[libc] Baremetal version of clock

[petrhosek]

This is analogous to the baremetal version of timespec_get using the __llvm_libc_timespec_get_active embedding interface.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Petr Hosek (petrhosek)

Changes

This is analogous to the baremetal version of timespec_get using the __llvm_libc_timespec_get_active embedding interface.

---

Full diff: https://github.com/llvm/llvm-project/pull/146417.diff

5 Files Affected:

• (modified) libc/config/baremetal/aarch64/entrypoints.txt (+1)
• (modified) libc/config/baremetal/arm/entrypoints.txt (+1)
• (modified) libc/config/baremetal/riscv/entrypoints.txt (+1)
• (modified) libc/src/time/baremetal/CMakeLists.txt (+11)
• (added) libc/src/time/baremetal/clock.cpp (+45)

diff --git a/libc/config/baremetal/aarch64/entrypoints.txt b/libc/config/baremetal/aarch64/entrypoints.txt
index a8e653fdd5159..c54d7d0d8e1bd 100644
--- a/libc/config/baremetal/aarch64/entrypoints.txt
+++ b/libc/config/baremetal/aarch64/entrypoints.txt
@@ -262,6 +262,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     # time.h entrypoints
     libc.src.time.asctime
     libc.src.time.asctime_r
+    libc.src.time.clock
     libc.src.time.ctime
     libc.src.time.ctime_r
     libc.src.time.difftime
diff --git a/libc/config/baremetal/arm/entrypoints.txt b/libc/config/baremetal/arm/entrypoints.txt
index acafef17fa5d1..de7549c57ff44 100644
--- a/libc/config/baremetal/arm/entrypoints.txt
+++ b/libc/config/baremetal/arm/entrypoints.txt
@@ -262,6 +262,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     # time.h entrypoints
     libc.src.time.asctime
     libc.src.time.asctime_r
+    libc.src.time.clock
     libc.src.time.ctime
     libc.src.time.ctime_r
     libc.src.time.difftime
diff --git a/libc/config/baremetal/riscv/entrypoints.txt b/libc/config/baremetal/riscv/entrypoints.txt
index 023826f12d723..7e8c186d52469 100644
--- a/libc/config/baremetal/riscv/entrypoints.txt
+++ b/libc/config/baremetal/riscv/entrypoints.txt
@@ -262,6 +262,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     # time.h entrypoints
     libc.src.time.asctime
     libc.src.time.asctime_r
+    libc.src.time.clock
     libc.src.time.ctime
     libc.src.time.ctime_r
     libc.src.time.difftime
diff --git a/libc/src/time/baremetal/CMakeLists.txt b/libc/src/time/baremetal/CMakeLists.txt
index bf0d42e265d6f..3072c8b14959d 100644
--- a/libc/src/time/baremetal/CMakeLists.txt
+++ b/libc/src/time/baremetal/CMakeLists.txt
@@ -1,3 +1,14 @@
+add_entrypoint_object(
+  clock
+  SRCS
+    clock.cpp
+  HDRS
+    ../clock.h
+  DEPENDS
+    libc.hdr.time_macros
+    libc.hdr.types.struct_timespec
+)
+
 add_entrypoint_object(
   timespec_get
   SRCS
diff --git a/libc/src/time/baremetal/clock.cpp b/libc/src/time/baremetal/clock.cpp
new file mode 100644
index 0000000000000..26b75d7829523
--- /dev/null
+++ b/libc/src/time/baremetal/clock.cpp
@@ -0,0 +1,45 @@
+//===-- Baremetal implementation of the clock function --------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/time/clock.h"
+#include "hdr/time_macros.h"
+#include "hdr/types/struct_timespec.h"
+#include "src/__support/CPP/limits.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/time/units.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+extern "C" bool __llvm_libc_timespec_get_active(struct timespec *ts);
+
+LLVM_LIBC_FUNCTION(clock_t, clock, ()) {
+  using namespace time_units;
+  struct timespec ts;
+  if (!__llvm_libc_timespec_get_active(&ts))
+    return clock_t(-1);
+
+  // The above call gets the CPU time in seconds plus nanoseconds.
+  // The standard requires that we return clock_t(-1) if we cannot represent
+  // clocks as a clock_t value.
+  constexpr clock_t CLOCK_SECS_MAX =
+      cpp::numeric_limits<clock_t>::max() / CLOCKS_PER_SEC;
+  if (ts.tv_sec > CLOCK_SECS_MAX)
+    return clock_t(-1);
+  if (ts.tv_nsec / 1_s_ns > CLOCK_SECS_MAX - ts.tv_sec)
+    return clock_t(-1);
+
+  // For the integer computation converting tv_nsec to clocks to work
+  // correctly, we want CLOCKS_PER_SEC to be less than 1000000000.
+  static_assert(1_s_ns > CLOCKS_PER_SEC,
+                "Expected CLOCKS_PER_SEC to be less than 1'000'000'000.");
+  return clock_t(ts.tv_sec * CLOCKS_PER_SEC +
+                 ts.tv_nsec / (1_s_ns / CLOCKS_PER_SEC));
+}
+
+} // namespace LIBC_NAMESPACE_DECL

[saturn691]

Thanks for the PR! Once we get this merged (and the semihosting library) I can close #146069, but I do have one comment.

[saturn691]

If you were to define this interface like so, downstream, I've have to do this:

bool __llvm_libc_timespec_get_active(struct timespec *ts) {
  long retval = semihosting_call(SYS_CLOCK, 0);
  if (retval == -1)
    return false;

  // Semihosting uses centiseconds
  ts->tv_sec = (retval / 100);
  ts->tv_nsec = (retval % 100) * (1'000'000'000 / 100);
  return true;
}

Pretty much reverse engineering your code. Perhaps this works better for you downstream, but it seems like some extra unnecessary work is being done (at least for me).

You could make the argument that:

• "there could be overflow", afaik hardware clocks are implemented as a single (usually unsigned long) value.
• "the performance gains would be minimal", true, but I think in principle we should not unnecessarily throw away performance