[PATCH v3] tests/qht-bench: Fix Clang 'implicit-int-float-conversion' warning

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

When building with Clang 10 on Fedora 32, we get:

  tests/qht-bench.c:287:29: error: implicit conversion from 'unsigned long' to 'double' changes value from 18446744073709551615 to 18446744073709551616 [-Werror,-Wimplicit-int-float-conversion]
          *threshold = rate * UINT64_MAX;
                            ~ ^~~~~~~~~~
  /usr/include/stdint.h:130:23: note: expanded from macro 'UINT64_MAX'
  # define UINT64_MAX             (__UINT64_C(18446744073709551615))
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/include/stdint.h:107:25: note: expanded from macro '__UINT64_C'
  #  define __UINT64_C(c) c ## UL
                          ^~~~~~~
  <scratch space>:14:1: note: expanded from here
  18446744073709551615UL
  ^~~~~~~~~~~~~~~~~~~~~~

Fix by using nextafter() from <math.h>:

  double nextafter( double from, double to );

      Returns the next representable value of 'from'
      in the direction of 'to'.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
Supersedes: <20200504144125.22435-1-philmd@redhat.com>

Since v2: corrected patch subject
Since v1: nextafterf() -> nextafter()

Cc: Emilio G. Cota <cota@braap.org>
Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Alexander Bulekov <alxndr@bu.edu>
---
 tests/qht-bench.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/qht-bench.c b/tests/qht-bench.c
index e3b512f26f..54ce1e8188 100644
--- a/tests/qht-bench.c
+++ b/tests/qht-bench.c
@@ -10,6 +10,7 @@
 #include "qemu/qht.h"
 #include "qemu/rcu.h"
 #include "qemu/xxhash.h"
+#include <math.h>
 
 struct thread_stats {
     size_t rd;
@@ -284,7 +285,7 @@ static void do_threshold(double rate, uint64_t *threshold)
     if (rate == 1.0) {
         *threshold = UINT64_MAX;
     } else {
-        *threshold = rate * UINT64_MAX;
+        *threshold = rate * nextafter(0x1p64, 0.0);
     }
 }
 
-- 
2.21.3