[Bug 89597] Implement SSBOs in GLSL front-end and i965

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89597

--- Comment #18 from Iago Toral <itoral at igalia.com> ---
Jason, I think I got it working for SIMD8 but I have a question regarding
SIMD16:

> (In reply to Jason Ekstrand from comment #16)
> > I think you are misunderstanding how these SIMD8/16 write messages work. 
> > I'll assume 8 in the following discussion but it all applies to 16.
> > 
> > As the shader executes, it is executes 8 pixels at a time.  Each
> > sub-register represents the same symbolic value in GLSL but for a different
> > pixel.  Suppose I have an SSBO declared as follows:
> > 
> > buffer Block {
> >     vec4 s[128];
> > };
> > 
> > And suppose I execute the line of code "s[i].xzw = foo;" where foo is some
> > vec3.  When the SIMD8 shader reaches this line, it stores 12 values in the
> > SSBO; 3 per pixel.  If the client doesn't want the values to stomp on each
> > other, it is up to the client to ensure that i is different for each pixel.
> > 
> > How does this work with the scattered read/write messages?  They are
> > designed for exactly a case like this.  When you get to this statement, you
> > will have one register that holds the value of i and three more for foo. 
> > Each of these registers has 8 sub-registers one for each SIMD channel (or
> > pixel).

In SIMD16 the instructions operate on 16 elements, but I understand that
registers still have 8 sub-registers, so this instruction:

mov(16)   g116<1>F     1.0F                { align1 1H };

is writing 1.0 in all sub-registers of g116 (8 elements) and all sub-registers
of g117 (8 elements). Is this correct? If I am correct, then I would expect
this assembly code for a SIMD16 scattered write to work:

mov(8)    g113<1>UD    g0<8,8,1>UD         { align1 WE_all 1Q compacted };
mov(1)    g113.2<1>UD  0x00000000UD        { align1 WE_all compacted };
mov(16)   g114<1>UD    g13<8,8,1>UD        { align1 1H compacted };
mov(16)   g116<1>F     1.0F                { align1 1H };
send(16)  g0<1>F       g113<8,8,1>F
        data ( DC DWORD scatterd write, 1, 3) mlen 5 rlen 0 { align1 1H };

The first mov(16) would write the offset payload to M1,M2 (g114,g115) and the
second mov(16) would write the data payload to M3,M4 (g116,g117). However, I
see that this does not produce correct writes into the buffer, I see writes to
the correct offsets but with wrong data, so I guess I am understanding
something wrong again?.

For the record, this same code works fine if I make the second mov(16) write to
g115 (like I do in SIMD8, where we want offsets in M1 and data in M2), but as
far as my understanding goes, this should actually be incorrect for SIMD16.

> > All you should have to do is build 3 messages each one of which is
> > i + some math for the address part and a component of foo for the payload
> > part.  Each scattered write writes 8 values but they are the different
> > values from the different SIMD channels, not from different components of
> > foo.  The first one will write all 8 of the s[i].x, the next one s[i].y, etc.

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