4 ms·

Do you think other compilers for other languages don't perform optimizations like this?

adgjlsfhk1 2y ago
Julia does all of this correctly (although there are a few 0 cost redundant move instructions presumably for ABI reasons). julia> f(x) = @fastmath sqrt.(x) julia > @code_native debuginfo=:none f.(Tuple(rand(Float32, 8))) vsqrtps ymm0, ymmword ptr [rsi] mov rbp, rsp mov rax, rdi vmovups ymmword ptr [rdi], ymm0 pop rbp vzeroupper ret julia> g() = f((1f0, 2f0, 3f0, 4f0)) julia> @code_native debuginfo=:none g() .LCPI0_0: .long 0x3f800000 # float 1 .long 0x3fb504f3 # float 1.41421354 .long 0x3fddb3d7 # float 1.73205078 .long 0x40000000 # float 2 .text .globl julia_g_600 .p2align 4, 0x90 .type julia_g_600,@function julia_g_600: # @julia_g_600 # %bb.0: # %top push rbp movabs rcx, offset .LCPI0_0 mov rbp, rsp mov rax, rdi vmovaps xmm0, xmmword ptr [rcx] vmovups xmmword ptr [rdi], xmm0 pop rbp ret
- g15jv2dp 2y ago
  ...so does LLVM, unless you specifically instruct it to do otherwise with -ffast-math. Don't be fooled by the name of this flag, if it were possible to just do math faster without compromising anything, it wouldn't be a flag, it would be the default. It seems that Julia has a flag with the same name but a different behavior. Okay? What's your point exactly?
  - chongli 2y ago
    But in this case, -ffast-math actually results in slower math, so the flag is badly named. Naming things is important (and hard). The naive expectation of a flag named -ffast-math is that the math should never be slower, sometimes be faster, and potentially less accurate and/or error prone. The fact that it can also be sometimes slower means the flag really should be named -fdifferent-math or (uncharitably) -funreliable-math.
    - tcbawo 2y ago
      It’s too bad that it hasn’t acquired the name of an author to describe this alternative functionality more concisely without being judgy (like Nagle’s algorithm). I looked around, and it seems that there isn’t any one designer/author that you could give credit to.
  - adgjlsfhk1 2y ago
    @fastmath in Julia is telling LLVM (basically) the same thing. I think the only difference is that Julia doesn't think rsqrt(x)*x with Newton iteration is a good way of computing sqrt

Julia does all of this correctly (although there are a few 0 cost redundant move instructions presumably for ABI reasons). julia> f(x) = @fastmath sqrt.(x) julia > @code_native debuginfo=:none f.(Tuple(rand(Float32, 8))) vsqrtps ymm0, ymmword ptr [rsi] mov rbp, rsp mov rax, rdi vmovups ymmword ptr [rdi], ymm0 pop rbp vzeroupper ret julia> g() = f((1f0, 2f0, 3f0, 4f0)) julia> @code_native debuginfo=:none g() .LCPI0_0: .long 0x3f800000 # float 1 .long 0x3fb504f3 # float 1.41421354 .long 0x3fddb3d7 # float 1.73205078 .long 0x40000000 # float 2 .text .globl julia_g_600 .p2align 4, 0x90 .type julia_g_600,@function julia_g_600: # @julia_g_600 # %bb.0: # %top push rbp movabs rcx, offset .LCPI0_0 mov rbp, rsp mov rax, rdi vmovaps xmm0, xmmword ptr [rcx] vmovups xmmword ptr [rdi], xmm0 pop rbp ret

g15jv2dp 2y ago
...so does LLVM, unless you specifically instruct it to do otherwise with -ffast-math. Don't be fooled by the name of this flag, if it were possible to just do math faster without compromising anything, it wouldn't be a flag, it would be the default. It seems that Julia has a flag with the same name but a different behavior. Okay? What's your point exactly?
- chongli 2y ago
  But in this case, -ffast-math actually results in slower math, so the flag is badly named. Naming things is important (and hard). The naive expectation of a flag named -ffast-math is that the math should never be slower, sometimes be faster, and potentially less accurate and/or error prone. The fact that it can also be sometimes slower means the flag really should be named -fdifferent-math or (uncharitably) -funreliable-math.
  - tcbawo 2y ago
    It’s too bad that it hasn’t acquired the name of an author to describe this alternative functionality more concisely without being judgy (like Nagle’s algorithm). I looked around, and it seems that there isn’t any one designer/author that you could give credit to.
- adgjlsfhk1 2y ago
  @fastmath in Julia is telling LLVM (basically) the same thing. I think the only difference is that Julia doesn't think rsqrt(x)*x with Newton iteration is a good way of computing sqrt