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Re: [Emacs-diffs] master bc78ff2 2/2: Use 0 for Qnil

From: Paul Eggert
Subject: Re: [Emacs-diffs] master bc78ff2 2/2: Use 0 for Qnil
Date: Wed, 07 Jan 2015 23:05:31 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.3.0

YAMAMOTO Mitsuharu wrote:
With the above macros, XUNTAG (a, type) is expanded to:

   ((void *) ((char *) (0) - (type) + (intptr_t) XLI (a)))

It is relative to the NULL pointer and clang seems to treat its
dereference as an undefined behavior.

Thanks for diagnosing that. This is arguably a bug in clang, but it's easy enough to work around as you suggest. I reproduced the problem with Fedora 21 x86-64 (clang 3.5.0) and installed the attached patch, which fixed it for me.

Attachment: 0001-Port-Qnil-0-XUNTAG-to-clang.patch
Description: Text Data

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