avoid chrono inclusion when clock_gettime is available (#695)

* avoid chrono inclusion when clock_gettime is available

* fix build

src/snmalloc/aal/aal.h

@@ -10,10 +10,19 @@
 #include "aal_concept.h"
 #include "aal_consts.h"
 
-#include <chrono>
+#if __has_include(<time.h>)
+#  include <time.h>
+#  ifdef CLOCK_MONOTONIC
+#    define SNMALLOC_TICK_USE_CLOCK_GETTIME
+#  endif
+#endif
 #include <cstdint>
 #include <utility>
 
+#ifndef SNMALLOC_TICK_USE_CLOCK_GETTIME
+#  include <chrono>
+#endif
+
 #if ( \
   defined(__i386__) || defined(_M_IX86) || defined(_X86_) || \
   defined(__amd64__) || defined(__x86_64__) || defined(_M_X64) || \
@@ -169,11 +178,27 @@ namespace snmalloc
       if constexpr (
         (Arch::aal_features & NoCpuCycleCounters) == NoCpuCycleCounters)
       {
+#ifdef SNMALLOC_TICK_USE_CLOCK_GETTIME
+        // the buf is populated by clock_gettime
+        SNMALLOC_UNINITIALISED timespec buf;
+        // we can skip the error checking here:
+        // * EFAULT: for out-of-bound pointers (buf is always valid stack
+        // memory)
+        // * EINVAL: for invalid clock_id (we only use CLOCK_MONOTONIC enforced
+        // by POSIX.1)
+        // Notice that clock_gettime is a usually a vDSO call, so the overhead
+        // is minimal.
+        ::clock_gettime(CLOCK_MONOTONIC, &buf);
+        return static_cast<uint64_t>(buf.tv_sec) * 1000'000'000 +
+          static_cast<uint64_t>(buf.tv_nsec);
+#  undef SNMALLOC_TICK_USE_CLOCK_GETTIME
+#else
         auto tick = std::chrono::high_resolution_clock::now();
         return static_cast<uint64_t>(
           std::chrono::duration_cast<std::chrono::nanoseconds>(
             tick.time_since_epoch())
             .count());
+#endif
       }
       else
       {

src/snmalloc/ds_core/defines.h

@@ -194,6 +194,15 @@ namespace snmalloc
 #  endif
 #endif
 
+// Used to suppress pattern filling for potentially unintialized variables with
+// automatic storage duration.
+// https://clang.llvm.org/docs/AttributeReference.html#uninitialized
+#ifdef __clang__
+#  define SNMALLOC_UNINITIALISED [[clang::uninitialized]]
+#else
+#  define SNMALLOC_UNINITIALISED
+#endif
+
 namespace snmalloc
 {
   /**

src/snmalloc/pal/pal_posix.h

@@ -6,6 +6,7 @@
 #if defined(SNMALLOC_BACKTRACE_HEADER)
 #  include SNMALLOC_BACKTRACE_HEADER
 #endif
+#include <cstdlib>
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>

src/snmalloc/pal/pal_timer_default.h

@@ -4,8 +4,6 @@
 #include "pal_consts.h"
 #include "pal_ds.h"
 
-#include <chrono>
-
 namespace snmalloc
 {
   template<typename PalTime>