Uros,
I did new patch with all comments addressed as proposed.
1. old option -mprefer-avx128 is Alias(mprefer-vector-width=, 128, none)
2. Simplified default initialization (as Bernhard proposed)
3. Fixed documentation (proposed by Sandra)
4. Several tests are changed to use new style of the option but many leaved with -mprefer-avx128 (one test with new style -mprefer-vector-width=128)


2017-11-21  Sergey Shalnov  <Sergey.Shalnov@intel.com>

gcc/
	* config/i386/i386-opts.h (enum prefer_vector_width): Added new enum
	for the new option -mprefer-vector-width=[none|128|256|512].
	* config/i386/i386.c (ix86_target_string): remove old style options
	-mprefer-avx256 and make -mprefer-avx128 as alias.
	(ix86_option_override_internal):  Apply defaults for the
	-mprefer-vector-width=[128|256] option.
	* config/i386/i386.h (TARGET_PREFER_AVX128, TARGET_PREFER_AVX256):
	Implement macros to work with -mprefer-vector-width=.
	* config/i386/i386.opt: Implemented option
	-mprefer-vector-width=[none|128|256|512].
	* doc/invoke.texi: Documentation for
	-mprefer-vector-width=[none|128|256|512].

gcc/testsuite/
	* g++.dg/ext/pr57362.C (__attribute__): Apply new option syntax.
	* g++.dg/torture/pr81249.C: Ditto.
	* gcc.target/i386/avx512f-constant-float-return.c: Ditto.
	* gcc.target/i386/avx512f-prefer.c: Ditto.
	* gcc.target/i386/pr82460-2.c: Ditto.

Please merge this patch if you think it is acceptable.
Thank you
Sergey


-----Original Message-----
From: Uros Bizjak [mailto:ubizjak@gmail.com] 
Sent: Tuesday, November 14, 2017 7:57 AM
To: Joseph Myers <joseph@codesourcery.com>
Cc: Shalnov, Sergey <sergey.shalnov@intel.com>; gcc-patches@gcc.gnu.org; kirill.yukhin@gmail.com; Koval, Julia <julia.koval@intel.com>; Senkevich, Andrew <andrew.senkevich@intel.com>; Peryt, Sebastian <sebastian.peryt@intel.com>; Ivchenko, Alexander <alexander.ivchenko@intel.com>
Subject: Re: [PATCH, i386] Refactor -mprefer-avx[128|256] options into common -mprefer-vector-width=[none|128|256|512]

On Tue, Nov 14, 2017 at 12:14 AM, Joseph Myers <joseph@codesourcery.com> wrote:
> On Mon, 13 Nov 2017, Uros Bizjak wrote:
>
>> [BTW: --mprefer-avx128 should be marked RejectNegative from the 
>> beginning; let's just assume nobody uses it in its (somehow weird) 
>> negative "-mno-prefer-avx128" form.]
>
> It's used in that form in various testcases that otherwise fail when 
> GCC is configured --with-arch= some CPU that defaults to -mprefer-avx128.

In this case, an even better choice would be:

Alias(mprefer-vector-width=, 128, none)

So, -mno-prefer-avx128 would just set the default back to none.

Uros.