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Re: [PATCH]: x86 gas: allow 'rep' prefix on 'bsf' and 'bsr' instructions
On Thu, Jun 21, 2012 at 4:01 PM, H.J. Lu <email@example.com> wrote:
> On Thu, Jun 21, 2012 at 2:36 PM, Roland McGrath <firstname.lastname@example.org> wrote:
>> 'rep; bsf ...'/'rep; bsr ...' are encoded the same as 'tzcnt ...'/'lzcnt ...'.
>> When not doing -mbmi, GCC (trunk) like to emit 'rep; bsf ...' on the
>> theory that since the two instructions have sufficiently similar
>> semantics for the purposes for which the compiler emits this,
>> BMI-capable hardware will decode it as 'tzcnt ...' and may execute that
>> faster than 'bsf ...', while older hardware will ignore the REP prefix
>> and decode it as 'bsf ...'.
>> When using .bundle_align_mode, the assembler might decide to insert some
>> nop padding between any two instructions, so the ';' could become some
>> number of nop instructions and break the encoding intended.
>> This change makes the assembler accept 'rep bsf ...' or 'rep bsr ...'
>> without complaint. ?The result is the same as using the 'tzcnt' or
>> 'lzcnt' mnemonic, but the 'rep' forms are accepted even under
>> -march=i686 or the like where 'tzcnt' and 'lzcnt' would be refused.
>> With this, I can change the compiler to use this syntax (when configured
>> with a new assembler) and remove the possibility of running afoul of
>> .bundle_align_mode nop-insertion.
>> No testsuite failures on an x86_64-linux-gnu host.
>> Ok for trunk?
>> (I've omitted the large patch to the generated file opcodes/i386-tbl.h,
>> so use --enable-maintainer-mode to test the patch.)
> Please make following changes:
> 1. Move RepPrefixOk next to HLEPrefixOk.
> 2. Add RepPrefixOk to all insns with IsString
> 3. Check repprefixok instead of isstring for REP prefix in
> OK with those changes.
Please also add a testcase for each change.