Hi, Recently, we discovered some floating point precision diffs when using GCC8 to build our application on arm64: After some investigation, it turns out that this is due to the -ffp-contract=fast option that is on by default. Therefore, we have to explicitly add -ffp-contract=off and do a full-rebuild.
GCC by default turns -ffp-contract=fast on. https://gcc.gnu.org/onlinedocs/gcc-8.5.0/gcc/Optimize-Options.html#Optimize-Options https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Optimize-Options.html#Optimize-Options " -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. " This can be shown by a small example for arm64 with gcc8.5 in https://godbolt.org/z/MxYfnG8TE. Only when adding -std=c89 explicitly, this transformaton is off. another exmaple also shows that Clang and MSVC only allow this transformation when speifiying ffast-math and fp:fast: https://godbolt.org/z/o54bYfPbP When searching online, we found that there were similar discussions recently on the exact same issue: https://github.com/dotnet/runtime/issues/64604 https://github.com/dotnet/runtime/issues/64591 a summary of these discussions is: 1. "fmadd" is a fused operation and will return a different result for many inputs; 2. therefore, -ffp-contract=fast is not a safe optimization to be on by default; 3. Clang and MSVC only allow this when specifying ffast-math and fp:fast since this is not an IEEE754 compliant optimization; 4. The reasons why GCC turns on this option by default are: A. GNU C language spec allows such transformation. B. this did not expose real problem for most X86/X64 apps previously since FMA instructions didn't exist until 2013 when the FMA3 instruction set was added, and also these instructions were not always available.. 5. Arm64 has fused multiply-add instructions as "baseline" and are always available. therefore -ffp-contract=fast exposed more serious problems on Arm64 platforms. our major question: Should GCC turn off -ffp-contract=fast by default since it's not IEEE754 compliant and more modern processors have the FMA instructions available by default? Thanks. Qing
