PR abseil#1644: unscaledcycleclock: remove RISC-V support

Imported from GitHub PR abseil#1644

Starting with Linux 6.6 [1], RDCYCLE is a privileged instruction on RISC-V and can't be used directly from userland. There is a sysctl option to change that as a transition period, but it will eventually disappear.

The RDTIME instruction is another less accurate alternative, however its frequency varies from board to board, and there is currently now way to get its frequency from userland [2].

Therefore this patch just removes the code for unscaledcycleclock on RISC-V. Without processor specific implementation, abseil relies on std::chrono::steady_clock::now().time_since_epoch() which is basically a wrapper around clock_gettime (CLOCK_MONOTONIC), which in turns use __vdso_clock_gettime(). On RISC-V this VDSO is just a wrapper around RDTIME correctly scaled to use nanoseconds units.

This fixes the testsuite on riscv64, tested on a VisionFive 2 board.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cc4c07c89aada16229084eeb93895c95b7eabaa3
[2] abseil#1631
Merge 43356a2 into 76f8011

Merging this change closes abseil#1644

COPYBARA_INTEGRATE_REVIEW=abseil#1644 from aurel32:rv64-no-unscaledcycleclock 43356a2
PiperOrigin-RevId: 618286262
Change-Id: Ie4120a727e7d0bb185df6e06ea145c780ebe6652

absl/base/internal/unscaledcycleclock.cc

@@ -118,18 +118,6 @@ double UnscaledCycleClock::Frequency() {
   return aarch64_timer_frequency;
 }
 
-#elif defined(__riscv)
-
-int64_t UnscaledCycleClock::Now() {
-  int64_t virtual_timer_value;
-  asm volatile("rdcycle %0" : "=r"(virtual_timer_value));
-  return virtual_timer_value;
-}
-
-double UnscaledCycleClock::Frequency() {
-  return base_internal::NominalCPUFrequency();
-}
-
 #elif defined(_M_IX86) || defined(_M_X64)
 
 #pragma intrinsic(__rdtsc)

absl/base/internal/unscaledcycleclock_config.h

@@ -21,8 +21,8 @@
 
 // The following platforms have an implementation of a hardware counter.
 #if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__) || \
-    defined(__powerpc__) || defined(__ppc__) || defined(__riscv) ||     \
-    defined(_M_IX86) || (defined(_M_X64) && !defined(_M_ARM64EC))
+    defined(__powerpc__) || defined(__ppc__) || defined(_M_IX86) ||     \
+    (defined(_M_X64) && !defined(_M_ARM64EC))
 #define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 1
 #else
 #define ABSL_HAVE_UNSCALED_CYCLECLOCK_IMPLEMENTATION 0
@@ -53,8 +53,8 @@
 #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) || \
-     defined(_M_IX86) || defined(_M_X64))
+#if (defined(__i386__) || defined(__x86_64__) || defined(_M_IX86) || \
+     defined(_M_X64))
 #define ABSL_INTERNAL_UNSCALED_CYCLECLOCK_FREQUENCY_IS_CPU_FREQUENCY
 #endif
 #endif