test: perf_tests: remove [[gnu::always_inline]] attribute from corout…

…ine perf tests

Unfortunately, gcc (11.2.1) emits the following error:
```
In file included from ../../tests/perf/coroutine_perf.cc:22:
../../include/seastar/testing/perf_tests.hh:367:37: error: 'always_inline' function might not be inlinable [-Werror=attributes]
  367 |     [[gnu::always_inline]] future<> test_##test_group##_##test_case::run()
```

This change removes the attribute and lets the compiler
decide about inlining the coroutine.

The results for gcc are:
test                                      iterations      median         mad         min         max      allocs       tasks        inst
coroutine_test.empty                        99179641    10.016ns     0.004ns    10.012ns    10.048ns       1.000       0.000       190.0
coroutine_test.without_preemption_check     90490151    11.131ns     0.021ns    11.049ns    11.151ns       1.000       0.000       209.0
coroutine_test.ready                        86973367    11.469ns     0.009ns    11.451ns    11.502ns       1.000       0.000       218.0
coroutine_test.maybe_yield                  92291054    10.829ns     0.002ns    10.822ns    10.948ns       1.000       0.000       200.0

And for clang:
before:
coroutine_test.empty                       293173628     3.414ns     0.004ns     3.404ns     3.420ns       0.000       0.000        58.0
coroutine_test.without_preemption_check    292630550     3.400ns     0.001ns     3.399ns     3.419ns       0.000       0.000        58.0
coroutine_test.ready                        82969765    12.059ns     0.016ns    12.043ns    12.145ns       1.000       0.000       233.0
coroutine_test.maybe_yield                  86828070    11.506ns     0.009ns    11.489ns    11.515ns       1.000       0.000       206.0

after:
coroutine_test.empty                       314302789     3.181ns     0.002ns     3.180ns     3.239ns       0.000       0.000        58.0
coroutine_test.without_preemption_check    314361925     3.386ns     0.002ns     3.384ns     3.465ns       0.000       0.000        58.0
coroutine_test.ready                        83549195    11.963ns     0.002ns    11.961ns    11.968ns       1.000       0.000       229.0
coroutine_test.maybe_yield                  87337650    11.431ns     0.005ns    11.426ns    11.451ns       1.000       0.000       202.0

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20220415061254.186016-1-bhalevy@scylladb.com>

include/seastar/testing/perf_tests.hh

@@ -360,18 +360,18 @@ void do_not_optimize(const T& v)
 
 #define PERF_TEST_C(test_group, test_case) \
     struct test_##test_group##_##test_case : test_group { \
-        [[gnu::always_inline]] inline future<> run(); \
+        inline future<> run(); \
     }; \
     static ::perf_tests::internal::test_registrar<test_##test_group##_##test_case> \
     test_##test_group##_##test_case##_registrar(#test_group, #test_case); \
-    [[gnu::always_inline]] future<> test_##test_group##_##test_case::run()
+    future<> test_##test_group##_##test_case::run()
 
 #define PERF_TEST_CN(test_group, test_case) \
     struct test_##test_group##_##test_case : test_group { \
-        [[gnu::always_inline]] inline future<size_t> run(); \
+        inline future<size_t> run(); \
     }; \
     static ::perf_tests::internal::test_registrar<test_##test_group##_##test_case> \
     test_##test_group##_##test_case##_registrar(#test_group, #test_case); \
-    [[gnu::always_inline]] future<size_t> test_##test_group##_##test_case::run()
+    future<size_t> test_##test_group##_##test_case::run()
 
 #endif // SEASTAR_COROUTINES_ENABLED