On Tue, Sep 21, 2021 at 3:07 PM Xi Ruoyao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > On Tue, 2021-09-21 at 12:41 +0100, Roger Sayle wrote: > > > > I was wondering if I may ask the C++ language experts for their > > opinion > > on whether (potential) floating point exceptions/traps can be ignored > > in constant expressions; this is related to PR c++/96862. I think my > > question boils down to whether (or not) the following is valid C++: > > > > constexpr float my_inf = 7.0 / 0.0; > > It's not. C++ disallows constexpr from invoking undefined behaviors in > Clauses 4 through 19, while division by zero is an undefined behavior in > Clause 8. > > > [This is currently an error with "-O2", but OK with "-O2 -ffast- > > math"!] > > > There's a long history of g++'s semantics being accidentally tied to > > the middle-end's constant folding, such that the current status quo > > is that some middle-end bugs can't be fixed without breaking C++, > > and vice versa. I'm hoping that the patch below (following Jakub's > > lead with rounding math) might be a next step to improving things, > > provided that my understanding of the desired/correct behaviour of > > the C++ front-end is correct. > > > > This patch has been tested on x86_64-pc-linux-gnu with a "make > > bootstrap" > > and "make -k check" with no new failures after tweaking two checks in > > g++.dg/ubsan/pr63956.C. > > > With this change the middle-end can become more > > strict about respecting flag_trapping_math without affecting C++'s > > behavior. Ideally, what the front-end considers valid should be > > independent of whether the user specified -fno-trapping-math (or > > -ffast-math) to the middle-end. > > I think we can allow a constexpr to contain floating div-by-zero with - > fno-trapping-math, if we consider -fno-trapping-math make the behavior > "no longer undefined" but I'm not sure.
Btw, -ftrapping-math also makes inexact operations possibly trapping so I'm not sure if any combination of -ftrapping-math and -ffinite-math-only can capture what C++ requires here. > However -ffast-math also enables -ffinite-math-only, which makes div-by- > zero absolutely undefined. So to me the expected behavior is: > > g++ t.cc -> Compile error > g++ t.cc -ffast-math -> Compile error > g++ t.cc -fno-trapping-math -> Ok > g++ t.cc -fno-trapping-math -ffinite-math-only -> Compile Error > > > Thoughts? Ok for mainline? > > Based on the reasoning above I think it's not OK. But anyway I'm not a > maintainer. > > > 2021-09-21 Roger Sayle <ro...@nextmovesoftware.com> > > > > gcc/cp/ChangeLog > > * constexpr.c (cxx_eval_outermost_const_expr): Temporarily > > disable > > the middle-end from honoring floating point exceptions/traps > > while > > folding "manifestly constant" expressions. > > > > gcc/testsuite/ChangeLog > > * g++.dg/ubsan/pr63956.C: Update to (always) allow floating > > point > > division in constexpr (if both operands are constexpr). > > > > Roger > > -- > > > > -- > Xi Ruoyao <xry...@mengyan1223.wang> > School of Aerospace Science and Technology, Xidian University
