I saw on stackoverflow, there are “Parallel bit deposit and extract” instructions, which are part of BMI2 (Bit Manipulation Instruction Set 2). Are these instructions (PDEP and PEXT) available to Julia / LLVM? algorithm - Find nth SET bit in an int - Stack Overflow ) x86 Bit manipulation instructi…

I’ve used this: function has_bmi2() CPUInfo = zeros(Int32, 4) ccall(:jl_cpuidex, Cvoid, (Ptr{Cint}, Cint, Cint), CPUInfo, 7, 0) CPUInfo[2] & 0x100 != 0 end Cf. this post . There’s also CpuId.jl .

There is an old issue Support AVX2 and BMI2 instructions , which is now closed. Does that mean that BMI2 instructions are now supported? @StefanKarpinski , I saw this old post from google groups, LLVM: trying to use x86 pext intrinsic Did you manage to get it working?

LLVM has had support for it for a long time and now that https://github.com/JuliaLang/julia/pull/21849 is merged the nightly that includes it should be able to use those for JIT. It’s then a matter of what code pattern LLVM recognizes.

OK thanks for that. I think I found how to call PDEP and PEXT explicitly: pdep(x::UInt32, y::UInt32) = ccall("llvm.x86.bmi.pdep.32", llvmcall, UInt32, (UInt32, UInt32), x, y) pdep(x::UInt64, y::UInt64) = ccall("llvm.x86.bmi.pdep.64", llvmcall, UInt64, (UInt64, UInt64), x, y) pext(x::UInt32, y::UI…

[image] greg_plowman: Firstly, is this the right way? A rought search through LLVM code (searching for pdep or PDEP) suggests that LLVM doesn’t have any code that do it automatically so this is likely the right/only way right now. [image] greg_plowman: Secondly, how can determine if a Ju…

[image] greg_plowman: @StefanKarpinski , I saw this old post from google groups, LLVM: trying to use x86 pext intrinsic Did you manage to get it working? I did, but only on a machine that supported that instruction set, of course.

Thanks Yichao. In the meantime I can use an environment variable (say "JULIA_USEBMI2") to opt-in to using these instructions.

[image] yuyichao: Secondly, how can determine if a Julia build / hardware supports this instruction? That’ll be the next step for my PR. Any update on this? Is there a way to programmatically determine if an intrinsic instruction (e.g. PDEP and PEXT) is supported?

Thanks! This is exactly what I was looking for.

Bit Manipulation Instruction Set

General Usage

Topic		Replies	Views
Calling AVX-512 intrinsics from Julia General Usage bit-twiddling	6	913	July 4, 2023
Julia equivalent of C compiler intrinsics? General Usage	23	3395	November 8, 2018
Check availability of AVX-512, AVX2 etc? General Usage	1	229	September 17, 2025
Bit manipulations with llvmcall give strange results General Usage llvm	11	430	March 9, 2024
How to compile AVX instructions General Usage simd	1	1362	May 14, 2017

Bit Manipulation Instruction Set

Related topics