On Mon, Dec 7, 2015 at 2:14 PM, David Majnemer <david.majne...@gmail.com> wrote: > > > On Mon, Dec 7, 2015 at 4:32 PM, Richard Smith via cfe-commits > <cfe-commits@lists.llvm.org> wrote: >> >> On Mon, Dec 7, 2015 at 7:25 AM, Joerg Sonnenberger via cfe-commits >> <cfe-commits@lists.llvm.org> wrote: >>> >>> On Thu, Dec 03, 2015 at 01:36:22AM -0000, Richard Smith via cfe-commits >>> wrote: >>> > Author: rsmith >>> > Date: Wed Dec 2 19:36:22 2015 >>> > New Revision: 254574 >>> > >>> > URL: http://llvm.org/viewvc/llvm-project?rev=254574&view=rev >>> > Log: >>> > PR17381: Treat undefined behavior during expression evaluation as an >>> > unmodeled >>> > side-effect, so that we don't allow speculative evaluation of such >>> > expressions >>> > during code generation. >>> > >>> > This caused a diagnostic quality regression, so fix constant expression >>> > diagnostics to prefer either the first "can't be constant folded" >>> > diagnostic or >>> > the first "not a constant expression" diagnostic depending on the kind >>> > of >>> > evaluation we're doing. This was always the intent, but didn't quite >>> > work >>> > correctly before. >>> > >>> > This results in certain initializers that used to be constant >>> > initializers to >>> > no longer be; in particular, things like: >>> > >>> > float f = 1e100; >>> > >>> > are no longer accepted in C. This seems appropriate, as such constructs >>> > would >>> > lead to code being executed if sanitizers are enabled. >>> >>> This leads to some pretty annoying regressions as it now seems to be >>> impossible to use NaN or infinites as constant initializers. Expressions >>> like 0.0 / 0.0, 1.0 / 0.0 and -1.0 / 0.0 are perfectly well defined >>> under normal IEEE rules, so they shouldn't be rejected. >> >> >> Well, we have a problem. The evaluation semantics of these expressions >> requires code to execute in some build modes (in particular, with sanitizers >> enabled), and thus has a side-effect. >> >> I'm inclined to relax the restriction added in this change for the >> specific case of global variables in C, since (as you say) there is a fair >> amount of code using divide-by-zero as a "portable" way of generating an inf >> or nan. >> >>> Worse, it seems >>> even using __builtin_nan() for example doesn't work. >> >> >> __builtin_nan() works fine for me, can you provide a testcase? >> >>> I'm not even sure about the example given in the commit message, how >>> exactly is that undefined behavior? >> >> >> C11 6.3.1.5/1: "If the value being converted is outside the range of >> values that can be represented, the behavior is undefined." > > > I don't think we want to make the UB here true UB. It would mean that code > which expected to get NaN might get undef, even outside of constant > expression evaluation. The implementation defined behavior of providing NaN > seems more friendly... IIRC, this broke Chrome recently because folks were > doing this in C++. Hans, do you remember the details?
Hmm, it doesn't ring a bell, but my memory sometimes fails me. Didn't you and Reid look at something like this the other day? (But maybe that was in internal code?) _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
