[Mesa-dev] [PATCH 11/21] glsl: Store ir_variable::ir_type in 8 bits instead of 32

[Ian Romanick]

On 05/28/2014 12:17 PM, Ian Romanick wrote:
> On 05/27/2014 08:28 PM, Matt Turner wrote:
>> On Tue, May 27, 2014 at 7:49 PM, Ian Romanick <idr at freedesktop.org> wrote:
>>> From: Ian Romanick <ian.d.romanick at intel.com>
>>>
>>> No change in the peak ir_variable memory usage in a trimmed apitrace of
>>> dota2 on 64-bit.
>>>
>>> No change in the peak ir_variable memory usage in a trimmed apitrace of
>>> dota2 on 32-bit.
>>>
>>> Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
>>> ---
>>>  src/glsl/ir.h | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/src/glsl/ir.h b/src/glsl/ir.h
>>> index 7faee74..bc02f6e 100644
>>> --- a/src/glsl/ir.h
>>> +++ b/src/glsl/ir.h
>>> @@ -92,12 +92,13 @@ enum ir_node_type {
>>>   */
>>>  class ir_instruction : public exec_node {
>>>  private:
>>> -   enum ir_node_type ir_type;
>>> +   uint8_t ir_type;
>>>
>>>  public:
>>>     inline enum ir_node_type get_ir_type() const
>>>     {
>>> -      return this->ir_type;
>>> +      STATIC_ASSERT(ir_type_max < 256);
>>> +      return (enum ir_node_type) this->ir_type;
>>>     }
>>>
>>>     /**
>>> --
>>> 1.8.1.4
>>
>> Instead of doing this, you can mark the enum type with the PACKED
>> attribute. I did this in a similar change in i965 already. See
>> http://lists.freedesktop.org/archives/mesa-dev/2014-February/054643.html
>>
>> This way we still get enum type checking and warnings out of switch
>> statements and such.
> 
> Hmm... that would mean that patch 10 wouldn't strictly be necessary.
> The disadvantage is that the next patch would need (right?) some changes
> for MSVC, especially on 32-bit.  I think it would need to be
> 
> #if sizeof(ir_node_type) < sizeof(void *)
> # define PADDING_BYTES (sizeof(void *) - sizeof(ir_node_type))
> #else
> # define PADDING_BYTES sizeof(void *)
> #  if (__GNUC__ >= 3)
> #    error "GCC did us wrong."
> #  endif
> #endif
> 
>   uint8_t padding[PADDING_BYTES];

After Patrick pointed out my obvious lack of C preprocessor skills, I
think the only alternative is:

   uint8_t padding[sizeof(ir_node_type) < sizeof(void *)
                   ? sizeof(void *) - sizeof(ir_node_type)
                   : sizeof(void *)];

I'd probably hide that with a #define and a big comment, but it's
still a bit ugly.

> Seems a little sketchy, but might still be better... hmm...
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