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SubjectGCC 4.x random code generator for x86_64 ?
Can anyone shed some light on the following:

Applying a patch which adds a yet unused "extern int bla(void);" line
to a header file results in something which is beyond me.

The .i files differ as expected in exaclty _one_ line.

--- 1.i 2009-12-07 16:23:35.000000000 +0100
+++ 2.i 2009-12-07 16:23:19.000000000 +0100
@@ -6566,6 +6566,8 @@
extern int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);
# 252 "/home/tglx/work/kernel/rt-new/linux-2.6-tip/include/linux/spinlock.h"
# 1 "/home/tglx/work/kernel/rt-new/linux-2.6-tip/include/linux/rwlock.h" 1
+# 17 "/home/tglx/work/kernel/rt-new/linux-2.6-tip/include/linux/rwlock.h"
+extern int bla(void);
# 253 "/home/tglx/work/kernel/rt-new/linux-2.6-tip/include/linux/spinlock.h" 2

One would expect that the generated code would be the same, but the
resulting code has following surprising deltas (removed the "offset:"
prefix to see the meat of the changes):

41 57 push %r15
41 56 push %r14
41 55 push %r13
+ 4c 8d ad 30 ff ff ff lea -0xd0(%rbp),%r13
41 54 push %r12
49 89 fc mov %rdi,%r12
53 push %rbx
....
48 85 c0 test %rax,%rax
49 89 c5 mov %rax,%r13
- 75 0c jne 85 <timecompare_offset+0x52>
- bb 0a 00 00 00 mov $0xa,%ebx
+ 75 0c jne 8c <timecompare_offset+0x59>
4c 8d ad 30 ff ff ff lea -0xd0(%rbp),%r13
+ bb 0a 00 00 00 mov $0xa,%ebx
45 31 ff xor %r15d,%r15d
45 31 f6 xor %r14d,%r14d
....
44 89 f3 mov %r14d,%ebx
4c 89 e9 mov %r13,%rcx
- 31 d2 xor %edx,%edx
31 c0 xor %eax,%eax
- eb 09 jmp 18f <timecompare_offset+0x15c>
+ 31 d2 xor %edx,%edx
+ eb 09 jmp 196 <timecompare_offset+0x163>
48 03 01 add (%rcx),%rax
ff c2 inc %edx
48 83 c1 10 add $0x10,%rcx
None of these changes makes any sense at all.

This is fully reproducible and the effect on the generated code
depends on the placement of the "extern int bla(void);" line in the
header file.

Note, I disabled BUG and printk and there is no code which might have
a __LINE__ reference which would be affected by the insertion of that
function declaration.

For enhanced fun the problem goes away, when I compile the same kernel
for 32bit with the very same GCC.

Looks to me that GCC has a builtin random code generator for
x86_64. I'd just like to know whether this can be turned off or if
there is some magic way to seed the RCG reliably.

Environment:
F11 x86_64
gcc (GCC) 4.4.1 20090725 (Red Hat 4.4.1-2)

The effect is reproducible with the patch below on top of Linus tree
and just compiling kernel/time/timecompare.o

Also verified that other 4.x versions of GCC have similar
randomization effects just in different source files.

Peter just provided:

@@ -62837,8 +62837,8 @@
3558a: 53 push %rbx
3558b: e8 00 00 00 00 callq 35590 <cleanup_srcu_struct+0x10>
35590: 4c 8b 35 00 00 00 00 mov 0x0(%rip),%r14
- 35597: 83 c9 ff or $0xffffffffffffffff,%ecx
- 3559a: 45 31 ed xor %r13d,%r13d
+ 35597: 45 31 ed xor %r13d,%r13d
+ 3559a: 83 c9 ff or $0xffffffffffffffff,%ecx
3559d: 49 89 fc mov %rdi,%r12
355a0: eb 14 jmp 355b6 <cleanup_srcu_struct+0x36>
Thanks,

tglx
---
include/linux/spinlock.h | 2 ++
1 file changed, 2 insertions(+)
Index: linux-2.6-tip/include/linux/spinlock.h
===================================================================
--- linux-2.6-tip.orig/include/linux/spinlock.h
+++ linux-2.6-tip/include/linux/spinlock.h
@@ -57,6 +57,8 @@

#include <asm/system.h>

+extern int bla(void);
+
/*
* Must define these before including other files, inline functions need them
*/

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