[patch,libgfortran,toplevel] Use libbacktrace in libgfortran

[FX <fxcoudert at gmail dot com>]

Use libbacktrace (instead of our own unwind-based code) to display backtraces from libgfortran upon error or user request.

  1. In toplevel Makefile.def, make libgfortran depend on libbacktrace (needs global reviewer approval)
  2. In gcc/fortran/config-lang.in, add libbacktrace to target_libs
  3. In libgfortran, we remove our own code and substitute calls to libbacktrace

Bootstrapped and regtested on x86_64-pc-linux-gnu (which has full libbacktrace support) and x86_64-apple-darwin14 (which has minimal libbacktrace support). OK to commit to trunk?

FX

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The fine print:

This is a great simplification of the library code, because we get rid of the unwind code, and donât need all the code to find out our executableâs absolute path, find addr2line, pipe-fork-exec addr2line, etc. Just libgfortran/runtime/backtrace.câs size is reduced by half. And we will benefit from future possible improvements in libbacktrace.

One choice I have made is to not display in backtraces the libgfortran functions, i.e. anything starting with _gfortran_ or _gfortrani_ is simply not printed. I think this is more convenient for some users, because it will avoid them being confused by symbols they donât recognize. It means, however, that runtime backtraces are a little less useful for us in debuggingâ but then, we know how to use gdb :)  I am open to being convinced otherwise and remove these few lines of code.

Other than that deliberate choice, no functionality is lost. On dwarf/elf targets (the majority), we get beautiful backtraces as before if code is compiled with â-gâ:

#0  0x4006f5 __foo_MOD_gee
	/home/fx/gcc/irun/a.f90:4
#1  0x400706 bar
	/home/fx/gcc/irun/a.f90:16
#2  0x40072e foo
	/home/fx/gcc/irun/a.f90:12
#3  0x400715 test
	/home/fx/gcc/irun/a.f90:9
#4  0x400765 main
	/home/fx/gcc/irun/a.f90:9

and if not compiled with â-gâ, we get the nonsymbolic backtrace, as before:

#0  0x4010bd ???
#1  0x4010ce ???
#2  0x4010f6 ???
#3  0x4010dd ???
#4  0x40112d ???
#5  0x42a6b3 ???
#6  0x400fbc ???


We will get backtraces on PE/COFF targets (i.e. Windows), where I donât think we had them before (but cannot test).
And on non-elf targets such as Mac OS X, we stay with the same as before, i.e. non-symbolic backtraces.



PS: This is probably one of the last of my âsummerâ gfortran projects for this year. The end of holidays is near, and Iâll have to get back to work. It was fun, as always!

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