Add trace_performance and trace_performance_since macros that print a
duration and an optional printf-formatted text to the file specified in
environment variable GIT_TRACE_PERFORMANCE.

These macros, in conjunction with getnanotime(), are intended to simplify
performance measurements from within the application (i.e. profiling via
manual instrumentation, rather than using an external profiling tool).

Unless enabled via GIT_TRACE_PERFORMANCE, these macros have no noticeable
impact on performance, so that test code for well known time killers may
be shipped in release builds. Alternatively, a developer could provide an
additional performance patch (not meant for master) that allows reviewers
to reproduce performance tests more easily, e.g. on other platforms or
using their own repositories.

Usage examples:

Simple use case (measure one code section):

  uint64_t start = getnanotime();
  /* code section to measure */
  trace_performance_since(start, "foobar");

Complex use case (measure repetitive code sections):

  uint64_t t = 0;
  for (;;) {
    /* ignore */
    t -= getnanotime();
    /* code section to measure */
    t += getnanotime();
    /* ignore */
  }
  trace_performance(t, "frotz");

Signed-off-by: Karsten Blees <bl...@dcon.de>
---
 trace.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 trace.h | 18 ++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/trace.c b/trace.c
index 88e05b9..65cd887 100644
--- a/trace.c
+++ b/trace.c
@@ -164,6 +164,27 @@ void trace_strbuf_fl(const char *file, int line, const 
char *key,
        print_trace_line(key, &buf);
 }
 
+static const char *GIT_TRACE_PERFORMANCE = "GIT_TRACE_PERFORMANCE";
+
+static void trace_performance_vprintf_fl(const char *file, int line,
+                                        uint64_t nanos, const char *format,
+                                        va_list ap)
+{
+       struct strbuf buf = STRBUF_INIT;
+
+       if (!prepare_trace_line(file, line, GIT_TRACE_PERFORMANCE, &buf))
+               return;
+
+       strbuf_addf(&buf, "performance: %.9f s", (double) nanos / 1000000000);
+
+       if (format && *format) {
+               strbuf_addstr(&buf, ": ");
+               strbuf_vaddf(&buf, format, ap);
+       }
+
+       print_trace_line(GIT_TRACE_PERFORMANCE, &buf);
+}
+
 #ifndef HAVE_VARIADIC_MACROS
 
 void trace_printf(const char *format, ...)
@@ -195,6 +216,23 @@ void trace_strbuf(const char *key, const struct strbuf 
*data)
        trace_strbuf_fl(NULL, 0, key, data);
 }
 
+void trace_performance(uint64_t nanos, const char *format, ...)
+{
+       va_list ap;
+       va_start(ap, format);
+       trace_performance_vprintf_fl(NULL, 0, nanos, format, ap);
+       va_end(ap);
+}
+
+void trace_performance_since(uint64_t start, const char *format, ...)
+{
+       va_list ap;
+       va_start(ap, format);
+       trace_performance_vprintf_fl(NULL, 0, getnanotime() - start,
+                                    format, ap);
+       va_end(ap);
+}
+
 #else
 
 void trace_printf_key_fl(const char *file, int line, const char *key,
@@ -215,6 +253,15 @@ void trace_argv_printf_fl(const char *file, int line, 
const char **argv,
        va_end(ap);
 }
 
+void trace_performance_fl(const char *file, int line, uint64_t nanos,
+                             const char *format, ...)
+{
+       va_list ap;
+       va_start(ap, format);
+       trace_performance_vprintf_fl(file, line, nanos, format, ap);
+       va_end(ap);
+}
+
 #endif /* HAVE_VARIADIC_MACROS */
 
 
diff --git a/trace.h b/trace.h
index c4964aa..f491471 100644
--- a/trace.h
+++ b/trace.h
@@ -21,6 +21,14 @@ extern void trace_argv_printf(const char **argv, const char 
*format, ...);
 
 extern void trace_strbuf(const char *key, const struct strbuf *data);
 
+/* Prints elapsed time (in nanoseconds) if GIT_TRACE_PERFORMANCE is enabled. */
+__attribute__((format (printf, 2, 3)))
+extern void trace_performance(uint64_t nanos, const char *format, ...);
+
+/* Prints elapsed time since 'start' if GIT_TRACE_PERFORMANCE is enabled. */
+__attribute__((format (printf, 2, 3)))
+extern void trace_performance_since(uint64_t start, const char *format, ...);
+
 #else
 
 /*
@@ -56,6 +64,13 @@ extern void trace_strbuf(const char *key, const struct 
strbuf *data);
 #define trace_strbuf(key, data) \
        trace_strbuf_fl(__FILE__, __LINE__, key, data)
 
+#define trace_performance(nanos, ...) \
+       trace_performance_fl(__FILE__, __LINE__, nanos, __VA_ARGS__)
+
+#define trace_performance_since(start, ...) \
+       trace_performance_fl(__FILE__, __LINE__, getnanotime() - (start), \
+                            __VA_ARGS__)
+
 /* backend functions, use non-*fl macros instead */
 __attribute__((format (printf, 4, 5)))
 extern void trace_printf_key_fl(const char *file, int line, const char *key,
@@ -65,6 +80,9 @@ extern void trace_argv_printf_fl(const char *file, int line, 
const char **argv,
                                 const char *format, ...);
 extern void trace_strbuf_fl(const char *file, int line, const char *key,
                            const struct strbuf *data);
+__attribute__((format (printf, 4, 5)))
+extern void trace_performance_fl(const char *file, int line,
+                                uint64_t nanos, const char *fmt, ...);
 
 #endif /* HAVE_VARIADIC_MACROS */
 
-- 
2.0.0.402.g13b8b25

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