Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Sun, 11 Aug 2002 00:38:38 -0700 | From | Andrew Morton <> | Subject | [patch 4/21] fix ARCH_HAS_PREFETCH |
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include/linux/prefetch.h does a strange thing: if the arch doesn't have the prefectch functions, this header defines no-op version of them and then defines ARCH_HAS_PREFETCH. So there's no way for mainline code to know if the architecture *really* has prefetch instructions.
This information loss is unfortunate. Examples:
for (i = 0; i < N; i++) prefetch(foo[i]);
Problem is, if `prefetch' is a no-op, the compiler will still generate an empty busy-wait loop. Which it must do. We need to know the truth about ARCH_HAS_PREFETCH to correctly elide that loop.
#ifdef ARCH_HAS_PREFETCH #define prefetch_prev_lru_page(_page, _base, _field) \ do { \ if ((_page)->lru.prev != _base) { \ struct page *prev; \ \ prev = list_entry(_page->lru.prev, \ struct page, lru); \ prefetch(&prev->_field); \ } \ } while (0) #else #define prefetch_prev_lru_page(_page, _base, _field) do { } while (0) #endif
Which needs a working ARCH_HAS_PREFETCH to avoid probable extra code generation on CPUs which don't have prefetch.
prefetch.h | 3 --- 1 files changed, 3 deletions(-)
--- 2.5.31/include/linux/prefetch.h~arch_has_prefetchw Sun Aug 11 00:20:08 2002 +++ 2.5.31-akpm/include/linux/prefetch.h Sun Aug 11 00:20:08 2002 @@ -39,17 +39,14 @@ */ #ifndef ARCH_HAS_PREFETCH -#define ARCH_HAS_PREFETCH static inline void prefetch(const void *x) {;} #endif #ifndef ARCH_HAS_PREFETCHW -#define ARCH_HAS_PREFETCHW static inline void prefetchw(const void *x) {;} #endif #ifndef ARCH_HAS_SPINLOCK_PREFETCH -#define ARCH_HAS_SPINLOCK_PREFETCH #define spin_lock_prefetch(x) prefetchw(x) #endif . - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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