linux-kernel - Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks.

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Date:	Wed, 27 Nov 2013 16:42:02 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, oleg@...hat.com, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...il.com, liming.wang@...driver.com, jkacur@...hat.com,
	harald.gustafsson@...csson.com, vincent.guittot@...aro.org,
	bruce.ashfield@...driver.com
Subject: Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks.


and even this works:

triton:~> cat test.c

struct foo {
        int a;
        int b;
};

int litter_our_stack(void)
{
        volatile struct foo x = { .a = 1, .b = 2 };

        return x.b;
}

int test_code(void)
{
        volatile struct foo x = { .a = 1, /* .b not initialized explicitly */ };

        return x.b;
}

int main(void)
{
        return litter_our_stack() + test_code();
}

triton:~> gcc -Wall -Wextra -O0 -o test test.c; ./test; echo $?
2
triton:~> 


The result is 2, so x.b in test_code() got explicitly set to 0.

If it was uninitialized, not only would we expect a compiler warning, 
but we'd also get a result of '4'. (the two functions have the same 
stack depth, so 'litter_our_stack()' initializes .b to 2.)

-O0 guarantees that GCC just dumbly implements these functions without 
any optimizations.

Thanks,

	ngo
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