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GCC doesn't default to non-conforming behaviour like -ffast-math -- that's Intel (at least a similar option). That's usually why people mistakenly think GCC ve

by gnufx 4y ago

GCC doesn't default to non-conforming behaviour like -ffast-math -- that's Intel (at least a similar option). That's usually why people mistakenly think GCC vectorization is deficient if they don't use -funsafe-math-optimizations in particular.

thxg 4y ago
Indeed GCC does not enable -ffast-math by default. Unfortunately, -ffast-math and -funsafe-math-optimizations (despite the name) are not the only options that prevent bit-for-bit-reproducible floating point. For example, -ffp-contract=fast is enabled by default [1], and it will lead to different floating-point roundings: Compare [2] which generates an FMA instruction, to [3] when -std=c99 is specified. As another example, -fexcess-precision=fast is also enabled by default. Similarly, [4] does intermediate calculations in the 80-bit x87 registers, while [5] has additional loads and stores to reduce the precision of intermediate results to 64 bits. In both examples, GCC generates code that does not conform to IEEE-754, unless -std=c99 is specified. [1] From the man page: -ffp-contract=style -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply- add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off. The default is -ffp-contract=fast. [2] https://godbolt.org/z/GKb7G4nW9 https://godbolt.org/z/GKb7G4nW9 [3] https://godbolt.org/z/KTnqcT6aW https://godbolt.org/z/KTnqcT6aW [4] https://godbolt.org/z/4q31oEe14 https://godbolt.org/z/4q31oEe14 [5] https://godbolt.org/z/qdf4hceca https://godbolt.org/z/qdf4hceca

by gnufx 4y ago

thxg 4y ago

Indeed GCC does not enable -ffast-math by default. Unfortunately, -ffast-math and -funsafe-math-optimizations (despite the name) are not the only options that prevent bit-for-bit-reproducible floating point. For example, -ffp-contract=fast is enabled by default [1], and it will lead to different floating-point roundings: Compare [2] which generates an FMA instruction, to [3] when -std=c99 is specified. As another example, -fexcess-precision=fast is also enabled by default. Similarly, [4] does intermediate calculations in the 80-bit x87 registers, while [5] has additional loads and stores to reduce the precision of intermediate results to 64 bits. In both examples, GCC generates code that does not conform to IEEE-754, unless -std=c99 is specified. [1] From the man page: -ffp-contract=style -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply- add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off. The default is -ffp-contract=fast. [2] https://godbolt.org/z/GKb7G4nW9 https://godbolt.org/z/GKb7G4nW9 [3] https://godbolt.org/z/KTnqcT6aW https://godbolt.org/z/KTnqcT6aW [4] https://godbolt.org/z/4q31oEe14 https://godbolt.org/z/4q31oEe14 [5] https://godbolt.org/z/qdf4hceca https://godbolt.org/z/qdf4hceca