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Re: [C/C++ PATCH] Fix -Wshift-overflow with sign bit

From: Jeff Law <law at redhat dot com>
To: Marek Polacek <polacek at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>, Jason Merrill <jason at redhat dot com>, Joseph Myers <joseph at codesourcery dot com>
Date: Wed, 12 Aug 2015 11:10:32 -0600
Subject: Re: [C/C++ PATCH] Fix -Wshift-overflow with sign bit
Authentication-results: sourceware.org; auth=none
References: <20150812153927 dot GK3335 at redhat dot com>

On 08/12/2015 09:39 AM, Marek Polacek wrote:

This patch fixes a defect in -Wshift-overflow.  We should only warn
about left-shifting 1 into the sign bit when -Wshift-overflow=2.  But
this doesn't apply only for 1 << 31, but also for 2 << 30, etc.
In C++14, never warn about this.

Neither existing tests nor documentation require updating, I think.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2015-08-12  Marek Polacek  <polacek@redhat.com>

	PR c++/55095
	* c-common.c (maybe_warn_shift_overflow): Properly handle
	left-shifting 1 into the sign bit.

	* c-c++-common/Wshift-overflow-6.c: New test.
	* c-c++-common/Wshift-overflow-7.c: New test.
	* g++.dg/cpp1y/left-shift-2.C: New test.

I didn't realize C++14 fixed this bit of lameness. I'd read that it wasunder consideration when I was looking at Mikael's sext_hwi patch, butdidn't follow up on the current status.


OK for the trunk.

jeff

References:
- [C/C++ PATCH] Fix -Wshift-overflow with sign bit
  - From: Marek Polacek

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