Examples on Compiler Explorer: By default, GCC generates FMA instructions on Haswell and later: https://godbolt.org/z/GKb7G4nW9 https://godbolt.org/z/GKb7G4nW

Examples on Compiler Explorer: By default, GCC generates FMA instructions on Haswell and later: https://godbolt.org/z/GKb7G4nW9 https://godbolt.org/z/GKb7G4nW9 but it does not with -std=c99 on the command line: https://godbolt.org/z/KTnqcT6aW https://godbolt.org/z/KTnqcT6aW Similarly on 32-bit x86, floating-point uses x87 by default, and some intermediate calculations are in 80-bit arithmetic: https://godbolt.org/z/4q31oEe14 https://godbolt.org/z/4q31oEe14 With -std=c99 instead, we can see additional load/stores, which round down results to 64 bits: https://godbolt.org/z/qdf4hceca https://godbolt.org/z/qdf4hceca