does clang define __GNUC__ as well? otherwise why !define(__clang__) ?
Yes, clang does defind __GNUC__ .
you use assembly directly, so optimize() should be irrelevant, no?
So we can make this non-GCC specific, right?
Yes, I'll make it in the next version.
is it guaranteed that nums address will end up in rdx and a in rax?...
I'd feel more comfortable if you explicitly set up rdx and rax in assembly, then add USDT with STAP_PROBE_ASM. That should be possible with embedded assembly, no?
I think it will in that the input operand constrain `"d"(nums), "a"(0)` implies the nums address will end up in rdx while the index 0 will be in rax.
So I think we don't need to explicitly set up rdx and rax again.
why these unnecessary comments embedded in the assembly?...
Because there is "before" and "after" in the `STAP_PROBE_ASM` example. It's OK to remove it.
At 2025-08-23 06:59:49, "Andrii Nakryiko" andrii.nakryiko@gmail.com wrote:
On Fri, Aug 22, 2025 at 8:16 AM Jiawei Zhao phoenix500526@163.com wrote:
When using GCC on x86-64 to compile an usdt prog with -O1 or higher optimization, the compiler will generate SIB addressing mode for global array and PC-relative addressing mode for global variable, e.g. "1@-96(%rbp,%rax,8)" and "-1@4+t1(%rip)".
In this patch:
- enrich subtest_basic_usdt test case to cover SIB addressing usdt argument spec handling logic
Signed-off-by: Jiawei Zhao phoenix500526@163.com
tools/testing/selftests/bpf/prog_tests/usdt.c | 44 ++++++++++++++++++- tools/testing/selftests/bpf/progs/test_usdt.c | 30 +++++++++++++ 2 files changed, 72 insertions(+), 2 deletions(-)
diff --git a/tools/testing/selftests/bpf/prog_tests/usdt.c b/tools/testing/selftests/bpf/prog_tests/usdt.c index 9057e983cc54..c04b416aa4a8 100644 --- a/tools/testing/selftests/bpf/prog_tests/usdt.c +++ b/tools/testing/selftests/bpf/prog_tests/usdt.c @@ -25,6 +25,10 @@ unsigned short test_usdt0_semaphore SEC(".probes"); unsigned short test_usdt3_semaphore SEC(".probes"); unsigned short test_usdt12_semaphore SEC(".probes");
+#if ((defined(__x86_64__) || defined(__i386__)) && defined(__GNUC__) && !defined(__clang__))
does clang define __GNUC__ as well? otherwise why !define(__clang__) ?
+unsigned short test_usdt_sib_semaphore SEC(".probes"); +#endif
static void __always_inline trigger_func(int x) { long y = 42;
@@ -40,12 +44,29 @@ static void __always_inline trigger_func(int x) { } }
+#if ((defined(__x86_64__) || defined(__i386__)) && defined(__GNUC__) && !defined(__clang__)) +static __attribute__((optimize("O1"))) void trigger_sib_spec(void)
you use assembly directly, so optimize() should be irrelevant, no?
So we can make this non-GCC specific, right?
+{
/* Base address + offset + (index * scale) *//* Force SIB addressing with inline assembly */asm volatile("# probe point with memory access\n"STAP_PROBE_ASM(test, usdt_sib, -2@(%%rdx,%%rax,2))is it guaranteed that nums address will end up in rdx and a in rax?...
I'd feel more comfortable if you explicitly set up rdx and rax in assembly, then add USDT with STAP_PROBE_ASM. That should be possible with embedded assembly, no?
"# end probe point"why these unnecessary comments embedded in the assembly?...
:: "d"(nums), "a"(0): "memory");+}
[...]
+int usdt_sib_called; +u64 usdt_sib_cookie; +int usdt_sib_arg_cnt; +int usdt_sib_arg_ret; +u64 usdt_sib_arg; +int usdt_sib_arg_size;
+// Note: usdt_sib is only tested on x86-related architectures, so it requires +// manual attach since auto-attach will panic tests under other architectures
don't use c++ style comments
+SEC("usdt") +int usdt_sib(struct pt_regs *ctx) +{
long tmp;if (my_pid != (bpf_get_current_pid_tgid() >> 32))return 0;__sync_fetch_and_add(&usdt_sib_called, 1);usdt_sib_cookie = bpf_usdt_cookie(ctx);usdt_sib_arg_cnt = bpf_usdt_arg_cnt(ctx);usdt_sib_arg_ret = bpf_usdt_arg(ctx, 0, &tmp);usdt_sib_arg = (short)tmp;usdt_sib_arg_size = bpf_usdt_arg_size(ctx, 0);return 0;+}
char _license[] SEC("license") = "GPL";
2.43.0