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Date: Sat, 18 Nov 2017 03:03:12 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Wei Wei <dotweiba@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>
Subject: Re: [Error after setting =?utf-8?Q?-Og=5D_?=
 =?utf-8?B?4oCYX19iYWRfY29weV9mcm9t4oCZIGRlY2xhcmU=?= =?utf-8?Q?d?= with
 attribute error: copy source size is too small
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On Thu, Nov 09, 2017 at 04:02:53PM -0500, Wei Wei wrote:
> Hi all,
> 
> I get a compile time error after setting -Og when compiling for the latest GitHub version.
> I am using `make defconfig’ to get the default x86_64 config. But previously I did this in v4.4, 
> it's fine.

; cat >a.c <<'EOF'
extern void __attribute((error("1"))) f1(void);
extern void __attribute((error("2"))) f2(void);

static inline __attribute__((always_inline)) void bar(const void *addr)
{
        int sz = __builtin_object_size(addr, 0);
        if (__builtin_expect(sz >= 0, 0))
                f1();
        if (sz >= 0)
                f2();
}

void foo(int *in)
{
        bar(in);
}
EOF
; gcc -O2 -c a.c
; gcc -Og -c a.c
In function ‘bar’,
    inlined from ‘foo’ at a.c:15:2:
a.c:8:3: error: call to ‘f1’ declared with attribute error: 1
   f1();
   ^
;

Note that the call of f2() _was_ eliminated.  Wrap the condition into
__builtin_expect() and with -Og it doesn't get eliminated until too
late.  It's really brittle and dependent not just upon the _result_
of optimizations (get rid of __attribute((error())) in those and
you'll see that with -Og it compiles into
        .file   "a.c"
        .text
        .globl  foo
        .type   foo, @function
foo:
.LFB1:
        .cfi_startproc
        rep ret
        .cfi_endproc
.LFE1:
        .size   foo, .-foo
getting rid of both calls, as it ought to); it depends upon the moment
when dead code elimination happens.  -Og leaves the sucker around for
too long.  If you replace that __builtin_object_size() with -1 (which
is what it evaluates to), you'll get elimination happening early enough
even with -Og; ditto for getting rid of inlining...