benchtests: Restore the clock_gettime option

commit 7621e38bf3
Author: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
Date:   Tue Jan 29 17:43:45 2019 +0000

    Add generic hp-timing support

removed the clock_gettime option.  Restore the clock_gettime option for
some x86 CPUs on which value from RDTSC may not be incremented at a fixed
rate.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

benchtests/Makefile

@@ -132,11 +132,17 @@ endif
 
 CPPFLAGS-nonlib += -DDURATION=$(BENCH_DURATION) -D_ISOMAC
 
+# Use clock_gettime to measure performance of functions.  The default is
+# to use the architecture-specific high precision timing instructions.
+ifdef USE_CLOCK_GETTIME
+CPPFLAGS-nonlib += -DUSE_CLOCK_GETTIME
+else
 # On x86 processors, use RDTSCP, instead of RDTSC, to measure performance
 # of functions.  All x86 processors since 2010 support RDTSCP instruction.
 ifdef USE_RDTSCP
 CPPFLAGS-nonlib += -DUSE_RDTSCP
 endif
+endif
 
 DETAILED_OPT :=
 

benchtests/README

@@ -27,7 +27,12 @@ BENCH_DURATION.
 
 The benchmark suite does function call measurements using architecture-specific
 high precision timing instructions whenever available.  When such support is
-not available, it uses clock_gettime (CLOCK_MONOTONIC).
+not available, it uses clock_gettime (CLOCK_MONOTONIC).  One can force the
+benchmark to use clock_gettime by invoking make as follows:
+
+  $ make USE_CLOCK_GETTIME=1 bench
+
+Again, one must run `make bench-clean' before changing the measurement method.
 
 On x86 processors, RDTSCP instruction provides more precise timing data
 than RDTSC instruction.  All x86 processors since 2010 support RDTSCP

benchtests/bench-timing.h

@@ -18,7 +18,11 @@
 
 #undef attribute_hidden
 #define attribute_hidden
+#ifdef USE_CLOCK_GETTIME
+# include <sysdeps/generic/hp-timing.h>
+#else
 # include <hp-timing.h>
+#endif
 #include <stdint.h>
 
 #define GL(x) _##x