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unscaledcycleclock: use RDTIME instead of RDCYCLE on RISC-V #1631

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unscaledcycleclock: use RDTIME instead of RDCYCLE on RISC-V #1631

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2 changes: 1 addition & 1 deletion absl/base/internal/unscaledcycleclock.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -125,7 +125,7 @@ double UnscaledCycleClock::Frequency() {

int64_t UnscaledCycleClock::Now() {
int64_t virtual_timer_value;
asm volatile("rdcycle %0" : "=r"(virtual_timer_value));
asm volatile("rdtime %0" : "=r"(virtual_timer_value));
return virtual_timer_value;
}

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2 changes: 1 addition & 1 deletion absl/base/internal/unscaledcycleclock_config.h
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Expand Up @@ -53,7 +53,7 @@
#if ABSL_USE_UNSCALED_CYCLECLOCK
// This macro can be used to test if UnscaledCycleClock::Frequency()
// is NominalCPUFrequency() on a particular platform.
#if (defined(__i386__) || defined(__x86_64__) || defined(__riscv) || \
#if (defined(__i386__) || defined(__x86_64__) || \
defined(_M_IX86) || defined(_M_X64))
#define ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
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We need an implementation of UnscaledCycleClock::Frequency() now that these are not the same thing. The code in this repo may not call that method, but code exists that does.

https://riscv.org/wp-content/uploads/2016/06/riscv-spec-v2.1.pdf says

The RDTIME pseudo-instruction reads the low XLEN bits of the time CSR, which counts wall-clock
real time that has passed from an arbitrary start time in the past. RDTIMEH is an RV32I-only instruction that reads bits 63–32 of the same real-time counter. The underlying 64-bit counter should
never overflow in practice. The execution environment should provide a means of determining the
period of the real-time counter (seconds/tick). The period must be constant. The real-time clocks
of all hardware threads in a single user application should be synchronized to within one tick of the
real-time clock. The environment should provide a means to determine the accuracy of the clock.

I have no idea how to get that.

If you are just trying to get something to build, -DABSL_USE_UNSCALED_CYCLECLOCK=0 should work.

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The frequency is implementation dependent, and there is no easy way to get it. Could it be measured instead? Basically how it is done for the ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY?

#endif
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