[Mesa-dev] [PATCH 2/2] glsl: validate sampler array indexing for 'constant-index-expression'

[Ian Romanick]

On 05/26/2015 02:53 AM, Tapani Pälli wrote:
> Hello;
> 
> I'd like to ping if this approach would be ok. We've had some
> discussions with Curro about it and overall it would seem nicer to move
> this check to happen at compile time. However, this seems quite a
> problematic move. I'll try explain below why;
> 
> The overall problem with the failing use cases in the bug is that loop
> unroll does not happen. It does not happen because loop analysis does
> not know how to deal with functions and there is a texture2D call inside
> the loop.
> 
> Now what follows is that because unroll does not happen the array index
> (loop induction variable) does not become constant during compilation,
> this will happen only after linking (where unroll finally happens due to
> function inlining which allows further optimization).
> 
> I have a hacky patch where I force unroll to happen early when only
> builtin calls are found inside loop and it works *but* unfortunately it
> does not help since in the unrolled result we still have sampler array
> indexing with a non-constant variable 'i', it will be constant only
> later after linking phase when function inlining and further
> optimizations happen. It looks like this (I modified ir print output a
> bit to fit in email):
> 
> 1st round:
> assign var_ref i constant_int 0
> call texture2D (constant int 0)
> 
> 2nd round:
> assign var_ref i (var_ref i + constant_int 1)
> call texture2D (var_ref i)
> 
> So at this point I got a bit tired of this approach. IMO linker check is
> sufficient and according the spec. Spec does not explicitly specify a
> compiler or linker error for this case but it does say:

I agree.  We could handle GLSL ES at compile time because there is only
one compilation unit per stage, but I'm not convinced handling it
special is worth any effort.

The linker check even here can be somewhat problematic.  Back-ends do
additional optimization, so they may be able to make some of these
accesses be non-dynamic.  Marek in particular has complained about this
before.  Perhaps add a flag to make the error a warning (see also my
comment below)?

> GLSL ES 1.0.17 spec (4.1.9 Arrays):
> 
> "Reading from or writing to an array with a non-constant index that is
> less than zero or greater than or equal to the array's size results in
> undefined behavior. It is platform dependent how bounded this undefined
> behavior may be. It is possible that it leads to instability of the
> underlying system or corruption of memory. However, a particular
> platform may bound the behavior such that this is not the case."
> 
> So according to spec, we should not really be checking anything but here
> I'm offering undefined behavior as extra linker check allowed by the
> last clause.

We have platforms that can fully do dynamic indexing of sampler arrays.
 I think the "undefined behavior" on those platforms should be "it just
works," perhaps with a portability warning.

One other comment far below.

> Any opinions appreciated;
> 
> // Tapani
> 
> 
> On 05/19/2015 03:01 PM, Tapani Pälli wrote:
>> Desktop GLSL < 130 and GLSL ES < 300 allow sampler array indexing where
>> index can contain a loop induction variable. This extra check makes sure
>> that all these indexes turn in to constant expressions during
>> compilation/linking.
>>
>> Signed-off-by: Tapani Pälli <tapani.palli at intel.com>
>> Cc: "10.5" and "10.6" <mesa-stable at lists.freedesktop.org>
>> ---
>>   src/glsl/linker.cpp | 71
>> +++++++++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 71 insertions(+)
>>
>> diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
>> index ecdc025..729b27f 100644
>> --- a/src/glsl/linker.cpp
>> +++ b/src/glsl/linker.cpp
>> @@ -346,6 +346,39 @@ private:
>>      bool uses_non_zero_stream;
>>   };
>>
>> +/* Class that finds array derefs and check if indexes are dynamic. */
>> +class dynamic_sampler_array_indexing_visitor : public
>> ir_hierarchical_visitor
>> +{
>> +public:
>> +   dynamic_sampler_array_indexing_visitor() :
>> +      dynamic_sampler_array_indexing(false)
>> +   {
>> +   }
>> +
>> +   ir_visitor_status visit_enter(ir_dereference_array *ir)
>> +   {
>> +      if (!ir->variable_referenced())
>> +         return visit_continue;
>> +
>> +      if (!ir->variable_referenced()->type->contains_sampler())
>> +         return visit_continue;
>> +
>> +      if (!ir->array_index->constant_expression_value()) {
>> +         dynamic_sampler_array_indexing = true;
>> +         return visit_stop;
>> +      }
>> +      return visit_continue;
>> +   }
>> +
>> +   bool uses_dynamic_sampler_array_indexing()
>> +   {
>> +      return dynamic_sampler_array_indexing;
>> +   }
>> +
>> +private:
>> +   bool dynamic_sampler_array_indexing;
>> +};
>> +
>>   } /* anonymous namespace */
>>
>>   void
>> @@ -2736,6 +2769,34 @@ build_program_resource_list(struct gl_context
>> *ctx,
>>       */
>>   }
>>
>> +/**
>> + * This check is done to make sure we allow only constant expression
>> + * indexing and "constant-index-expression" (indexing with an expression
>> + * that includes loop induction variable).
>> + */
>> +bool
>> +validate_sampler_array_indexing(struct gl_shader_program *prog)
>> +{
>> +   dynamic_sampler_array_indexing_visitor v;
>> +   for (unsigned i = 0; i < MESA_SHADER_STAGES; i++) {
>> +      if (prog->_LinkedShaders[i] == NULL)
>> +     continue;
>> +
>> +      /* Search for array derefs in shader. */
>> +      v.run(prog->_LinkedShaders[i]->ir);
>> +
>> +      if (v.uses_dynamic_sampler_array_indexing()) {
>> +         linker_error(prog,
>> +                      "sampler arrays indexed with non-constant "
>> +                      "expressions is forbidden in GLSL %s %u",
>> +                      prog->IsES ? "ES" : "", prog->Version);
>> +         return false;
>> +      }
>> +   }
>> +
>> +   return true;
>> +}
>> +
>>
>>   void
>>   link_shaders(struct gl_context *ctx, struct gl_shader_program *prog)
>> @@ -2948,6 +3009,16 @@ link_shaders(struct gl_context *ctx, struct
>> gl_shader_program *prog)
>>         lower_const_arrays_to_uniforms(prog->_LinkedShaders[i]->ir);
>>      }
>>
>> +   /* Validation for special cases where we allow sampler array indexing
>> +    * with loop induction variable. This makes sure that all such cases
>> +    * have been turned in to constant expressions.
>> +    */
>> +   if ((!prog->IsES && prog->Version < 130) ||

What about the gpu_shader5 case?

>> +       (prog->IsES && prog->Version < 300)) {
>> +      if (!validate_sampler_array_indexing(prog))
>> +         goto done;
>> +   }
>> +
>>      /* Check and validate stream emissions in geometry shaders */
>>      validate_geometry_shader_emissions(ctx, prog);
>>
>>