On Wed, 7 Feb 2001, Linus Torvalds wrote:
>
> I'd rather not do these kinds of things that the compiler should be able
> to trivially do for us.
>
> (gcc sometimes _does_ do these things. I've seen it. Why doesn't it do it
> here? Did you check the code? Have you asked the gcc lists?)
The "(1<<PG_bitshift)" part of it is done, sure; but I've rechecked
activate_page_nolock() compiled -O2 -march=i686 with egcs-2.91.66 (RH7.0
kgcc), gcc-2.96-69 (RH7.0 gcc+fixes), gcc-2.97 (gcc-snapshot-20010207-1).
None of those optimizes this: I believe the semantics of "||" (don't
try next test if first succeeds) forbid the optimization "|" gives?
2.91 and 2.96 give three movs (two unnecessary), three tests,
three jumps (first two not usually taken):
232: 8b 43 18 mov 0x18(%ebx),%eax
235: a8 40 test $0x40,%al
237: 75 0f jne 248 <activate_page_nolock+0x4c>
239: 8b 43 18 mov 0x18(%ebx),%eax
23c: a8 80 test $0x80,%al
23e: 75 08 jne 248 <activate_page_nolock+0x4c>
240: 8b 43 18 mov 0x18(%ebx),%eax
243: f6 c4 08 test $0x8,%ah
246: 74 19 je 261 <activate_page_nolock+0x65>
2.97 is jumpier: mov and je mov test jne mov test jne jmp.
That looks worse to me: David, earlier on you advertized
http://www.codesourcery.com/gcc-snapshots/
Is this something worth your pursuing with the gcc guys?
Hugh
--- linux-2.4.2-pre1/include/linux/swap.h Wed Feb 7 15:21:13 2001
+++ linux/include/linux/swap.h Wed Feb 7 17:21:25 2001
@@ -200,8 +200,8 @@
* with the pagemap_lru_lock held!
*/
#define DEBUG_ADD_PAGE \
- if (PageActive(page) || PageInactiveDirty(page) || \
- PageInactiveClean(page)) BUG();
+ if ((page)->flags & ((1<<PG_active)|(1<<PG_inactive_dirty)| \
+ (1<<PG_inactive_clean))) BUG();
#define ZERO_PAGE_BUG \
if (page_count(page) == 0) BUG();
-
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