[Bug c++/43498] New: -fasynchronous-unwind-tables does not seem to do anything

[Bug c++/43498] New: -fasynchronous-unwind-tables does not seem to do anything

[lat at cern dot ch]

I have compiled various C and C++ source files in attempt to generate async
unwind tables for better stack trace accuracy with asynchronous signals, adding
-fasynchronous-unwind-tables to compile options. I have so far been completely
unable to come up with any example where the option would produce any
difference in the generated binaries, and unwind tables (.eh_frame)
specifically.

For example please compile the following source file foo.cc:

      struct A1 { virtual ~A1(); virtual double foo(void); };
      struct A2 { virtual ~A2(); virtual double bar(void); };
      struct B : A1, A2 { ~B(); virtual double foo(void); };
      A1::~A1() {}
      double A1::foo() { return 3.14; }
      A2::~A2() {}
      double A2::bar() { return 2.17; }
      B::~B() {}
      double B::foo() { return A1::foo() + A2::bar(); }

    with and without -fasynchronous-unwind-tables:

      c++ -fasynchronous-unwind-tables -O2 -fPIC -W -Wall -shared -o libfooa.so
foo.cc
      c++ -O2 -fPIC -W -Wall -shared -o libfoob.so foo.cc

    then compare unwind information:

      for x in a b; do
        objdump -w -x -d libfoo$x.so > foo$x-disasm.txt
        readelf -Wa --debug=frames libfoo$x.so > foo$x-info.txt
      done

      diff -u foo{a,b}-disasm.txt
      --- fooa-disasm.txt       2010-03-23 18:28:46.000000000 +0100
      +++ foob-disasm.txt       2010-03-23 18:28:46.000000000 +0100
      @@ -1,6 +1,6 @@

      -libfooa.so:     file format elf64-x86-64
      -libfooa.so
      +libfoob.so:     file format elf64-x86-64
      +libfoob.so
       architecture: i386:x86-64, flags 0x00000150:
       HAS_SYMS, DYNAMIC, D_PAGED
       start address 0x0000000000000c90

      diff -u foo{a,b}-info.txt  
      (no output)

    note that there is basically no difference at all, and there is
    no unwind info for the thunks, disassembly has:

      0000000000000df0 <_ZN1B3fooEv>:
       df0: 53                      push   %rbx
       df1: 48 89 fb                mov    %rdi,%rbx
       df4: 48 83 c3 08             add    $0x8,%rbx
       df8: 48 83 ec 10             sub    $0x10,%rsp
       dfc: e8 7f fe ff ff          callq  c80 <_ZN2A13fooEv@plt>
       e01: 48 89 df                mov    %rbx,%rdi
       e04: f2 0f 11 44 24 08       movsd  %xmm0,0x8(%rsp)
       e0a: e8 41 fe ff ff          callq  c50 <_ZN2A23barEv@plt>
       e0f: f2 0f 58 44 24 08       addsd  0x8(%rsp),%xmm0
       e15: f2 0f 11 44 24 08       movsd  %xmm0,0x8(%rsp)
       e1b: 48 83 c4 10             add    $0x10,%rsp
       e1f: 5b                      pop    %rbx
       e20: c3                      retq   
       e21: 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00    nopw  
%cs:0x0(%rax,%rax,1)

      0000000000000e30 <_ZThn8_N1BD1Ev>:
       e30: 48 83 c7 f8             add    $0xfffffffffffffff8,%rdi
       e34: eb 0a                   jmp    e40 <_ZN1BD1Ev>
       e36: 66 2e 0f 1f 84 00 00 00 00 00   nopw   %cs:0x0(%rax,%rax,1)

      0000000000000e40 <_ZN1BD1Ev>:
       e40: 48 89 6c 24 f8          mov    %rbp,-0x8(%rsp)
       e45: 48 89 5c 24 f0          mov    %rbx,-0x10(%rsp)
       e4a: 48 83 ec 18             sub    $0x18,%rsp
       [snip]

    and unwind info has a gap at 0xe30, the thunk, and does not describe
    the epilogue at address 0xe1b:

      000000b0 0000001c 000000b4 FDE cie=00000000 pc=00000df0..00000e21
        Augmentation data:     00 00 00 00

        DW_CFA_advance_loc: 1 to 00000df1
        DW_CFA_def_cfa_offset: 16
        DW_CFA_offset: r3 (rbx) at cfa-16
        DW_CFA_advance_loc: 11 to 00000dfc
        DW_CFA_def_cfa_offset: 32
        DW_CFA_nop
        DW_CFA_nop
        DW_CFA_nop

      000000d0 0000001c 000000d4 FDE cie=00000000 pc=00000e40..00000e99
        Augmentation data:     83 00 00 00

        DW_CFA_advance_loc: 14 to 00000e4e
        DW_CFA_def_cfa_offset: 32
        DW_CFA_offset: r3 (rbx) at cfa-24
        DW_CFA_offset: r6 (rbp) at cfa-16
        DW_CFA_nop
        DW_CFA_nop
        DW_CFA_nop
        DW_CFA_nop

I believe the part about missing unwind info for epilogues is a known problem
and patches have been committed to the head. Similarly lack of unwind
information for global ctors and dtors seems to be known. But how about the
missing unwind information for the thunks?

More exactly where should I expect the -fasynchronous-unwind-tables option to
make a difference? Any example code / output available?

The compiler is GCC 4.3.4 with binutils 2.19.1 on RHEL5-derived system.

$ c++ -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../configure
--prefix=/build/3jm/slc5_amd64_gcc434/external/gcc/4.3.4
--enable-languages=c,c++,fortran
--with-gmp=/build/3jm/slc5_amd64_gcc434/external/gcc/4.3.4/tmp/gmp
--with-mpfr=/build/3jm/slc5_amd64_gcc434/external/gcc/4.3.4/tmp/mpfr
--enable-shared
Thread model: posix
gcc version 4.3.4 (GCC) 

$ as -v /dev/null
GNU assembler version 2.19.1 (x86_64-unknown-linux-gnu) using BFD version (GNU
Binutils) 2.19.1

$ ld -v 
GNU ld (GNU Binutils) 2.19.1


-- 
           Summary: -fasynchronous-unwind-tables does not seem to do
                    anything
           Product: gcc
           Version: 4.3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: lat at cern dot ch
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug c++/43498] -fasynchronous-unwind-tables does not seem to do anything

[jakub at gcc dot gnu dot org]

------- Comment #1 from jakub at gcc dot gnu dot org  2010-03-23 18:47 -------
-fasynchronous-unwind-tables is the default on x86_64.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug c++/43498] -fasynchronous-unwind-tables does not seem to do anything

[lat at cern dot ch]

------- Comment #2 from lat at cern dot ch  2010-03-23 18:52 -------
Created an attachment (id=20174)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=20174&action=view)
Random C source file for testing

Random C source file for additional testing. Compile on Linux with and without
-fasynchronous-unwind-tables, with normal compile "gcc -g -std=gnu99 -W -Wall
-O2 islocal.c -o islocal". No difference in "readelf --debug=frames" output.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug c++/43498] -fasynchronous-unwind-tables does not seem to do anything

[lat at cern dot ch]

------- Comment #3 from lat at cern dot ch  2010-03-23 18:54 -------
Ah, that would explain it. Does it mean that all omissions and inaccuracies in
unwind information are bugs? Like the lack of unwind info for thunks?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug middle-end/43498] -fasynchronous-unwind-tables does not seem to do anything

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2010-03-23 18:56 -------
(In reply to comment #3)
> Ah, that would explain it. Does it mean that all omissions and inaccuracies in
> unwind information are bugs? Like the lack of unwind info for thunks?

yes thunks should have unwind tables also.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug middle-end/43498] -fasynchronous-unwind-tables does not seem to do anything

[lat at cern dot ch]

------- Comment #5 from lat at cern dot ch  2010-03-23 18:58 -------
OK, reopening bug then.


-- 

lat at cern dot ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug middle-end/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[lat at cern dot ch]

------- Comment #6 from lat at cern dot ch  2010-03-23 19:04 -------
Changing subject to be more descriptive of the actual bug.


-- 

lat at cern dot ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|-fasynchronous-unwind-tables|No unwind info for thunks
                   |does not seem to do anything|even with -fasynchronous-
                   |                            |unwind-tables


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug target/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2010-03-23 19:13 -------
The x86-64 target writes directly out asm rather than going through some RTL
code which causes unwind tables not to be written out for the thunks.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
          Component|middle-end                  |target
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2010-03-23 19:13:40
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug target/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[jakub at gcc dot gnu dot org]

------- Comment #8 from jakub at gcc dot gnu dot org  2010-03-23 22:10 -------
At least when using CFI asm this is solvable easily, see how I've changed i386
PIC setup generation.  Without CFI asm it would be much harder though.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug target/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug target/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[jakub at gcc dot gnu dot org]

------- Comment #9 from jakub at gcc dot gnu dot org  2010-03-24 11:59 -------
Actually, as the thunks are emitted before .eh_frame, it is much easier.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |jakub at gcc dot gnu dot org
                   |dot org                     |
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2010-03-23 19:13:40         |2010-03-24 11:59:53
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug target/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[jakub at gcc dot gnu dot org]

------- Comment #10 from jakub at gcc dot gnu dot org  2010-03-24 12:06 -------
Created an attachment (id=20182)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=20182&action=view)
gcc45-pr43498.patch

Patch to emit unwind info for thunks on x86, x86-64 and s390{,x}.
ppc/ppc64, sparc/sparc64 and ia64 (from arches that I care at least a little
bit about) should be outputting unwind info already before this change.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug target/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[jakub at gcc dot gnu dot org]

------- Comment #11 from jakub at gcc dot gnu dot org  2010-03-24 20:31 -------
Subject: Bug 43498

Author: jakub
Date: Wed Mar 24 20:31:09 2010
New Revision: 157706

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=157706
Log:
        PR target/43498
        * config/i386/i386.c (x86_output_mi_thunk): Call final_start_function
        at the beginning and final_end_function at the end.
        * config/s390/s390.c (s390_output_mi_thunk): Likewise.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/i386/i386.c
    trunk/gcc/config/s390/s390.c


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498

[Bug target/43498] No unwind info for thunks even with -fasynchronous-unwind-tables

[jakub at gcc dot gnu dot org]

------- Comment #12 from jakub at gcc dot gnu dot org  2010-03-24 20:52 -------
Should be fixed.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43498