Re: [PATCH] rs6000: Rework option -mpowerpc64 handling [PR106680]

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

On Wed, Sep 28, 2022 at 01:30:46PM +0800, Kewen.Lin wrote:
> PR106680 shows that -m32 -mpowerpc64 is different from
> -mpowerpc64 -m32, this is determined by the way how we
> handle option powerpc64 in rs6000_handle_option.
> 
> Segher pointed out this difference should be taken as
> a bug and we should ensure that option powerpc64 is
> independent of -m32/-m64.  So this patch removes the
> handlings in rs6000_handle_option and add some necessary
> supports in rs6000_option_override_internal instead.

Thanks!

> With this patch, if users specify -m{no-,}powerpc64, the
> specified value is honoured, otherwise, for 64bit it
> always enables OPTION_MASK_POWERPC64 while for 32bit
> it disables OPTION_MASK_POWERPC64 if OS_MISSING_POWERPC64.

If the user says -m64 -mno-powerpc64 it should error, and perhaps -m32
-mpowerpc64 should warn if OS_MISSING_POWERPC64?

> -  /* Some OSs don't support saving the high part of 64-bit registers on context
> -     switch.  Other OSs don't support saving Altivec registers.  On those OSs,
> -     we don't touch the OPTION_MASK_POWERPC64 or OPTION_MASK_ALTIVEC settings;
> -     if the user wants either, the user must explicitly specify them and we
> -     won't interfere with the user's specification.  */
> +  /* Some OSs don't support saving Altivec registers.  On those OSs, we don't
> +     touch the OPTION_MASK_POWERPC64 or OPTION_MASK_ALTIVEC settings; if the
> +     user wants either, the user must explicitly specify them and we won't
> +     interfere with the user's specification.  */
> 
>    set_masks = POWERPC_MASKS;
> -#ifdef OS_MISSING_POWERPC64
> -  if (OS_MISSING_POWERPC64)
> -    set_masks &= ~OPTION_MASK_POWERPC64;
> -#endif

As I said elsewhere, it probably is helpful if we still warn here for
-m32 -mpowerpc64 with OS_MISSING_POWERPC64 (or without the -m32 even,
same thing).

> +  /* With option powerpc64 specified explicitly (either on or off), even if
> +     being compiled for 64 bit we don't need to check if it's disabled here,
> +     since subtargets will check and raise an error message if necessary
> +     later.  But without option powerpc64 specified explicitly, we need to
> +     ensure powerpc64 enabled for 64 bit and disabled on those OSes with
> +     OS_MISSING_POWERPC64, since they don't support saving the high part of
> +     64-bit registers on context switch.  */
> +  if (!(rs6000_isa_flags_explicit & OPTION_MASK_POWERPC64))
> +    {
> +      if (TARGET_64BIT)
> +	/* Make sure we always enable it by default for 64 bit.  */
> +	rs6000_isa_flags |= OPTION_MASK_POWERPC64;
> +#ifdef OS_MISSING_POWERPC64
> +      else if (OS_MISSING_POWERPC64)
> +	/* It's unexpected to have OPTION_MASK_POWERPC64 on for OSes which
> +	   miss powerpc64 support, so disable it.  */
> +	rs6000_isa_flags &= ~OPTION_MASK_POWERPC64;
> +#endif
> +    }

Aha.  Please don't, just warn instead?  Silently disabling such stuff is
the worst option :-(

> +/* { dg-error "'-m64' requires a PowerPC64 cpu" "PR106680" { target powerpc*-*-linux* powerpc-*-rtems* } 0 } */

Everything except AIX even?  So it will include Darwin as well (and the
BSDs, and powerpc*-elf, etc.)

> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/powerpc/pr106680-4.c
> @@ -0,0 +1,16 @@
> +/* Skip this on aix, otherwise it emits the error message like "64-bit
> +   computation with 32-bit addressing not yet supported" on aix.  */
> +/* { dg-skip-if "" { powerpc*-*-aix* } } */
> +/* { dg-require-effective-target ilp32 } */
> +/* { dg-options "-mpowerpc64 -m32 -O2" } */

If you have -m32 you don't need ilp32, and the other way around.

> +/* Verify option -m32 doesn't override option -mpowerpc64.
> +   If option -mpowerpc64 gets overridden, the assembly would
> +   end up with addc and adde.  */
> +/* { dg-final { scan-assembler-not "addc" } } */
> +/* { dg-final { scan-assembler-not "adde" } } */

Lol, nice :-)

"adde" is a frequent substring, use \m \M please?  You will always get
these exact insns anyway.  And you could add a -times {\madd\M} 1 ?

 - - -

The Darwin problem might be something in darwin*.h, but I don't see it.
Maybe it is a more generic problem?


Segher