linux-kernel - Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

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Message-ID: <20190226112521.GH32534@hirez.programming.kicks-ass.net>
Date:   Tue, 26 Feb 2019 12:25:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>
Subject: Re: [RFC PATCH] tools/memory-model: Remove (dep ; rfi) from ppo

On Tue, Feb 26, 2019 at 12:21:33PM +0100, Peter Zijlstra wrote:

> Also; we need to find a GCC person to find/give us a knob to kill this
> entire class of nonsense. This is just horrible broken shit:
> 
> 
> ~/tmp# gcc -O2 -fno-strict-aliasing  -o ptr ptr.c ; ./ptr
> p=0x5635dd3d5034 q=0x5635dd3d5034
> x=1 y=2 *p=11 *q=2
> ~/tmp# cat ptr.c
> #include <stdio.h>
> #include <string.h>
> int y = 2, x = 1;
> int main (int argc, char **argv) {
> 	int *p = &x + argc;

damn; wrong version; that should've been: s/argc/1/
same result though.

> 	int *q = &y;
> 	printf("p=%p q=%p\n", p, q);
> 	if (!memcmp(&p, &q, sizeof(p))) {
> 		*p = 11;
> 		printf("x=%d y=%d *p=%d *q=%d\n", x, y, *p, *q);
> 	}
> }