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Date: Wed, 27 Nov 2013 16:42:02 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, Juri Lelli <juri.lelli@gmail.com>,
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Subject: Re: [PATCH 08/14] sched: add latency tracing for -deadline tasks.
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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