[Mesa-dev] [PATCH 8/7] i965: Accurately bail on SIMD16 compiles.

[Ian Romanick]

On 03/12/2014 11:30 AM, Kenneth Graunke wrote:
> On 03/12/2014 02:51 AM, Ilia Mirkin wrote:
>> On Wed, Mar 12, 2014 at 4:28 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
>>> Ideally, we'd like to never even attempt the SIMD16 compile if we could
>>> know ahead of time that it won't succeed---it's purely a waste of time.
>>> This is especially important for state-based recompiles, which happen at
>>> draw time.
>>>
>>> The fragment shader compiler has a number of checks like:
>>>
>>>    if (dispatch_width == 16)
>>>       fail("...some reason...");
>>>
>>> This patch introduces a new no16() function which replaces the above
>>> pattern.  In the SIMD8 compile, it sets a "SIMD16 will never work" flag.
>>> Then, brw_wm_fs_emit can check that flag, skip the SIMD16 compile, and
>>> issue a helpful performance warning if INTEL_DEBUG=perf is set.  (In
>>> SIMD16 mode, no16() calls fail(), for safety's sake.)
>>>
>>> The great part is that this is not a heuristic---if the flag is set, we
>>> know with 100% certainty that the SIMD16 compile would fail.  (It might
>>> fail anyway if we run out of registers, but it's always worth trying.)
>>>
>>> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
>>> ---
>>>  src/mesa/drivers/dri/i965/brw_fs.cpp         | 69 +++++++++++++++++++++++-----
>>>  src/mesa/drivers/dri/i965/brw_fs.h           |  4 ++
>>>  src/mesa/drivers/dri/i965/brw_fs_visitor.cpp | 44 +++++++++---------
>>>  3 files changed, 83 insertions(+), 34 deletions(-)
>>>
>>> I forgot to send this one out...it applies on top of the previous 7 patches.
>>>
>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>> index 62848be..9ad80c5 100644
>>> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>>> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>> @@ -647,9 +647,8 @@ fs_visitor::emit_shader_time_write(enum shader_time_shader_type type,
>>>  }
>>>
>>>  void
>>> -fs_visitor::fail(const char *format, ...)
>>> +fs_visitor::vfail(const char *format, va_list va)
>>>  {
>>> -   va_list va;
>>>     char *msg;
>>>
>>>     if (failed)
>>> @@ -657,9 +656,7 @@ fs_visitor::fail(const char *format, ...)
>>>
>>>     failed = true;
>>>
>>> -   va_start(va, format);
>>>     msg = ralloc_vasprintf(mem_ctx, format, va);
>>> -   va_end(va);
>>>     msg = ralloc_asprintf(mem_ctx, "FS compile failed: %s\n", msg);
>>>
>>>     this->fail_msg = msg;
>>> @@ -669,6 +666,49 @@ fs_visitor::fail(const char *format, ...)
>>>     }
>>>  }
>>>
>>> +void
>>> +fs_visitor::fail(const char *format, ...)
>>> +{
>>> +   va_list va;
>>> +
>>> +   va_start(va, format);
>>> +   vfail(format, va);
>>> +   va_end(va);
>>> +}
>>> +
>>> +/**
>>> + * Mark this program as impossible to compile in SIMD16 mode.
>>> + *
>>> + * During the SIMD8 compile (which happens first), we can detect and flag
>>> + * things that are unsupported in SIMD16 mode, so the compiler can skip
>>> + * the SIMD16 compile altogether.
>>> + *
>>> + * During a SIMD16 compile (if one happens anyway), this just calls fail().
>>> + */
>>> +void
>>> +fs_visitor::no16(const char *format, ...)
>>> +{
>>> +   va_list va;
>>> +
>>> +   va_start(va, format);
>>> +
>>> +   if (dispatch_width == 16) {
>>> +      vfail(format, va);
>>
>> I think there's a va_end() missing in this path. Not sure what the
>> end-effect of that is, but I'm pretty sure that the recommendation is
>> to always end the list before returning.
> 
> Good catch, thanks!
> 
> For v2, I've changed the code to:
> 
> void
> fs_visitor::no16(const char *format, ...)
> {
>    va_list va;
> 
>    va_start(va, format);
> 
>    if (dispatch_width == 16) {
>       vfail(format, va);
>    } else {
>       simd16_unsupported = true;
> 
>       if (INTEL_DEBUG & DEBUG_PERF) {
>          if (no16_msg)
>             ralloc_vasprintf_append(&no16_msg, format, va);
>          else
>             no16_msg = ralloc_vasprintf(mem_ctx, format, va);
>       }
>    }
> 
>    va_end(va);
> }
> 
> which should also address Ian's concern.  (I'm not sure whether it's
> necessary, but it's easy enough to do, so why not play it safe?)

I can get behind that. :)

> --Ken


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