Scheduling x86 dispatch windows

H.J. Lu hjl.tools@gmail.com
Fri Jun 11 19:17:00 GMT 2010


On Fri, Jun 11, 2010 at 12:09 PM, Quentin Neill
<quentin.neill.gnu@gmail.com> wrote:
> On Fri, Jun 11, 2010 at 10:58 AM, Daniel Jacobowitz
> <dan@codesourcery.com> wrote:
>> On Thu, Jun 10, 2010 at 09:48:24PM -0500, Quentin Neill wrote:
> [snip]
>>> Does this qualify as a form of what you are suggesting?  Because this
>>> is exactly what is being proposed:
>>>
>>> .balign 8                  # start window
>>>     insn op, op          # 67 67 XX YY ZZ  - padded with 2 prefixes to make 8
>>>     insn2 op, op        # AA BB CC
>>> .padalign 8              # window boundary
>>>     insn4 op
>>>     . . .
>>
>> No, this is quite different.  These are directives that tell the
>> assembler to make changes.  I'm talking about assertions, not
>> directives.  Something like this:
>>
>>  mov r0, r1 @ [length 2]
>>  add ip, lr, ip @ [length 4]
>>  mov r0, r1 @ [length 4] <-- assembler error 'insn has length 2'
>>
>> GCC can output length information, but it is never exact, and it is
>> not in a form recognized by the assembler.
>>
>> On x86, I have no idea how this would work.
>>
>> --
>> Daniel Jacobowitz
>> CodeSourcery
>>
>
> I see.
>
> Currently GCC doesn't compute the current encoding offset (doesn't
> know mnemonic/opcode lengths), nor does it perform a relaxation pass
> (to resolve forward displacement/branch offsets).   Without these it
> so cannot accurately formulate such assertions.
>
> Our proposal is to let the assembler itself (knowing best the details
> of the encoding stream, offsets, and the processor) aligns
> instructions, with hints about the structure (block starts, ends,
> instruction sets) using macros/assertions/tokens if needed.
>
> Another option would be to expose some subset of the assembler
> functionality as a plugin to GCC (similar to how gold is used) to
> extract the instruction sizes.   Any comments on that approach?
>

I would suggest generating object code directly, totally bypassing
assembler. Many compilers do it. But it is a HUGE effort.


-- 
H.J.



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