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x86/function-graph: fix constraint for recording old return value

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After upgrading from gcc 4.2.2 to 4.4.0, the function graph tracer broke.
Investigating, I found that in the asm that replaces the return value,
gcc was using the same register for the old value as it was for the
new value.

	mov	(addr), old
	mov	new, (addr)

But if old and new are the same register, we clobber new with old!
I first thought this was a bug in gcc 4.4.0 and reported it:

  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40132

Andrew Pinski responded (quickly), saying that it was correct gcc behavior
and the code needed to denote old as an "early clobber".

Instead of "=r"(old), we need "=&r"(old).

[Impact: keep function graph tracer from breaking with gcc 4.4.0 ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt 2009-05-13 13:52:19 -04:00 committed by Steven Rostedt
parent 091bf7624d
commit aa512a27e9
1 changed files with 1 additions and 1 deletions
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arch/x86/kernel/ftrace.c
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@ -442,7 +442,7 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
_ASM_EXTABLE(1b, 4b)
_ASM_EXTABLE(2b, 4b)
: [old] "=r" (old), [faulted] "=r" (faulted)
: [old] "=&r" (old), [faulted] "=r" (faulted)
: [parent] "r" (parent), [return_hooker] "r" (return_hooker)
: "memory"
);