[Bug 92760] Add FP64 support to the i965 shader backends

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=92760

--- Comment #26 from Samuel Iglesias <siglesias at igalia.com> ---
I found an issue related to having interleaved uniform definitions of 32-bit
and 64-bit data types in the push constant buffer.

The bug is easily shown when we have a double defined just after a 32-bit data
type. For example, we have following definition in a GLSL fragment shader:

    uniform double arg0;
    uniform bool arg1;
    uniform double arg2;

The generated code that copies those push constant values does the following in
SIMD16:

mov(8)          g19<1>DF        g2<0,1,0>DF
mov(8)          g23<1>DF        g2<0,1,0>DF
mov(16)         g9<1>D          g2.2<0,1,0>D
mov(8)          g5<1>DF         g2.1<0,1,0>DF
mov(8)          g7<1>DF         g2.1<0,1,0>DF

As you see, there is a misalignment in the memory access that copies 'arg2'
contents: we are copying the 32 bits of arg1 into the copy of arg2 (notice that
g2.1<0,1,0>DF is at the same offset than g2.2<0,1,0>D).

My proposal is to do a 64-bit alignment when uploading push constant doubles
and when reading them from the push constant buffer. The 32-bit push constants'
upload and access would not be changed. So the generated code for the same
example would be like:

mov(8)          g19<1>DF        g2<0,1,0>DF
mov(8)          g23<1>DF        g2<0,1,0>DF
mov(16)         g9<1>D          g2.2<0,1,0>D
mov(8)          g5<1>DF         g2.2<0,1,0>DF
mov(8)          g7<1>DF         g2.2<0,1,0>DF

This solution has the drawback of adding padding inside push constant buffer
when we have a mixture of 32 bits and 64-bit data type constants, so it is not
memory efficient; plus take it into account to avoid exceeding the push buffer
size limitation. The advantage is that it does not add new instructions in the
generated code.

Do you like the proposed solution? Or do you have other solution in mind?

BTW, I expect to have a similar problem when reading doubles from the pull
constant buffer contents but I have not checked it yet.

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