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Re: [PATCH] PR58143/58227 wrong code at -O3
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Bernd Edlinger <bernd dot edlinger at hotmail dot de>
- Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 29 Aug 2013 11:54:22 +0200
- Subject: Re: [PATCH] PR58143/58227 wrong code at -O3
- Authentication-results: sourceware.org; auth=none
- References: <DUB122-W47A045BD4341642F7403DDE44B0 at phx dot gbl>
On Wed, Aug 28, 2013 at 11:24 PM, Bernd Edlinger
> The lim pass (aka loop invariant motion) can move conditional expressions with possible undefined behavior out of the if statement inside a loop which may cause the loop optimization to silently generate wrong code as PR tree-optimization/58143 and PR tree-optimization/58227 demonstrate.
> This patch prevents lim from moving code that might cause undefined behavior.
> This triggered a minor regression in gcc.target/i386/pr53397-1.c:
> Here lim used to move the expression "2*step" out of the loop, but this may cause undefined behavior on case of overflow, I propose to resolve this by adding -fno-strict-overflow. The test case looks pretty constructed anyway.
> The patch was boot-strapped and regression tested on i686-pc-linux-gnu and
> OK for trunk?
2013-08-28 Bernd Edlinger <email@example.com>
Prevent lim from moving conditional expressions
if that could trigger undefined behavior.
* gimple.h (gimple_could_trap_p_2): New function.
* gimple.c (gimple_could_trap_p_2): New function.
(gimple_could_trap_p_1): Call gimple_could_trap_p_2.
* tree-ssa-loop-im.c (movement_possibility): Likewise.
Please fold the overall comment into the movement_possibility changelog.
Can you instead of introducing gimple_could_trap_p_2 simply change the
signature and name of gimple_could_trap_p_1 please? It is only called
A proper name could be gimple_could_trap_or_invoke_undefined_p.
That you need to change the two testcases makes me nervous (also please
use -fwrapv, not -fno-strict-overflow) - did you check what happens for them?
> Bernd Edlinger