MIPS: IEEE 754-2008 features support

On Wed, 17 Jul 2013, Richard Sandiford wrote:

> "Maciej W. Rozycki" <macro@codesourcery.com> writes:
> >> The patch mostly looks good apart from that, but please use a single
> >> enum for the 2008/legacy thing, both in mips.h and mips.opt.
> >
> >  Also mips.c and mips.md (and last but not least mips-opts.h).  Done.
> 
> I wasn't trying to list all the places that use the C enum.
> I said mips.opt because I thought we should be able to use a single
> mips.opt Enum too, a bit like we share mips_arch_opt_value for both
> -march= and -mtune=.

 Doh, I *knew* it was doable somehow.  Fixed now.  Thanks for persistence.  
Please check if the adjusted wording of the help messages is fine with 
you.

> >  Please also note that the writability of the individual new (HAS2008) 
> > FCSR bits is optional e.g. a conforming processor may have NAN2008 
> > hardwired to 1 and ABS2008 hardwired to 0 (or likewise with NAN2008 
> > writable).
> 
> OK, I'd missed that this was allowed, sorry.  It just seems really
> unfortunate...

 Well, I'm not really sure what the Big Plan here is.  It looks to me like 
the non-arithmetic ABS/NEG feature is a really good thing, while the 2008 
NaN encoding has its shortcomings, e.g. unlike with the legacy encoding 
there's no single sNaN bit pattern to preset FPRs or variables with to 
catch uninitialised use that would work across all the floating-point 
formats (S, D and PS).  So it seems to me like there's no single superior 
setting we could make the default for a group option.

> >> > +# Return 1 if this is a MIPS target supporting -mnan=.
> >> > +# Old versions of binutils may not support this option.
> >> > +
> >> > +proc check_effective_target_mips_nan { } {
> >> > +    if { ![istarget mips*-*-*] } {
> >> > +	return 0
> >> > +    }
> >> > +    return [check_no_compiler_messages mips_nan object {
> >> > +	int dummy;
> >> > +    } "-mnan=2008"]
> >> > +}
> >> 
> >> The tests you added are dg-do compile tests, which stop after assembly
> >> generation, so this guard shouldn't be needed.
> >
> >  It is needed in case the compiler was built without support for this 
> > option i.e. configured against old binutils.  I verified that it indeed 
> > triggered in this case:
> >
> > Executing on host: mips-linux-gnu-gcc  -fno-diagnostics-show-caret -fdiagnostics-color=never  -mnan=2008 -c  -o mips_nan21474.o mips_nan21474.c    (timeout = 300)
> > mips-linux-gnu-gcc: error: unrecognized command line option '-mnan=2008'
> > compiler exited with status 1
> > output is:
> > mips-linux-gnu-gcc: error: unrecognized command line option '-mnan=2008'
> 
> That seems like bad practice though.  In other cases we leave the
> assembler to report options that it doesn't understand.  I think that's
> better because it's then clearer that the assembler needs to be upgraded.
> 
> Within config/mips, the configure test should just control whether it's
> safe to use .nan when no -mnan option has been given.  (This is what we
> did for -mmicromips vs ".set nomicromips" FWIW.)

 I can see your point and acknowledge the preexisting practice, but I 
don't feel particularly convinced, especially in this case where we have a 
feature that's never going to raise the user's attention when 
miconfigured, but it also applies to the microMIPS case.  The two issues I 
see with it are:

1. Conceptually I see the toolchain as a whole and I don't see a value in 
   GCC producing known-unsupported assembly and relying on the assembler 
   (or the linker if applicable) to complain.  I agree pointing at the 
   other tool being incapable or obsolete is a useful practice, but I also 
   think a clear message from GCC itself would be more appropriate (e.g. 
   "`-mfoo' unsupported, please reconfigure against current binutils").

2. Technically I think we have an actual problem here, e.g. in the example 
   you referred we have a situation where GCC supports microMIPS 
   compilation in all cases, however non-microMIPS code is different
   depending on whether the compiler has been configured against modern or 
   obsolete binutils.  Now the latter case may prompt someone to upgrade
   binutils, but there is nothing to prompt that person to reconfigure GCC
   afterwards.  As a result we have two cases of a toolchain comprised of 
   the same versions of both GCC and binutils, but depending on the 
   "history" of the GCC binaries code produced will be different.  I think
   this is subtler and riskier than just rejecting the relevant compiler 
   option outright.

Applying this to the 2008-NaN case the compiler would have to refrain from 
producing the .nan directive in the legacy case if built against old 
binutils but would produce the directive regardless in the 2008 case.  I 
don't feel safe with such an arrangement.

2013-07-18  Maciej W. Rozycki  <macro@codesourcery.com>

	gcc/
	* config/mips/linux.h (GLIBC_DYNAMIC_LINKER): Handle `-mnan=2008'.
	(UCLIBC_DYNAMIC_LINKER): New macro.
	* config/mips/linux64.h (GLIBC_DYNAMIC_LINKER32): Handle 
	`-mnan=2008'.
	(GLIBC_DYNAMIC_LINKER64, GLIBC_DYNAMIC_LINKERN32): Likewise.
	(UCLIBC_DYNAMIC_LINKER32): Undefine macro first.  Handle 
	`-mnan=2008'.
	(UCLIBC_DYNAMIC_LINKER64): Redefine macro.
	(UCLIBC_DYNAMIC_LINKERN32): Likewise.
	* config/mips/mips-modes.def: Remove RESET_FLOAT_FORMAT calls
	for SF and DF modes.  Use ieee_quad_format for TF mode.
	* config/mips/mips-opts.h (mips_ieee_754_setting): New enum.
	* config/mips/mips.c (mips_file_start) [HAVE_AS_NAN]: Output a 
	`.nan' directive.
	(mips_option_override): Handle `-mnan=legacy'.
	* config/mips/mips.h (TARGET_CPU_CPP_BUILTINS): Handle 
	`-mabs=2008' and `-mnan=2008'.
	(OPTION_DEFAULT_SPECS): Add "nan" default.
	(ASM_SPEC): Handle `-mnan='.
	* config/mips/mips.md (abs<mode>2): Handle `-mabs=2008', update
	comment accordingly.
	(neg<mode>2): Likewise.
	* config/mips/mips.opt (mabs, mnan): New options.
	* doc/install.texi (Configuration): Document `--with-nan=' option.
	* doc/invoke.texi (Option Summary): List MIPS `-mabs=' and 
	`-mnan=' options.
	(MIPS Options): Document them.
	* config.gcc <mips*-*-*>: Handle `--with-nan='.
	* configure.ac <mips*-*-*>: Check for GAS `-mnan=2008' support.
	* configure: Regenerate.
	* config.in: Regenerate.

	gcc/testsuite/
	* gcc.target/mips/fabs-2008.c: New test case.
	* gcc.target/mips/fabs-legacy.c: New test case.
	* gcc.target/mips/fabsf-2008.c: New test case.
	* gcc.target/mips/fabsf-legacy.c: New test case.
	* gcc.target/mips/fneg-2008.c: New test case.
	* gcc.target/mips/fneg-legacy.c: New test case.
	* gcc.target/mips/fneg-2008.c: New test case.
	* gcc.target/mips/fneg-legacy.c: New test case.
	* gcc.target/mips/nan-2008.c: New test case.
	* gcc.target/mips/nan-legacy.c: New test case.
	* gcc.target/mips/nanf-2008.c: New test case.
	* gcc.target/mips/nanf-legacy.c: New test case.
	* gcc.target/mips/nans-2008.c: New test case.
	* gcc.target/mips/nans-legacy.c: New test case.
	* gcc.target/mips/nansf-2008.c: New test case.
	* gcc.target/mips/nansf-legacy.c: New test case.
	* gcc.target/mips/mips.exp: Handle `-mabs=' and `-mnan='.
	* lib/target-supports.exp (check_effective_target_mips_nan): New 
	procedure.

  Maciej

gcc-mips-nan2008.diff

MIPS: IEEE 754-2008 features support

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