Mailing List Archive

* tests/bench-slope.c (vone): New.
(auto_ghz_bench): Remove memory barrier usage; Generate constant
values from volatile variable.
--

Alderlake P-core is able of fuse "immediate to register
addition" instructions with other instructions. This feature
broke auto-GHZ detection in bench-slope. Patch forces generation
of "register to register addition" by generating constant values
from volatile source, which prevents compiler from known the
values in constant value variables.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
---
tests/bench-slope.c | 26 +++++++++++---------------
1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/tests/bench-slope.c b/tests/bench-slope.c
index b8818aed..71bc97d0 100644
--- a/tests/bench-slope.c
+++ b/tests/bench-slope.c
@@ -515,6 +515,8 @@ err_free:

/********************************************* CPU frequency auto-detection. */

+static volatile size_t vone = 1;
+
static int
auto_ghz_init (struct bench_obj *obj)
{
@@ -535,6 +537,9 @@ auto_ghz_free (struct bench_obj *obj)
static void
auto_ghz_bench (struct bench_obj *obj, void *buf, size_t buflen)
{
+ size_t one = vone;
+ size_t two = one + vone;
+
(void)obj;
(void)buf;

@@ -544,21 +549,12 @@ auto_ghz_bench (struct bench_obj *obj, void *buf, size_t buflen)
* function will give cycles/iteration result 1024.0 on high-end CPUs.
* With turbo, result will be less and can be used detect turbo-clock. */

-#ifdef HAVE_GCC_ASM_VOLATILE_MEMORY
- /* Auto-ghz operation takes two CPU cycles to perform. Memory barriers
- * are used to prevent compiler from optimizing this loop away. */
- #define AUTO_GHZ_OPERATION \
- asm volatile ("":"+r"(buflen)::"memory"); \
- buflen ^= 1; \
- asm volatile ("":"+r"(buflen)::"memory"); \
- buflen -= 2
-#else
- /* TODO: Needs alternative way of preventing compiler optimizations.
- * Mix of XOR and subtraction appears to do the trick for now. */
- #define AUTO_GHZ_OPERATION \
- buflen ^= 1; \
- buflen -= 2
-#endif
+ /* Auto-ghz operation takes two CPU cycles to perform. Variables are
+ * generated through volatile object and therefore compiler is unable
+ * to optimize these operations away. */
+#define AUTO_GHZ_OPERATION \
+ buflen ^= one; \
+ buflen -= two

#define AUTO_GHZ_OPERATION_2 \
AUTO_GHZ_OPERATION; \
--
2.40.1


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