Hi, (Do you really need to post to gcc@, fortran@ and gcc-patches@? Shouldn't be one of the list sufficient – like fortran@?)
On 2/28/20 6:02 AM, Jiufu Guo wrote:
When I check a PR93709, I find the testcase maxlocval_4.f90 […] With -ffast-math -O3, this case `STOP 3` on a few platforms, e.g. ppc64le/x86.
To my knowledge, this test case is never be run with -ffast-math; for sure, it is not run with that option on my x86-64-gnu-linux system. Given that it uses NaN and INF, it is also not suitable for -ffast-math (cf. below) – and probably no one has tested or intended to run it with -ffast-math. Thus, it is not that unexpected that it fails with -ffast-math. As it does what it is supposed to do with -O3 and no -ffast-math. Hence, I fail to understand the problem.
The test case does not check NaN explicitly. So, strictly speaking, this code may need more stronger to check NaN, otherwise it may STOP during execution, and this STOP is acceptable. Right?
In terms of GCC, "-ffast-math: Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast" See "man gcc" or https://gcc.gnu.org/onlinedocs/ → GCC Manual → "Option Index" or Invoking → Optimization And in the test case, the neither "mnan" nor "minf" falls under "finite-math-only". Regarding -fast-math etc, see also: https://gcc.gnu.org/wiki/FloatingPointMath https://gcc.gnu.org/wiki/x87note https://gcc.gnu.org/wiki/Math_Optimization_Flags and, talking about FP math in general, the Goldberg paper, http://www.validlab.com/goldberg/paper.pdf and similar but both less famous and written more recently: a paper by Monniaux, https://hal.archives-ouvertes.fr/hal-00128124 Cheers, Tobias PS: Using -ffast-math is fine, if one is careful – but one should then strongly avoid trickery with NaN, INF etc. BTW: Talking about optimizations, -Ofast (which was not used by you) also enables -ffast-math but additionally turns implies "-fno-protect-parens", which otherwise are protected according to the Fortran standard. ----------------- Mentor Graphics (Deutschland) GmbH, Arnulfstraße 201, 80634 München / Germany Registergericht München HRB 106955, Geschäftsführer: Thomas Heurung, Alexander Walter
