-ffast-math is global though and can break other libraries. I mean local, vectorization compatible but portable syntax, without actually writing it (let compiler do the work)

According to https://stackoverflow.com/questions/40699071/can-i-make-my-c... you annotate it with: __attribute__((optimize("-ffast-math")))

I tried it with:

  double sum1(double arr[128]) {
    double tot = 0.0;
    for (int i=0; i<128; i++) {
      tot += arr[i];
    }
    return tot;
  }

  __attribute__((optimize("-ffast-math")))
  double sum2(double arr[128]) {
    double tot = 0.0;
    for (int i=0; i<128; i++) {
      tot += arr[i];
    }
    return tot;
  }

The Compiler Explorer (gcc 13.2, --std=c++20 -march=native -O3) generates two different bodies for those:

    sum1(double*):
        lea     rax, [rdi+1024]
        vxorpd  xmm0, xmm0, xmm0
    .L2:
        vaddsd  xmm0, xmm0, QWORD PTR [rdi]
        add     rdi, 32
        vaddsd  xmm0, xmm0, QWORD PTR [rdi-24]
        vaddsd  xmm0, xmm0, QWORD PTR [rdi-16]
        vaddsd  xmm0, xmm0, QWORD PTR [rdi-8]
        cmp     rax, rdi
        jne     .L2
        ret
    sum2(double*):
        lea     rax, [rdi+1024]
        vxorpd  xmm0, xmm0, xmm0
    .L6:
        vaddpd  ymm0, ymm0, YMMWORD PTR [rdi]
        add     rdi, 32
        cmp     rax, rdi
        jne     .L6
        vextractf64x2   xmm1, ymm0, 0x1
        vaddpd  xmm1, xmm1, xmm0
        vunpckhpd       xmm0, xmm1, xmm1
        vaddpd  xmm0, xmm0, xmm1
        vzeroupper
        ret

It is still compiler-specific and non-portable, but at least it is not global.

I'm guilty of having used it, but form the doc: " The optimize attribute should be used for debugging purposes only. It is not suitable in production code."

It can be very unreliable.

One of the things I struggled with in Rust’s portable_simd is that I think fadd_fast and simd intrinsics don’t mix well. This makes writing dot products only ok, rather than really easy. Here’s (one) reference: https://internals.rust-lang.org/t/suggestion-ffastmath-intri...

Totally fair. I'm with you.