LLVMDev would be the right mailing list. Alternatively you could probably also look at the x86 patterns and see which one matches the instruction you're trying to generate.
On Mon, Apr 21, 2014 at 11:02 AM, Stefan Karpinski <[email protected]>wrote: > LLVM seems to figure out the rotation idiom just fine here: > > julia> rot8(x) = (x << 8) | (x >>> 56) > rot8 (generic function with 1 method) > > julia> code_llvm(rot8,(Uint,)) > > define i64 @julia_rot818217(i64) { > top: > %1 = shl i64 %0, 8, !dbg !5098 > %2 = lshr i64 %0, 56, !dbg !5098 > %3 = or i64 %1, %2, !dbg !5098, !julia_type !5099 > ret i64 %3, !dbg !5098 > } > > julia> code_native(rot8,(Uint,)) > .section __TEXT,__text,regular,pure_instructions > Filename: none > Source line: 1 > push RBP > mov RBP, RSP > Source line: 1 > rol RDI, 8 > mov RAX, RDI > pop RBP > ret > > > If there are cases where it's not detecting this idiom, we could always > provide Julia intrinsics for bit rotation that emit instructions that we're > sure LLVM will translate to x86 rotation instructions. Seems kind of silly, > but that's one way to do it. The bit reversing thing is much harder - I'm > not sure if there is an idiom that LLVM can detect. We could ask on the > appropriate LLVM list. Keno, Jameson - you guys know where it would be good > to ask about this? > > > On Sun, Apr 20, 2014 at 8:39 PM, Laszlo Hars <[email protected]> wrote: > >> Stefan, do you know about any plans of LLVM to support rotations and bit >> reversals in the near future? Those are the most important ones missing. I >> tried rot8(x) = x<<8 | x>>>56, and in my Windows-7 x64 machine it was slow >> (more than twice slower than a shift alone). Bit reversing Int64 takes >> about 40 machine instructions when coded in C or Julia (without huge lookup >> tables), instead of the appropriate single machine instruction in Intel >> CPUs. >> > >
