PEXT instruction

ericlang · July 18, 2025, 12:43pm

I saw in another post and in the zig-todo’s that PEXT will be added as a builtin.
I am going to experiment with that instruction.
How would I do a pext right now in Zig?

ppipelin · July 18, 2025, 2:59pm

Here is the active PR for this fyi : Implement @depositBits and @extractBits by bnuuydev · Pull Request #23474 · ziglang/zig · GitHub

Validark · July 18, 2025, 3:38pm

I took out the polyfill for now but you can get the intrinsic here:

github.com/Validark/Accelerated-Zig-Parser

src/main.zig

main


      
                      '&' => left & right,
                      '^' => left ^ right,
                      '|' => left | right,
                      else => unreachable,
                  };
              }
          
              return .{ pos, left };
          }
          
          fn pext(a: anytype, b: anytype) if (@bitSizeOf(@TypeOf(a)) >= @bitSizeOf(@TypeOf(b))) @TypeOf(a) else @TypeOf(b) {
              const T = if (@bitSizeOf(@TypeOf(a)) >= @bitSizeOf(@TypeOf(b))) @TypeOf(a) else @TypeOf(b);
          
              if (@inComptime() or !HAS_FAST_PDEP_AND_PEXT) {
                  @compileError("pls add pext implementation");
                  // var src = a;
                  // var mask = b;
                  // var result: T = 0;
          
                  // while (true) {
                  //     // 1. isolate the lowest set bit of mask

ericlang · July 30, 2025, 2:46pm

That is some unknown dark art. Anyway this enables me to do my pext experiment. Thanks.

    const methods = struct
    {
        extern fn @"llvm.x86.bmi.pext.32"(u32, u32) u32;
        extern fn @"llvm.x86.bmi.pext.64"(u64, u64) u64;
        extern fn @"llvm.ppc.pextd"(u64, u64) u64;
    };

Validark · July 30, 2025, 3:21pm

Come to the dark side, we have cookies!

voroskoi · September 15, 2025, 9:13pm

When I try to use these functions I get an error starting like this:
LLVM ERROR: Cannot select: 0x40efc630: i64 = X86ISD::PEXT Constant:i64<0>, 0x417cd150...

I assume that I need some configuration here. Just putting extern before those functions seems a bit magic to me.

Sze · September 15, 2025, 9:52pm

Not sure if thats it, but it might, are you sure you are using the llvm backend?
From 0.15.1 Release Notes ⚡ The Zig Programming Language :

you can use LLVM backend for Debug builds by passing -fllvm on the command-line or by setting .use_llvm = true when creating a std.Build.Step.Compile

voroskoi · September 20, 2025, 6:58pm

Yes, I am using llvm. I have ended up using asm, because I was unable to figure out how this should work.