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This provides some background the design of the generated headers. We
started out trying to generate bit fields but it evolved into the pack
functions because of a few limitations:
1) Bit fields still generate terrible code today. Even with modern
optimizing compilers you get multiple load+mask+store operations
to the same dword in memory as you set individual bits. The
compiler also has to generate code to mask out overflowing values
(for example, if you assign 200 to a 2 bit field). Our driver
never writes overflowing values so that's not needed. On the
other hand, most compiler recognize that the template struct we
use is a temporary variable and copy propagate the individual
fields and do amazing constant folding. You should take a look
at the code that gets generated when you compile in release mode
with optimizations.
2) For some types we need to have overlapping bit fields. For
example, some values are 64 byte aligned 32 bit offsets. The
lower 5 bits of the offset are always zero, so the hw packs in a
few misc bits in the lower 5 bits there. Other times a field can
be either a u32 or a float. I tried to do this with overlapping
anonymous unions and it became a big mess. Also, when using
initializers, you can only initialize one union member so this
just doesn't work with out approach.
The pack functions on the other hand allows us a great deal of
flexibility in how we combine things. In the case of overlapping
fields (the u32 and float case), if we only set one of them in
the pack function, the compiler will recognize that the other is
initialized to 0 and optimize out the code to or it it.
3) Bit fields (and certainly overlapping anonymous unions of bit
fields) aren't generally stable across compilers in how they're
laid out and aligned. Our pack functions let us control exactly
how things get packed, using only simple and unambiguous bitwise
shifting and or'ing that works on any compiler.
Once we have the pack function it allows us to hook in various
transformations and validation as we go from template struct to dwords
in memory:
1) Validation: As I said above, our driver isn't supposed to write
overflowing values to the fields, but we've of course had lots of
cases where we make mistakes and write overflowing values. With
the pack function, we can actually assert on that and catch it at
runtime. bitfields would just silently truncate.
2) Type conversions: some times it's just a matter of writing a
float to a u32, but we also convert from bool to bits, from
floats to fixed point integers.
3) Relocations: whenever we have a pointer from one buffer to
another (for example a pointer from the meta data for a texture
to the raw texture data), we have to tell the kernel about it so
it can adjust the pointer to point to the final location. That
means extra work we have to do extra work to record and annotate
the dword location that holds the pointer. With bit fields, we'd
have to call a function to do this, but with the pack function we
generate code in the pack function to do this for us. That's a
lot less error prone and less work.
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