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[libc] Baremetal version of clock #146417

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[libc] Baremetal version of clock #146417

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1 change: 1 addition & 0 deletions libc/config/baremetal/aarch64/entrypoints.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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1 change: 1 addition & 0 deletions libc/config/baremetal/arm/entrypoints.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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1 change: 1 addition & 0 deletions libc/config/baremetal/riscv/entrypoints.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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11 changes: 11 additions & 0 deletions libc/src/time/baremetal/CMakeLists.txt
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Original file line number Diff line number Diff line change
@@ -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
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45 changes: 45 additions & 0 deletions libc/src/time/baremetal/clock.cpp
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@@ -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);
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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


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
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