[Mesa-dev] [PATCH v2 26/82] glsl: Don't do copy propagation on buffer variables

[Iago Toral]

On Mon, 2015-06-22 at 14:28 -0700, Jordan Justen wrote:
> 24-26 once again makes me wonder if these optimization *can* be used
> with SSBOs based on the same ext spec wording I referenced before:
> 
> "The ability to write to buffer objects creates the potential for
>  multiple independent shader invocations to read and write the same
>  underlying memory. The same issue exists with the
>  ARB_shader_image_load_store extension provided in OpenGL 4.2, which
>  can write to texture objects and buffers. In both cases, the
>  specification makes few guarantees related to the relative order of
>  memory reads and writes performed by the shader invocations."
> 
> In these patches "other threads" were specifically mentioned.
> 
> Did these patches also prevent bad things from happening in generated
> code? (Like mentioned for patch 23.)

I think the problem here is the possibility for shaders to use
memoryBarrier():

"SHADER_STORAGE_BARRIER_BIT:  Memory accesses using shader buffer
variables issued after the barrier will reflect data written by
shaders prior to the barrier.  Additionally, assignments to and atomic
operations performed on shader buffer variables after the barrier will
not execute until all memory accesses (e.g., loads, stores, texture
fetches, vertex fetches) initiated prior to the barrier complete."

I think in these cases we can't allow these optimizations to kick in. 

That said, maybe we can check if we are using any memorybarriers in the
shader code and decide if we want to enable these optimizations for
ssbos based on that. I think we can try to do that in a later patch.

Iago

> -Jordan
> 
> On 2015-06-03 00:01:16, Iago Toral Quiroga wrote:
> > Since the backing storage for these is shared we cannot ensure that the
> > value won't change by writes from other threads.
> > ---
> >  src/glsl/opt_copy_propagation.cpp | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/glsl/opt_copy_propagation.cpp b/src/glsl/opt_copy_propagation.cpp
> > index 806027b..f206995 100644
> > --- a/src/glsl/opt_copy_propagation.cpp
> > +++ b/src/glsl/opt_copy_propagation.cpp
> > @@ -330,7 +330,7 @@ ir_copy_propagation_visitor::add_copy(ir_assignment *ir)
> >           */
> >          ir->condition = new(ralloc_parent(ir)) ir_constant(false);
> >          this->progress = true;
> > -      } else {
> > +      } else if (lhs_var->data.mode != ir_var_shader_storage) {
> >          entry = new(this->acp) acp_entry(lhs_var, rhs_var);
> >          this->acp->push_tail(entry);
> >        }
> > -- 
> > 1.9.1
> > 
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