpu is specified; likewise
> @@ -54,7 +55,7 @@
>     --with-float is ignored if -mhard-float or -msoft-float are
>       specified.  */
>  #define OPTION_DEFAULT_SPECS \
> -  {"abi", "%{!mabi=elfv*:-mabi=%(VALUE)}" }, \
> +  {"abi", "%{!mabi=*:-mabi=%(VALUE)}" }, \
>    {"tune", "%{!mtune=*:%{!mcpu=*:-mtune=%(VALUE)}}" }, \
>    {"tune_32", "%{" OPT_ARCH32 ":%{!mtune=*:%{!mcpu=*:-mtune=%(VALUE)}}}" }, \
>    {"tune_64", "%{" OPT_ARCH64 ":%{!mtune=*:%{!mcpu=*:-mtune=%(VALUE)}}}" }, \

So this patch will make a difference to people who use --with-abi= for
one of the ABI variant things.

But it does not seem correct.  -mabi=optionA should not override the
-mabi=optionB set in --with-abi=, where A and B are independent, nor
should it override the base ABI.

Please open a PR for the problem you want to solve here, so we do not
lose track of it?

Thanks,


Segher