[Mesa-dev] [PATCH 11/20] i965/fs: Append uniform entries to the gather table

[Francisco Jerez]

Matt Turner <mattst88 at gmail.com> writes:

> On Fri, Oct 23, 2015 at 11:23 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>> Abdiel Janulgue <abdiel.janulgue at linux.intel.com> writes:
>>
>>> This patch generates the gather table entries for ordinary uniforms
>>> if they are present. The uniform constants here will later be packed
>>> together with UBO constants.
>>>
>>> Signed-off-by: Abdiel Janulgue <abdiel.janulgue at linux.intel.com>
>>> ---
>>>  src/mesa/drivers/dri/i965/brw_fs.cpp | 18 +++++++++++++++++-
>>>  1 file changed, 17 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>> index d240371..e39d821 100644
>>> --- a/src/mesa/drivers/dri/i965/brw_fs.cpp
>>> +++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
>>> @@ -1863,8 +1863,11 @@ fs_visitor::assign_constant_locations()
>>>        }
>>>     }
>>>
>>> -   stage_prog_data->nr_params = num_push_constants;
>>>     stage_prog_data->nr_pull_params = num_pull_constants;
>>> +   stage_prog_data->nr_params = 0;
>>
>> I doubt it's useful to set this to zero here only to calculate it again
>> below.
>>
>>> +
>>> +   unsigned const_reg_access[uniforms];
>>
>> VLAs are a non-standard extension to C++.  Usually I'd recommend to use
>> the C++ standard library instead, but because it's banned from the i965
>> back-end I guess the best you can get is new[]/delete[].
>
> So... why not just use the extension to C++? Does some compiler we
> care about not support them?
>
> GCC does: https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html
>
> Looks like this use would be allowed by clang as well:
> http://clang.llvm.org/compatibility.html#vla

I believe only GCC has a more or less reasonable implementation, Clang's
implementation doesn't allow much of what GCC's allows, like objects
with user-defined constructors or anything non-POD (seems mainly
intended for compatibility with C99 code).  MS VC++ doesn't support it
at all.

The C99 VLA feature was BTW made optional again in C11, and the similar
feature proposed for C++14 (runtime-sized arrays) was rejected by the
standards committee.  Anyway it doesn't seem like a big loss for C++,
there are already standards-compliant ways to get a type-safe array of
automatic duration and dynamically determined size, which support
non-POD C++ objects as element type and behave as first-class C++
objects themselves unlike in the various VLA-like language extensions.
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