[thxg]

Examples on Compiler Explorer:

By default, GCC generates FMA instructions on Haswell and later:

https://godbolt.org/z/GKb7G4nW9

but it does not with -std=c99 on the command line:

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

With -std=c99 instead, we can see additional load/stores, which round down results to 64 bits:

https://godbolt.org/z/qdf4hceca