llvm dev - Jul 2013 - [LLVMdev] [PATCH] x86: disambiguate unqualified btr, bts

Jim Grosbach wrote:
> Also, please elaborate on why this is a good change. Because gas accepts it
> isn’t sufficient reason in and of itself.
That they're valid instructions isn't sufficient reason?  Should I
additionally say that linux.git uses them?

I wrote:
> The instructions btr and bts are perfectly valid, and have existed since
> Intel 386.

On Jul 10, 2013, at 1:44 PM, Ramkumar Ramachandra <artagnon at gmail.com>
wrote:
> Jim Grosbach wrote:
>> Also, please elaborate on why this is a good change. Because gas
accepts it
>> isn’t sufficient reason in and of itself.
> 
> That they're valid instructions isn't sufficient reason?  Should I
> additionally say that linux.git uses them?
> 
Is the diagnostic incorrect?

To say that another way, is the assembler correctly diagnosing a previously
unnoticed problem in the project source code, or is the assembler not behaving
correctly according the the documented Intel assembly mnemonics? If the former,
the fix belongs in the project, not the assembler. If the latter, then we should
absolutely fix the assembler. From your emails, it is unclear to me which is the
case.

-Jim
> I wrote:
>> The instructions btr and bts are perfectly valid, and have existed
since
>> Intel 386.
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Jim Grosbach wrote:
> To say that another way, is the assembler correctly diagnosing a previously
> unnoticed problem in the project source code, or is the assembler not
> behaving correctly according the the documented Intel assembly mnemonics?
Where are the authoritative instruction set pages?  If such a thing
were readily available, why are there gaps in the current
implementation?  A quick Googling gets me [1], but I can't say it's
authoritative.  What's important is that there certainly are
architectures where btr/bts are valid instructions, and they must be
supported.  btr/bts are certainly not invalid instructions that we're
bending over backwards to support, because linux.git/gas works with
them.

[1]: http://web.itu.edu.tr/kesgin/mul06/intel/index.html