Incorrect usage of PERCPU_USE_RSEQ_ASM_GOTO macro

[ppadevski]

Hi,

I've been trying to build tcmalloc with clang 8 to no avail. The compilation error is:

tcmalloc/internal/percpu_tcmalloc.h:255:7: error: 'asm goto' constructs are not supported yet
   asm goto(

The problem seems to be the following check

#ifdef PERCPU_USE_RSEQ_ASM_GOTO
  asm goto(
#else

and specifically the fact that PERCPU_USE_RSEQ_ASM_GOTO is always defined but has different values: 1 or 0. In my case I have it defined as 0 from here:

 42
 43 #else
 44 #define PERCPU_USE_RSEQ_ASM_GOTO 0 # <<<<<<<<<<<<<<<<
 45 #endif
 46 #else
 47 #define PERCPU_USE_RSEQ_ASM_GOTO 0
 48 #endif

You can easily reproduce the issue

#include <iostream>

#if 0
#define PERCPU_USE_RSEQ_ASM_GOTO 1
#else
#define PERCPU_USE_RSEQ_ASM_GOTO 0
#endif

int
main()
{
#ifdef PERCPU_USE_RSEQ_ASM_GOTO
   std::cout << "defined, value: " << PERCPU_USE_RSEQ_ASM_GOTO << std::endl;
#endif
}

[ckennelly]

This should be easy to fix and would make the macros actually behave as expected. I think there's a second issue behind it, though. We use flag output constraints (=@ccae and =@ccbe) to avoid using an extra register.

Clang 8 isn't supported and doesn't see testing, as a result, which leads to these creeping in.