[virglrenderer-devel] Float /integer interaction with undefined behavior

[Lepton Wu]

FYI,  I upload a WIP CL to show the idea in my mind to fix this issue:

My plan is:  introduce integer type in generated GLSL, also I also plan to
change the default type to int instead of float for all temp registers and
if it's possible (for example, a flat inout between shaders), also change
those generic registers to integer.  The idea hea is:  it's safe to  call
floatbitsToInt while intBitsToFloat doesn't work all the time.



On Tue, May 12, 2020 at 6:55 PM Tao Wu(吴涛@Eng) <lepton at google.com> wrote:

> Hi,
>
> I am trying run virglrender on mali and this uncovered one issue in virgl,
> I am trying to get a fix but I'd like to hear from you since
> that could save lots of time.
>
> Currently in virgl, most register are used as "float", so for in/out in
> guest shader like "in ivec4 in1" or "out ivec4 out1", we actually
> are creating shaders like "in vec4 in1" and "out vec4 out1", but this
> could work because 2 undefined behavior (according to opengl
> document):
>
> The 1st thing is: intBitsToFloat and uintBitsToFloat return the encoding
> passed in parameter x as a highp floating-point value. If the encoding of a
> NaN is passed in x, it will not signal and the resulting value will be
> undefined. That means actually for some GPU,  we can't really get back x
> with  floatBitsToInt(intBitsToFloat(x)) for some X.
>
> The 2nd thing is: The general type of attribute used in the vertex shader
> must match the general type provided by the attribute array. This is
> governed by which glVertexAttribPointer function you use. For
> floating-point attributes, you must use glVertexAttribPointer. For integer
> (both signed and unsigned), you must use glVertexAttribIPointer. And for
> double-precision attributes, where available, you must use
> glVertexAttribLPointer.  For the issue I am hitting, it's sightly different,
> but I believe it's similar: it seems we are calling glVertexAttribIFormat
> while the attibute in vertex shader was defined as float, We also have
> similar thing for output.
> For example, fsout_c0 is defined as float in fragment shader while the
> surface is in GL_R32I format.
>
> All of these works fine with intel gpu backends. But according to spec,
> it's undefined behavior and cause trouble on mali.
>
> It looks like there is no type information in tgsi  (or actually they are
> there already?) for fixing this, any suggestions?
>
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