On Fri, 2010-01-29 at 02:01 -0800, Andrew Morton wrote:
> On Fri, 29 Jan 2010 09:28:04 GMT tip-bot for Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
>
> > +#define HWEIGHT8(w) \
> > + ( (!!((w) & (1ULL << 0))) + \
> > + (!!((w) & (1ULL << 1))) + \
> > + (!!((w) & (1ULL << 2))) + \
> > + (!!((w) & (1ULL << 3))) + \
> > + (!!((w) & (1ULL << 4))) + \
> > + (!!((w) & (1ULL << 5))) + \
> > + (!!((w) & (1ULL << 6))) + \
> > + (!!((w) & (1ULL << 7))) )
> > +
> > +#define HWEIGHT16(w) (HWEIGHT8(w) + HWEIGHT8(w >> 8))
> > +#define HWEIGHT32(w) (HWEIGHT16(w) + HWEIGHT16(w >> 16))
> > +#define HWEIGHT64(w) (HWEIGHT32(w) + HWEIGHT32(w >> 32))
>
> Would be nice if it had a comment explaining why it exists. If people
> accidentally use this with non-constant arguments, the generated code
> will be pretty ghastly.
*sigh* and here I though it being placed right next to hweight_long()
which uses the arch hweightN() would be clue enough.
If people are so clueless, who says they'll read a comment.. but sure I
guess I can add one.
> Or add some barf-if-not-__constant_p() thing, perhaps.
I've actually sneaked one non-constant usage in, but since its in an
init path I didn't care to fix that, but I guess here goes:
---
Subject: bitops: Dummyify the compile-time hweight versions
Because it seems allowed to not think and write kernel code.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
Index: linux-2.6/arch/x86/kernel/cpu/perf_event.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/cpu/perf_event.c
+++ linux-2.6/arch/x86/kernel/cpu/perf_event.c
@@ -93,13 +93,16 @@ struct cpu_hw_events {
struct perf_event *event_list[X86_PMC_IDX_MAX]; /* in enabled order */
};
-#define EVENT_CONSTRAINT(c, n, m) { \
+#define __EVENT_CONSTRAINT(c, n, m, w) {\
{ .idxmsk64[0] = (n) }, \
.code = (c), \
.cmask = (m), \
- .weight = HWEIGHT64((u64)(n)), \
+ .weight = (w), \
}
+#define EVENT_CONSTRAINT(c, n, m) \
+ __EVENT_CONSTRAINT(c, n, m, HWEIGHT(n))
+
#define INTEL_EVENT_CONSTRAINT(c, n) \
EVENT_CONSTRAINT(c, n, INTEL_ARCH_EVTSEL_MASK)
@@ -2646,7 +2649,8 @@ void __init init_hw_perf_events(void)
register_die_notifier(&perf_event_nmi_notifier);
unconstrained = (struct event_constraint)
- EVENT_CONSTRAINT(0, (1ULL << x86_pmu.num_events) - 1, 0);
+ __EVENT_CONSTRAINT(0, (1ULL << x86_pmu.num_events) - 1,
+ 0, x86_pmu.num_events);
pr_info("... version: %d\n", x86_pmu.version);
pr_info("... bit width: %d\n", x86_pmu.event_bits);
Index: linux-2.6/include/linux/bitops.h
===================================================================
--- linux-2.6.orig/include/linux/bitops.h
+++ linux-2.6/include/linux/bitops.h
@@ -45,19 +45,29 @@ static inline unsigned long hweight_long
return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
}
-#define HWEIGHT8(w) \
- ( (!!((w) & (1ULL << 0))) + \
- (!!((w) & (1ULL << 1))) + \
- (!!((w) & (1ULL << 2))) + \
- (!!((w) & (1ULL << 3))) + \
- (!!((w) & (1ULL << 4))) + \
- (!!((w) & (1ULL << 5))) + \
- (!!((w) & (1ULL << 6))) + \
+/*
+ * Clearly slow versions of the hweightN() functions, their benefit is
+ * of course compile time evaluation of constant arguments.
+ */
+#define HWEIGHT8(w) \
+ ( BUILD_BUG_ON_ZERO(!__builtin_constant_p(w)) + \
+ (!!((w) & (1ULL << 0))) + \
+ (!!((w) & (1ULL << 1))) + \
+ (!!((w) & (1ULL << 2))) + \
+ (!!((w) & (1ULL << 3))) + \
+ (!!((w) & (1ULL << 4))) + \
+ (!!((w) & (1ULL << 5))) + \
+ (!!((w) & (1ULL << 6))) + \
(!!((w) & (1ULL << 7))) )
-#define HWEIGHT16(w) (HWEIGHT8(w) + HWEIGHT8(w >> 8))
-#define HWEIGHT32(w) (HWEIGHT16(w) + HWEIGHT16(w >> 16))
-#define HWEIGHT64(w) (HWEIGHT32(w) + HWEIGHT32(w >> 32))
+#define HWEIGHT16(w) (HWEIGHT8(w) + HWEIGHT8((w) >> 8))
+#define HWEIGHT32(w) (HWEIGHT16(w) + HWEIGHT16((w) >> 16))
+#define HWEIGHT64(w) (HWEIGHT32(w) + HWEIGHT32((w) >> 32))
+
+/*
+ * For us lazy bastards
+ */
+#define HWEIGHT(w) HWEIGHT64((u64)(w))
/**
* rol32 - rotate a 32-bit value left
--
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