Andre and I had debated over whether the shader pipeline should support condition codes (like most processors) or not (like MIPS). With MIPS, this reduces the amount of state that flows from one instruction to the next. But our GPU shader doesn't need any restrictions on this since we do away with pipeline hazards by having more round-robin threads than there are pipeline stages. After some discussion, I am entertaining the idea of not only having condition codes but also predicating every instruction's execution based on them, just like ARM's 32-bit ISA. Predicating like Itanium didn't work our well, because it has rather limited utility for such an elaborate system, but in the limited scope to which ARM has put it, it has worked out well. So, what conditions? I didn't bother looking up ARM's conditions. Instead, I pulled out an MC68000 manual. We need a 4-bit field in the instruction, plus a 5th bit to indicate whether or not the instruction changes the codes.
~C -- carry clear C -- carry set ~Z -- not equal Z -- equal ~V -- not overflow V -- overflow ~N -- non-negative N -- negative N&V + ~N&~V -- greater than or equal (for signed ints) N&~V + ~N&V -- less than (signed) ~C&~Z -- greater than (unsigned) Z + N&~V + ~N&V -- less than or equal (signed) C + Z -- less than or equal (unsigned) N&V&~Z + ~N&~V&~Z -- greater than (signed) T -- always true F -- always false (no-op) FCMP will generate codes that have equivalent meaning. The hardware for this requires an 8-input MUX and an additional XOR for the output: D0 = 1 D1 = C D2 = Z D3 = V D4 = N D5 = N xor V D6 = C or Z D7 = (N xor V) or Z BTW, when I say that I recommend something like this, what I mean is that I suggest adding it as an option that can be enabled. At the very least, we'll want predicated move and we'll want min/max instructions to avoid branching. In a GPU, conditional branching is to be avoided, because it can lead to branch divergence. On a completely unrelated note, when I was studying fuzzy logic back in the 90's, I came up with these formulas, which we'd probably never want to use in a hardware implementation. They're just helpful for doing algebra with min/max fuzzy logic. min(a,b) = (a + b - abs(a-b)) / 2 max(a,b) = (a + b + abs(a-b)) / 2 -- Timothy Normand Miller http://www.cse.ohio-state.edu/~millerti Open Graphics Project _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
