Re: [PATCH][SPARC] sparc: switch -fasynchronous-unwind-tables on by default.

[jose.marchesi@oracle.com (Jose E. Marchesi)]

    > Consider the attached test program.  When built with -g in sparc64-*-*
    > the resulting binary contains:
    > 
    > - A .eh_frame segment containing CFA information for __libc_csu_init and
    >   __libc_csu_fini.
    > 
    > - A .debug_frame segment containing CFA information for func2, func1 and
    >   main.
    > 
    > The backtrace(3) implementation for sparc contains a simple unwinder
    > that works well in most cases, but that unwinder is not used if
    > libgcc_s.so can be dlopened and it provides _Unwind_Backtrace.  Now,
    > _Unwind_Backtrace uses .eh_frame but not .debug_frame.  Thus,
    > backtrace(3) is only useful in programs built with
    > -fasynchronous-unwind-tables even if -g provides CFA info in
    > .debug_frame.
    
    How does that work for e.g. PowerPC or MIPS?  Why not do the same for SPARC?

The glibc PowerPC port doesn't use the libgcc_s unwinder to implement
backtrace().  It has a little ad-hoc unwinder.

MIPS is like x86_64: it exclusively relies on libgcc_s _Unwind_Backtrace
to unwind the stack.

As far as I can tell, -fasynchronous-unwind-tables is disabled in MIPS
by default.  Therefore unless -fasynchronous-unwind-tables is used at
build time backtrace() probably has the same problem than sparc (can't
tell for sure, as I don't have access to any mips host where to test).