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Re: [C++ PATCH] Detect UB in shifts in constexpr functions

From: Jason Merrill <jason at redhat dot com>
To: Marek Polacek <polacek at redhat dot com>
Cc: Jakub Jelinek <jakub at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
Date: Tue, 02 Dec 2014 09:59:26 -0500
Subject: Re: [C++ PATCH] Detect UB in shifts in constexpr functions
Authentication-results: sourceware.org; auth=none
References: <20141124135114 dot GO29446 at redhat dot com> <20141124154925 dot GI1674 at tucnak dot redhat dot com> <20141124155822 dot GQ29446 at redhat dot com> <20141124160508 dot GJ1674 at tucnak dot redhat dot com> <20141124164148 dot GR29446 at redhat dot com> <5476063A dot 7020200 at redhat dot com> <20141127142652 dot GC15555 at redhat dot com> <54788673 dot 9060000 at redhat dot com> <20141202141426 dot GQ15555 at redhat dot com>

On 12/02/2014 09:14 AM, Marek Polacek wrote:

On Fri, Nov 28, 2014 at 09:28:03AM -0500, Jason Merrill wrote:

I was thinking even more detailed: one diagnostic for negative count, one
diagnostic for count larger than the precision of the lhs, and then a third
for overflow.


Alright, done.

Thanks. These errors also need to be conditional on (!ctx->quiet), andif we see one of these conditions we need to set *non_constant_p.

This was a little bit problematic, because I don't really know where
that formula comes from.  We use it in c-ubsan.c, so I've just taken
it from there.  It was Jakub who came with that formula, so maybe he
remembers its origin.  I wrote something more to the comment describing
it, even though that might not be enough.


Looks good.

Jason

Follow-Ups:
- Re: [C++ PATCH] Detect UB in shifts in constexpr functions
  - From: Marek Polacek

References:
- Re: [C++ PATCH] Detect UB in shifts in constexpr functions
  - From: Marek Polacek

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