> Namely, you need to know "statically, at code gen time",
> the shuffle constants that are given as "immediates" to
> the instructions!
Can you clarify where the extra difficulty is?
I'm ignorant of GHC, but I'd think that from the compilers POV all that matters is that that the operand is a constant. Then it's just a matter of putting the value in instead of the name. In GCC inline assembly, you can use the poorly documented '%c' prefix to have a number treated as an immediate, so I'd guess this must be possible internally too. Also possibly worth noting is that unlike the others, PSHUFB works from a register rather a value encoded into the instruction.
There's more than one shuffle instruction, in fact there's quite a few! You're right that some can take register args, but those aren't the ones I care about as much.
You're right, there are hacks in c that handle that. My goal for ghc is to actually have a systematic solution for handling any sort of constant literal expression at compile time. This includes making it easy to add new primops that require compile time literal data.
There's some interesting implications if you want that restriction to be typecheckable! This includes having a "static data kind". Part of why you want that is also because ghc is great at common sub expression elimination, and I consider any implementation strategy that could be broken by compiler optimization to be unacceptable.
[edit to clarify: just naively using normal literal values would likely be subject to cse optimization, and having code gen need to look around to lookup a variable rather than being a localized process is somewhat horrifying]
One particular end goal of mine is this: SIMD isn't that complicated, and it's really easy to experiment with (but only if you can cope with c). I want to make experimenting with SIMD much more accessible.
Interestingly enough, the notion of static data I want seems like it might be an especially strong version of the notion of static data that Cloud Haskell (the distributed process lib) would like. So there may be some trickle Down there!
One really cool optimization having a proper notion of static literals might enable is making it much easier to generate things like static lookup tables and related data structures that are small and perf sensitive
Edit: also if you want to try and stare at the source for a serious compiler, ghc (while huge) is pretty darn readable. Just pick a piece you want to understand and stare at the code for a while!
Edit: I should add that Geoff mainland has some great preliminary work adding experimental simd support to ghc that's in head/ pending 7.8. That said, ghc support for interesting simd won't be ready for prime time till 7.10 in a yearish
Comments
I'm ignorant of GHC, but I'd think that from the compilers POV all that matters is that that the operand is a constant. Then it's just a matter of putting the value in instead of the name. In GCC inline assembly, you can use the poorly documented '%c' prefix to have a number treated as an immediate, so I'd guess this must be possible internally too. Also possibly worth noting is that unlike the others, PSHUFB works from a register rather a value encoded into the instruction.
There's more than one shuffle instruction, in fact there's quite a few! You're right that some can take register args, but those aren't the ones I care about as much.
You're right, there are hacks in c that handle that. My goal for ghc is to actually have a systematic solution for handling any sort of constant literal expression at compile time. This includes making it easy to add new primops that require compile time literal data.
There's some interesting implications if you want that restriction to be typecheckable! This includes having a "static data kind". Part of why you want that is also because ghc is great at common sub expression elimination, and I consider any implementation strategy that could be broken by compiler optimization to be unacceptable.
[edit to clarify: just naively using normal literal values would likely be subject to cse optimization, and having code gen need to look around to lookup a variable rather than being a localized process is somewhat horrifying]
One particular end goal of mine is this: SIMD isn't that complicated, and it's really easy to experiment with (but only if you can cope with c). I want to make experimenting with SIMD much more accessible.
Interestingly enough, the notion of static data I want seems like it might be an especially strong version of the notion of static data that Cloud Haskell (the distributed process lib) would like. So there may be some trickle Down there!
One really cool optimization having a proper notion of static literals might enable is making it much easier to generate things like static lookup tables and related data structures that are small and perf sensitive
Edit: also if you want to try and stare at the source for a serious compiler, ghc (while huge) is pretty darn readable. Just pick a piece you want to understand and stare at the code for a while!
Edit: I should add that Geoff mainland has some great preliminary work adding experimental simd support to ghc that's in head/ pending 7.8. That said, ghc support for interesting simd won't be ready for prime time till 7.10 in a yearish