We use the same strategy for computing the system time as on x86 now.

The time base conversion factor is the 32 bit value
  2^32 * 1000000 / time base frequency,
so the system time can be computed by
  system time = time base * conversion factor / 2^32.
The expression in system_time() looks more complicated now, but is
actually much faster (factor 2.5 on my Mac mini). I'm positively
surprised, how good the assembly looks, that GCC 4 generates. There's
not that much potential for optimization by hand-coding the function.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@15863 a95241bf-73f2-0310-859d-f6bbb57e9c96

headers/private/kernel/arch/ppc/arch_cpu.h

@@ -105,7 +105,7 @@ extern void reset_dbats(void);
 //extern void setl2cr(unsigned int val);
 extern long long get_time_base(void);
 
-void __ppc_setup_system_time(vint64 *cvFactor);
+void __ppc_setup_system_time(vint32 *cvFactor);
 	// defined in libroot: os/arch/system_time.c
 int64 __ppc_get_time_base(void);
 	// defined in libroot: os/arch/system_time_asm.S

headers/private/kernel/arch/ppc/arch_real_time_data.h

@@ -15,7 +15,7 @@ struct ppc_real_time_data {
 
 struct arch_real_time_data {
 	struct ppc_real_time_data	data[2];
-	vint64						system_time_conversion_factor;
+	vint32						system_time_conversion_factor;
 	vint32						version;
 		// Since there're no cheap atomic_{set,get,add}64() on PPC 32 (i.e. one
 		// that doesn't involve a syscall), we can't have just a single

src/system/kernel/arch/ppc/arch_real_time_clock.cpp

@@ -22,8 +22,12 @@ arch_rtc_init(kernel_args *args, struct real_time_data *data)
 
 	// init the arch specific part of the real_time_data
 	data->arch_data.data[0].system_time_offset = 0;
+	// cvFactor = 2^32 * 1000000 / tbFreq
+	// => (tb * cvFactor) >> 32 = (tb * 2^32 * 1000000 / tbFreq) >> 32
+	//    = tb / tbFreq * 1000000 = time in us
 	data->arch_data.system_time_conversion_factor
-		= args->arch_args.time_base_frequency;
+		= uint32((uint64(1) << 32) * 1000000
+			/ args->arch_args.time_base_frequency);
 	data->arch_data.version = 0;
 
 	// init spinlock

src/system/libroot/os/arch/ppc/system_time.c

@@ -10,10 +10,10 @@
 #include <real_time_data.h>
 
 
-static vint64 *sConversionFactor;
+static vint32 *sConversionFactor;
 
 void
-__ppc_setup_system_time(vint64 *cvFactor)
+__ppc_setup_system_time(vint32 *cvFactor)
 {
 	sConversionFactor = cvFactor;
 }
@@ -26,13 +26,7 @@ bigtime_t
 system_time(void)
 {
 	uint64 timeBase = __ppc_get_time_base();
-	// TODO: The multiplication doesn't look that nice. The value can easily
-	// overflow when timeBase gets big enough. The limit for timebase is
-	// about 2^(64 - 20). This might sound a lot, but the conversion factor
-	// might be quite big. Assuming a worst case factor of 2^32,
-	// this would leave us with only about 2^12 = 4096 seconds we can
-	// represent. The actual factor for my Mac mini is about 40 * 10^6, i.e.
-	// the overflow limit is ca. 100 times greater, but that isn't more than
-	// five days either.
-	return (timeBase * 1000000ULL) / *sConversionFactor;
+
+	uint32 cv = *sConversionFactor;
+	return (timeBase >> 32) * cv + (((timeBase & 0xffffffff) * cv) >> 32);
 }