[llvm-exegesis] Don't try to use SYS_rseq if it's not defined.

[Romain-Geissler-1A]

Summary

When compiling against recent glibc (>= 2.35) but old kernel headers (< 4.18), SYS_rseq is not defined and thus llvm-exegesis fails to build. So also check that SYS_rseq is defined before trying to use it.

Fixes https://github.com/llvm/llvm-project/issues/64456

[Romain-Geissler-1A]

@MaskRay Thanks for reviewing this.

It's reported above in this UI that it causes some build failure, but I doubt the build failure are related to this. Is it ok to ignore them ? Also, can you please post this patch for me, as I have no right to push in the github repository ?

[gchatelet]

Thx for the patch @Romain-Geissler-1A . I'll submit the patch on your behalf.

[aidengrossman]

Since this is probably a configuration that people will be running the 17.x release on, I've opened up https://github.com/llvm/llvm-project/issues/64482#issuecomment-1667352876 to backport this change. Thanks for the patch!

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

 #include <sys/ptrace.h>
 #include <sys/socket.h>
 #include <sys/syscall.h>
 #include <sys/wait.h>
 #include <unistd.h>
 
 #if defined(__GLIBC__) && __has_include(<sys/rseq.h>) && defined(HAVE_BUILTIN_THREAD_POINTER)
 #include <sys/rseq.h>
-#ifdef RSEQ_SIG
+#if defined(RSEQ_SIG) && defined(SYS_rseq)
 #define GLIBC_INITS_RSEQ
 #endif
 #endif
 #endif
 
 namespace llvm {
 namespace exegesis {