[Bug target/60788] Miscompilation of __builtin_clz with -mlzcnt

["linux at carewolf dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60788

--- Comment #1 from Allan Jensen <linux at carewolf dot com> ---
Sorry. The optimization has nothing to do with it, it just causes the constant
expressions used for testing to be evaluated at compile time.

The real issue is that the lzcnt instruction does not return the number of
leading zeroes, but the bitwidth minus the number of leading zeroes. So the
mapping of _builtin_clz to lzcnt is simply naïve and incorrect.
>From gcc-bugs-return-448548-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org Tue Apr 08 16:31:29 2014
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From: "murali.marimekala at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/60459] Crash seen in _Unwind_VRS_Pop() for ARM platform
Date: Tue, 08 Apr 2014 16:31:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`459

Murali <murali.marimekala at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |murali.marimekala at gmail dot com

--- Comment #5 from Murali <murali.marimekala at gmail dot com> ---
Hi Ramana,

I am working with Raghu and looking into the same crash issue.

Thanks for your suggestions till now. Due to some constraints , we cant move to
a new toolchain. We are looking forward to fix this issue in ARM9 toolchain
4.2.1.

Though we got a fix for this issue online, we need your expertise to port it to
our toolchain.

When we were searching for inputs, we came across following article.
https://sourceware.org/ml/libc-ports/2011-03/msg00045.html

In this article,  Ryosei Takagi was facing the same issue (at exact location)
and has provided a fix for it in __longjump.S.

This particular __longjump.S is not available in toolchain 4.2.1. I searched
for relevant code and couldnt find it.

Since we are not much familiar with this toolchain code, its taking more time
than expected to understand and port the code. So, need your valuable advise to
find the relevant location to port this fix.

If you need additional information, please let me know.

Thank you,
Murali