[Mesa-dev] [PATCH v4 (part2) 49/59] glsl: Do not allow assignments to read-only variables

[Iago Toral]

On Wed, 2015-08-05 at 22:22 +1000, Timothy Arceri wrote:
> On Wed, 2015-08-05 at 13:45 +0200, Iago Toral wrote:
> > On Wed, 2015-08-05 at 20:04 +1000, Timothy Arceri wrote:
> > > On Wed, 2015-08-05 at 10:30 +0200, Iago Toral Quiroga wrote:
> > > > ---
> > > >  src/glsl/ast_to_hir.cpp | 9 ++++++++-
> > > >  1 file changed, 8 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/src/glsl/ast_to_hir.cpp b/src/glsl/ast_to_hir.cpp
> > > > index e834a46..518612d 100644
> > > > --- a/src/glsl/ast_to_hir.cpp
> > > > +++ b/src/glsl/ast_to_hir.cpp
> > > > @@ -811,8 +811,15 @@ do_assignment(exec_list *instructions, struct 
> > > > _mesa_glsl_parse_state *state,
> > > >     }
> > > >  
> > > >     ir_variable *lhs_var = lhs->variable_referenced();
> > > > -   if (lhs_var)
> > > > +   if (lhs_var) {
> > > > +      if (lhs_var->data.image_read_only) {
> > > 
> > > It looks like data.read_only is always set to true for images so wouldn't 
> > > this
> > > already be caught already by the existing read-only check?
> > > 
> > >       else if (lhs_var != NULL && lhs_var->data.read_only) {
> > >          _mesa_glsl_error(&lhs_loc, state,
> > >                           "assignment to read-only variable '%s'",
> > >                           lhs_var->name);
> > 
> > Not as it is now, because with SSBOs we only set image_read_only and not
> > read_only when the readonly qualifier is used. I suppose this is what we
> > are expected to do since the SSBO spec says that behavior for these
> > qualifiers on SSBOs is the same as for images:
> > https://www.opengl.org/registry/specs/ARB/shader_storage_buffer_object.txt
> > 
> > "Modify Section 4.10, Memory Qualifiers (p. 71)"
> > (...)
> > "(insert after third paragraph, p. 73) The memory qualifiers "coherent",
> > "volatile", "restrict", "readonly", and "writeonly" may be used in the
> > declaration of buffer variables (i.e., members of shader storage blocks).
> > When a buffer variable is declared with a memory qualifier, the behavior
> > specified for memory accesses involving image variables described above
> > applies identically to memory accesses involving that buffer variable.  It
> > is an error to assign to a buffer variable qualified with "readonly" or to
> > read from a buffer variable qualified with "writeonly".
> > 
> > What is a bit confusing for me is that images seem to set
> > image_read_only depending on whether we used the readonly qualifier or
> > not (like ssbos) but then they also set read_only to true
> > unconditionally, so I guess there is a difference between both fields,
> 
> Asking what the difference is was originally going to be my first question to
> you :)
> 
> > but I don't know what it is exactly, specially since you can also use
> > writeonly on images, for example.
> 
> So I really dont know much about images but after some reading the conclusion
> I've come to is the qualifiers (image_read_only) are meant to limit how you
> can use imageStore(), imageLoad() and imageAtomic*() etc.

Looking at ARB_shader_image_load_store that seems consistent...  In that
case I imagine that we could just set read_only for buffer variables
with the readonly qualifier instead of image_read_only and drop this
patch. We will need to add, at least, write_only to ir_variable as well
I guess... I imagine that the 3 other fields (image_coherent,
image_restrict, image_volatile) do not have image-specific semantics
like image_read_only and image_write_oly and can be shared with ssbos
we do not have to replicate them in ir_variable as well (in that case we
might want to rename them so it is clear that image_read_only and
image_write_only really are special and specific to images)

Curro, what do you think?

> On the other hand read_only is the usual uniform restriction stoping you from
> assigning to the variable directly e.g myImage = 1; which is why its always
> set to true.
> 
> If I'm correct I dont think this patch is needed.
> 
> > 
> > In any case, since we have both read_only and image_read_only in
> > ir_variable at present, I think it makes sense to have checks for both
> > of them, if one of them ends up being redundant the right thing to do
> > would be to kill it completely I guess, otherwise it only gets (even)
> > more confusing.
> > 
> > Iago
> > 
> > > 
> > > > +         _mesa_glsl_error(&lhs_loc, state,
> > > > +                          "assignment to read-only variable `%s'",
> > > > +                          lhs_var->name);
> > > > +         error_emitted = true;
> > > > +      }
> > > >        lhs_var->data.assigned = true;
> > > > +   }
> > > >  
> > > >     if (!error_emitted) {
> > > >        if (non_lvalue_description != NULL) {
> > > 
> > 
> > 
>