According to https://stackoverflow.com/questions/40699071/can-i-make-my-compiler-use-fast-math-on-a-per-function-basis https://stackoverflow.com/questions/40699

According to https://stackoverflow.com/questions/40699071/can-i-make-my-compiler-use-fast-math-on-a-per-function-basis 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.