corsix
Joined 12 years ago
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234
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54
Member since
Sep 23
Recent submissions
Comment · 1 year ago
In this case, "atomic 128-bit store" is the special instruction, with the twist that half of those 128 bits contain a pointer.
Comment · 1 year ago
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.
Comment · 2 years ago
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
Comment · 2 years ago
For an implementation of logical immediate encoding without the loop, see https://github.com/LuaJIT/LuaJIT/blob/04dca7911ea255f37be799...
Comment · 2 years ago
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.
Comment · 2 years ago
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!!
endComment · 2 years ago
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.
Comment · 2 years ago
close() is documented as a cancellation point, and is the kind of syscall that might crop up in a destructor.
Comment · 3 years ago
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).
Comment · 3 years ago
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.
Comment · 3 years ago
https://www.corsix.org/content/whirlwind-tour-aarch64-vector... is my take on NEON, albeit not quite the same form factor as the OP.
Comment · 3 years ago
I think https://github.com/LuaJIT/LuaJIT/commit/6a2163a6b45d6d251599... improved things a bit, notably making automatic tarballs work again.
Comment · 3 years ago
I’m pleased to see FUTEX2_SIZE_U64, but saddened that it isn’t actually implemented. It has always seemed like a very useful primitive to have.