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    SubjectRe: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression
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    On Tuesday 03 October 2006 23:32, Andrew Morton wrote:

    > It might help, but we still don't know what's going on (I think).
    >
    > I mean, if cache misses against __warn_once were sufficiently high for it
    > to affect performance, then __warn_once would be, err, in cache?

    Yes, of course. I'm embarrassed.

    I took a look at the generated code, and GCC is having difficulty
    optimizing WARN_ON_ONCE. Here is the start of __local_bh_enable:

    00000130 <__local_bh_enable>:
    130: 83 ec 10 sub $0x10,%esp
    133: 8b 15 04 00 00 00 mov 0x4,%edx <-+
    139: 89 e0 mov %esp,%eax |
    13b: 25 00 e0 ff ff and $0xffffe000,%eax | !!!
    140: 8b 40 14 mov 0x14(%eax),%eax |
    143: 25 00 00 ff 0f and $0xfff0000,%eax |
    148: 85 d2 test %edx,%edx <-+
    14a: 74 04 je 150 <__local_bh_enable+0x20>
    14c: 85 c0 test %eax,%eax
    14e: 75 35 jne 185 <__local_bh_enable+0x55>
    150: 89 e0 mov %esp,%eax
    152: 8b 0d 00 00 00 00 mov 0x0,%ecx <-+
    158: 25 00 e0 ff ff and $0xffffe000,%eax |
    15d: 8b 40 14 mov 0x14(%eax),%eax | !!!
    160: 25 00 ff 00 00 and $0xff00,%eax |
    165: 3d 00 01 00 00 cmp $0x100,%eax |
    16a: 0f 94 c0 sete %al |
    16d: 85 c9 test %ecx,%ecx <-+
    16f: 0f b6 c0 movzbl %al,%eax
    172: 74 04 je 178 <__local_bh_enable+0x48>
    174: 85 c0 test %eax,%eax
    176: 75 42 jne 1ba <__local_bh_enable+0x8a>
    178: b8 00 01 00 00 mov $0x100,%eax
    17d: 83 c4 10 add $0x10,%esp
    180: e9 fc ff ff ff jmp 181 <__local_bh_enable+0x51>
    185: c7 44 24 0c 3e 00 00 movl $0x3e,0xc(%esp)
    ...

    WARN_ON is better, but still has problems:

    000011a0 <do_exit>:
    11a0: 55 push %ebp
    11a1: 57 push %edi
    11a2: 56 push %esi
    11a3: 53 push %ebx
    11a4: 83 ec 30 sub $0x30,%esp
    11a7: 89 44 24 18 mov %eax,0x18(%esp)
    11ab: 89 e0 mov %esp,%eax
    11ad: 25 00 e0 ff ff and $0xffffe000,%eax
    11b2: 8b 30 mov (%eax),%esi
    11b4: 8b 86 58 0a 00 00 mov 0xa58(%esi),%eax
    11ba: 89 44 24 2c mov %eax,0x2c(%esp) <-+
    11be: 8b 44 24 2c mov 0x2c(%esp),%eax <-+
    11c2: 85 c0 test %eax,%eax | !!!
    11c4: 0f 85 65 07 00 00 jne 192f <do_exit+0x78f> |
    11ca: 8b 44 24 2c mov 0x2c(%esp),%eax <-+
    11ce: 89 e0 mov %esp,%eax
    ...

    This is gcc (GCC) 4.0.3 (Ubuntu 4.0.3-1ubuntu5), -O2.

    I don't know why this would show up as cache misses, but it does look
    like a compiler bug is being tickled.

    Andrew Wade
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