From: "Floyd, Paul" <paulf@free.fr>
To: "gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: Safe transposition of logical and operands
Date: Mon, 18 Sep 2023 16:46:47 +0200 [thread overview]
Message-ID: <d5f5c721-46a9-4ca9-8195-a2df602a1f51@free.fr> (raw)
In-Reply-To: <CAFiYyc1BVi-AWrQVng7PM+zMnJQ59SrmYJcLWc=HhDugcP1e_Q@mail.gmail.com>
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Hi Richard and Jonathan
On 18/09/2023 10:00, Richard Biener wrote:
> On Mon, Sep 18, 2023 at 9:24 AM Jonathan Wakely via Gcc<gcc@gcc.gnu.org> wrote:
>> Yes, GCC assumes that the reference is bound to a valid object, because C++
>> requires that to be true. Of course memcheck can't assume that, because one
>> of its main reasons to exist is to find undefined behaviour where that
>> isn't true!
It's even worse than that. This transformation is being done in VEX
(which unfortunately
is also the bit I know the least). Not normally where we'd do
accessibility checks.
>> I think what GCC is doing is a valid transformation, in the context of a
>> valid C++ program. But I'm not sure that helps valgrind, which doesn't have
>> the liberty of assuming a valid program.
> More specifically GCC thinks it's fine to speculate loads (given it can prove
> doing so doesn't trap)
I don't think that will be easy for us to prove. We just don't know
enough about stack variables.
A+
Paui
next prev parent reply other threads:[~2023-09-18 14:46 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-09-17 19:33 Paul Floyd
2023-09-17 20:51 ` Jonathan Wakely
2023-09-18 7:03 ` Paul Floyd
2023-09-18 7:23 ` Jonathan Wakely
2023-09-18 8:00 ` Richard Biener
2023-09-18 14:46 ` Floyd, Paul [this message]
2023-09-18 14:55 ` Richard Biener
2023-09-18 17:56 ` Paul Floyd
2023-09-18 19:09 ` Martin Uecker
2023-09-18 20:15 ` Paul Floyd
2023-09-18 20:52 ` Martin Uecker
2023-09-19 5:03 ` Paul Floyd
2023-09-18 9:36 ` Jonathan Wakely
2023-09-18 10:30 ` Andreas Schwab
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