corsix

In this case, "atomic 128-bit store" is the special instruction, with the twist that half of those 128 bits contain a pointer.

corsix

vorig jaardiscuss

You can start with this idea, and then make it _very_ performant by using bit counting instructions. See https://www.corsix.org/content/higher-quality-random-floats for an exposition.

corsix

2 jaar geledendiscuss

Indeed, LuaJIT support for Windows/Arm64 was added in https://github.com/LuaJIT/LuaJIT/issues/593, and there’s experimental out-of-tree support for Windows/Arm64EC in https://github.com/LuaJIT/LuaJIT/issues/1096

corsix

2 jaar geledendiscuss

For an implementation of logical immediate encoding without the loop, see https://github.com/LuaJIT/LuaJIT/blob/04dca7911ea255f37be799...

corsix

2 jaar geledendiscuss

AArch64 NEON has the URSQRTE instruction, which gets closer to the OP's question than you might think; view a 32-bit value as a fixed-precision integer with 32 fractional bits (so the representable range is evenly spaced 0 through 1-ε, where ε=2^-32), then URSQRTE computes the approximate inverse square root, halves it, then clamps it to the range 0 through 1-ε. Fixed-precision integers aren't quite integers, and approximate inverse square root isn't quite square root, but it might get you somewhere close.

The related FRSQRTE instruction is much more conventional, operating on 32-bit floats, again giving approximate inverse square root.

corsix

2 jaar geledendiscuss

Unfortunately things aren't so simple, as when doing JIT compilation, LuaJIT _will_ try to shorten the lifetimes of local variables. Using the latest available version of LuaJIT (https://github.com/LuaJIT/LuaJIT/commit/0d313b243194a0b8d239...), the following reliably fails for me:

  local ffi = require"ffi"
  local function collect_lots()
    for i = 1, 20 do collectgarbage() end
  end
  local function f(s)
    local blob = ffi.new"int[2]"
    local interior = blob + 1
    interior[0] = 13 -- should become the return value
    s:gsub(".", collect_lots)
    return interior[0] -- kept alive by blob?
  end
  for i = 1, 60 do
    local str = ("x"):rep(i - 59)
    assert(f(str) == 13) -- can fail!!
  end

corsix

3 jaar geledendiscuss

Some compute might be on the AMX units (dedicated matrix multiplication coprocessor, closely attached to the CPU, distinct from both ANE and GPU). They gained bf16 support in M2.

corsix

3 jaar geledendiscuss

close() is documented as a cancellation point, and is the kind of syscall that might crop up in a destructor.

corsix

3 jaar geledendiscuss

Using your cdf framing, while there is a point about [0, 1) versus (0, 1] intervals, the bigger point of the article is about whether said cdf holds for any IEEE-754 double-precision p, or whether it only holds for p of the form i*2^-53 (for integer i).

corsix

3 jaar geledendiscuss

Oh, cute. It looks like they ever-so-slightly overweight the probability of values whose mantissa is entirely zeroes though. For example, the probability of hitting exactly 0.5 should be 2^-54 + 2^-55, whereas zig looks to give it 2^-54 + 2^-54.

corsix

3 jaar geledendiscuss

https://www.corsix.org/content/whirlwind-tour-aarch64-vector... is my take on NEON, albeit not quite the same form factor as the OP.

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