<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Mon, Dec 7, 2015 at 4:32 PM, Richard Smith via cfe-commits <span dir="ltr"><<a href="mailto:cfe-commits@lists.llvm.org" target="_blank">cfe-commits@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><span>On Mon, Dec 7, 2015 at 7:25 AM, Joerg Sonnenberger via cfe-commits <span dir="ltr"><<a href="mailto:cfe-commits@lists.llvm.org" target="_blank">cfe-commits@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><span>On Thu, Dec 03, 2015 at 01:36:22AM -0000, Richard Smith via cfe-commits wrote:<br>
> Author: rsmith<br>
> Date: Wed Dec 2 19:36:22 2015<br>
> New Revision: 254574<br>
><br>
> URL: <a href="http://llvm.org/viewvc/llvm-project?rev=254574&view=rev" rel="noreferrer" target="_blank">http://llvm.org/viewvc/llvm-project?rev=254574&view=rev</a><br>
> Log:<br>
> PR17381: Treat undefined behavior during expression evaluation as an unmodeled<br>
> side-effect, so that we don't allow speculative evaluation of such expressions<br>
> during code generation.<br>
><br>
> This caused a diagnostic quality regression, so fix constant expression<br>
> diagnostics to prefer either the first "can't be constant folded" diagnostic or<br>
> the first "not a constant expression" diagnostic depending on the kind of<br>
> evaluation we're doing. This was always the intent, but didn't quite work<br>
> correctly before.<br>
><br>
> This results in certain initializers that used to be constant initializers to<br>
> no longer be; in particular, things like:<br>
><br>
> float f = 1e100;<br>
><br>
> are no longer accepted in C. This seems appropriate, as such constructs would<br>
> lead to code being executed if sanitizers are enabled.<br>
<br>
</span>This leads to some pretty annoying regressions as it now seems to be<br>
impossible to use NaN or infinites as constant initializers. Expressions<br>
like 0.0 / 0.0, 1.0 / 0.0 and -1.0 / 0.0 are perfectly well defined<br>
under normal IEEE rules, so they shouldn't be rejected.</blockquote><div><br></div></span><div>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.</div><div><br></div><div>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.</div><span><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Worse, it seems<br>
even using __builtin_nan() for example doesn't work.<br></blockquote><div><br></div></span><div>__builtin_nan() works fine for me, can you provide a testcase?</div><span><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">
I'm not even sure about the example given in the commit message, how<br>
exactly is that undefined behavior?</blockquote><div><br></div></span><div>C11 <a href="http://6.3.1.5/1" target="_blank">6.3.1.5/1</a>: "If the value being converted is outside the range of values that can be represented, the behavior is undefined."</div></div></div></div></blockquote><div><br></div><div>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?</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_extra"><div class="gmail_quote"><div><br></div><div>We also have C11 6.6/4: "Each constant expression shall evaluate to a constant that is in the range of representable values for its type."</div></div></div></div>
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