On Donnerstag, 21. Mai 2020 17:46:01 CEST Marc Glisse wrote: > On Thu, 21 May 2020, Jonathan Wakely wrote: > > On 27/04/20 17:09 +0200, Matthias Kretz wrote: > >> From: Matthias Kretz <kr...@kde.org> > >> > >> PR libstdc++/84949 > >> * include/std/limits: Let is_iec559 reflect whether > >> __GCC_IEC_559 says float and double support IEEE 754-2008. > >> * testsuite/18_support/numeric_limits/is_iec559.cc: Test IEC559 > >> mandated behavior if is_iec559 is true. > >> * testsuite/18_support/numeric_limits/infinity.cc: Only test inf > >> behavior if is_iec559 is true, otherwise there is no guarantee > >> how arithmetic on inf behaves. > >> * testsuite/18_support/numeric_limits/quiet_NaN.cc: ditto for > >> NaN. > >> * testsuite/18_support/numeric_limits/denorm_min-1.cc: Compile > >> with -ffast-math. > >> * testsuite/18_support/numeric_limits/epsilon-1.cc: ditto. > >> * testsuite/18_support/numeric_limits/infinity-1.cc: ditto. > >> * testsuite/18_support/numeric_limits/is_iec559-1.cc: ditto. > >> * testsuite/18_support/numeric_limits/quiet_NaN-1.cc: ditto. > > > > I'm inclined to go ahead and commit this (to master only, obviously). > > It certainly seems more correct to me, and we'll probably never find > > out if it's "safe" to do unless we actually change it and see what > > happens. > > > > Marc, do you have an opinion? > > I don't have a strong opinion on this. I thought we were refraining from > changing numeric_limits based on flags (like -fwrapv for modulo) because > that would lead to ODR violations when people link objects compiled with > different flags. There is a value in libstdc++.so, which may have been > compiled with different flags than the application.
But these ODR violations happen in any case: The floating-point types are different types with or without -ffast-math (and related) flags. They behave differently. Compiling a function in multiple TUs with different flags produces observably different results. Choosing a single one of them is obviously fragile and broken. That's the spirit of an ODR violation... It would sometimes be useful to have different types: float, float_no_nan, float_no_nan_no_signed_zero, ... -- ────────────────────────────────────────────────────────────────────────── Dr. Matthias Kretz https://mattkretz.github.io GSI Helmholtz Centre for Heavy Ion Research https://gsi.de std::experimental::simd https://github.com/VcDevel/std-simd ──────────────────────────────────────────────────────────────────────────
