[Bug c++/53380] .ehframe could be smaller
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53380 Andrew Pinski pinskia at gcc dot gnu.org changed: What|Removed |Added Status|UNCONFIRMED |WAITING Last reconfirmed||2012-05-22 Ever Confirmed|0 |1
[Bug c++/53380] .ehframe could be smaller
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53380 --- Comment #2 from Andrew Pinski pinskia at gcc dot gnu.org 2012-05-22 06:01:32 UTC --- Did -fno-asynchronous-unwind-tables do what you wanted it to do? In that disable the unwinding tables when not using exceptions?
[Bug c++/53380] .ehframe could be smaller
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53380
--- Comment #3 from msharov at users dot sourceforge.net 2012-05-22 11:21:41
UTC ---
Did -fno-asynchronous-unwind-tables do what you wanted it to do? In that
disable the unwinding tables when not using exceptions?
No, it did not. For example:
#include stdio.h
int calculate (int x, int y) { return (x * y); }
void print (void) { printf (%d, calculate(1,2)); }
int main (void) { print(); return (0); }
g++ -Os -c -fno-asynchronous-unwind-tables -o tes.o tes.cc
readelf --debug-dump=frames tes.o
Contents of the .eh_frame section:
0014 CIE
Version: 1
Augmentation: zR
Code alignment factor: 1
Data alignment factor: -8
Return address column: 16
Augmentation data: 1b
DW_CFA_def_cfa: r7 (rsp) ofs 8
DW_CFA_offset: r16 (rip) at cfa-8
DW_CFA_nop
DW_CFA_nop
0018 0010 001c FDE cie= pc=..0006
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
002c 0010 0030 FDE cie= pc=0006..0017
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
0040 0014 0044 FDE cie= pc=..000a
DW_CFA_advance_loc: 1 to 0001
DW_CFA_def_cfa_offset: 16
DW_CFA_advance_loc: 8 to 0009
DW_CFA_def_cfa_offset: 8
DW_CFA_nop
As you can see, all three functions still have unwind entries emitted.
According to documentation I saw on the web, -fno-asynchronous-unwind-tables
increases unwind information granularity to function-level, meaning that it
supposedly avoids stepping cfa unless there is a function call there. While I
don't regularly read the unwind tables, I was under impression that this was
happening by default.
[Bug c++/53380] .ehframe could be smaller
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53380 Jakub Jelinek jakub at gcc dot gnu.org changed: What|Removed |Added CC||jakub at gcc dot gnu.org --- Comment #4 from Jakub Jelinek jakub at gcc dot gnu.org 2012-05-22 12:55:30 UTC --- Adding default handling if there is no FDE is an ABI change, so can't be done on existing architectures (except those that have that in their ABI already). Why do you care about the .eh_frame size so much? If you don't use the unwind info, it often won't be even paged in, or can be discarded from RAM if needed. And if you need it, it better be accurrate.
[Bug c++/53380] .ehframe could be smaller
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53380 --- Comment #5 from msharov at users dot sourceforge.net 2012-05-22 18:53:32 UTC --- (In reply to comment #4) Adding default handling if there is no FDE is an ABI change, so can't be done on existing architectures (except those that have that in their ABI already). I was not suggesting doing it by default. A switch like -fno-asynchronous-unwind-tables would be perfectly acceptable. Why do you care about the .eh_frame size so much? I don't care so much. I was merely suggesting a way of making it smaller, for the use of the same people who prefer to use -Os instead -O3. Yes, I probably could write a post-link tool to do this, but it would be much more work since I would have to implement a disassembler, etc. to find out which functions do not need unwind info. In the compiler you already have all that info in the parse tree, so it is just a matter of adding a couple of if statements. If you don't use the unwind info, it often won't be even paged in, or can be discarded from RAM if needed. Removing unnecessary entries would make lookup faster. Making .eh_frame smaller would also help the exception path by having less to page in. And if you need it, it better be accurrate. It would still be accurate. Making the common case default does not remove any useful information.
[Bug c++/53380] .ehframe could be smaller
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53380 --- Comment #1 from Andrew Pinski pinskia at gcc dot gnu.org 2012-05-16 23:49:09 UTC --- Try -fno-asynchronous-unwind-tables . Also the ABI for x86_64 requires the unwind tables IIRC. Also i?86 enables the unwinding tables if frame pointers are disable in 4.6 and above.
