3 ms·

C++ exceptions have a runtime overhead, because they have to set up a handler (insert a record into a linked list) upon entering the try block and remove it whe

C++ exceptions have a runtime overhead, because they have to set up a handler (insert a record into a linked list) upon entering the try block and remove it when leaving the try block. Now of course checking return values introduces a lot of branches, so go figure out which one of them is worse... still it is very advisable to have likely pragmas around if conditions that check return values. The big problem with exceptions is that it introduces a lot of implicit code paths, and lots of possibilities for resource leaks (if raw pointers are involved).

mzs 10y ago
That's one way they can be implemented, but now they tend to be zero-cost implementation where only when an exception occurs does it figure-out where it was and what to do.
- MichaelMoser123 10y ago
  the article speaks about 'embedded' systems - these are not x84-64 systems (intel cpus need too much power) so they don't have a 'zero cost' exception table (that is also not exactly zero cost) Can you show me an embedded system with zero cost exceptions?
  - deleted 10y ago
    [deleted]
  - mzs 10y ago
    $ CXX=g++ppc CXXFLAGS=-g make -f /dev/null foo.o g++ppc -g -c -o foo.o foo.cc $ objdumpppc -drS foo.o foo.o: file format elf32-powerpc-vxworks Disassembly of section .text: 00000000 <_Z3fooi>: int foo(int i) 0: 94 21 ff d0 stwu r1,-48(r1) 4: 7c 08 02 a6 mflr r0 8: 90 01 00 34 stw r0,52(r1) c: 93 e1 00 2c stw r31,44(r1) 10: 7c 3f 0b 78 mr r31,r1 14: 90 7f 00 18 stw r3,24(r31) { try { if (i < 0) throw (i); } 18: 80 1f 00 18 lwz r0,24(r31) 1c: 2f 80 00 00 cmpwi cr7,r0,0 20: 40 9c 00 74 bge- cr7,94 <_Z3fooi+0x94> 24: 38 60 00 04 li r3,4 28: 48 00 00 01 bl 28 <_Z3fooi+0x28> 28: R_PPC_REL24 __cxa_allocate_exception 2c: 7c 60 1b 78 mr r0,r3 30: 7c 0b 03 78 mr r11,r0 34: 7d 69 5b 78 mr r9,r11 38: 80 1f 00 18 lwz r0,24(r31) 3c: 90 09 00 00 stw r0,0(r9) 40: 7d 63 5b 78 mr r3,r11 44: 3d 20 00 00 lis r9,0 46: R_PPC_ADDR16_HA _ZTIi 48: 38 89 00 00 addi r4,r9,0 4a: R_PPC_ADDR16_LO _ZTIi 4c: 38 a0 00 00 li r5,0 50: 48 00 00 01 bl 50 <_Z3fooi+0x50> 50: R_PPC_REL24 __cxa_throw 54: 90 7f 00 1c stw r3,28(r31) 58: 7c 80 23 78 mr r0,r4 5c: 2f 80 00 01 cmpwi cr7,r0,1 60: 41 9e 00 0c beq- cr7,6c <_Z3fooi+0x6c> 64: 80 7f 00 1c lwz r3,28(r31) 68: 48 00 00 01 bl 68 <_Z3fooi+0x68> 68: R_PPC_REL24 _Unwind_Resume catch (int e) { i = -e; } 6c: 80 7f 00 1c lwz r3,28(r31) 70: 48 00 00 01 bl 70 <_Z3fooi+0x70> 70: R_PPC_REL24 __cxa_begin_catch 74: 7c 60 1b 78 mr r0,r3 78: 7c 09 03 78 mr r9,r0 7c: 80 09 00 00 lwz r0,0(r9) 80: 90 1f 00 08 stw r0,8(r31) 84: 80 1f 00 08 lwz r0,8(r31) 88: 7c 00 00 d0 neg r0,r0 8c: 90 1f 00 18 stw r0,24(r31) 90: 48 00 00 01 bl 90 <_Z3fooi+0x90> 90: R_PPC_REL24 __cxa_end_catch return (i); 94: 80 1f 00 18 lwz r0,24(r31) } 98: 7c 03 03 78 mr r3,r0 9c: 81 61 00 00 lwz r11,0(r1) a0: 80 0b 00 04 lwz r0,4(r11) a4: 7c 08 03 a6 mtlr r0 a8: 83 eb ff fc lwz r31,-4(r11) ac: 7d 61 5b 78 mr r1,r11 b0: 4e 80 00 20 blr $ g++ppc -v 2>&1 | tail -1 gcc version 4.3.3 (Wind River VxWorks G++ 4.3-315) $

mzs 10y ago

That's one way they can be implemented, but now they tend to be zero-cost implementation where only when an exception occurs does it figure-out where it was and what to do.

MichaelMoser123 10y ago
the article speaks about 'embedded' systems - these are not x84-64 systems (intel cpus need too much power) so they don't have a 'zero cost' exception table (that is also not exactly zero cost) Can you show me an embedded system with zero cost exceptions?
- deleted 10y ago
  [deleted]
- mzs 10y ago
  $ CXX=g++ppc CXXFLAGS=-g make -f /dev/null foo.o g++ppc -g -c -o foo.o foo.cc $ objdumpppc -drS foo.o foo.o: file format elf32-powerpc-vxworks Disassembly of section .text: 00000000 <_Z3fooi>: int foo(int i) 0: 94 21 ff d0 stwu r1,-48(r1) 4: 7c 08 02 a6 mflr r0 8: 90 01 00 34 stw r0,52(r1) c: 93 e1 00 2c stw r31,44(r1) 10: 7c 3f 0b 78 mr r31,r1 14: 90 7f 00 18 stw r3,24(r31) { try { if (i < 0) throw (i); } 18: 80 1f 00 18 lwz r0,24(r31) 1c: 2f 80 00 00 cmpwi cr7,r0,0 20: 40 9c 00 74 bge- cr7,94 <_Z3fooi+0x94> 24: 38 60 00 04 li r3,4 28: 48 00 00 01 bl 28 <_Z3fooi+0x28> 28: R_PPC_REL24 __cxa_allocate_exception 2c: 7c 60 1b 78 mr r0,r3 30: 7c 0b 03 78 mr r11,r0 34: 7d 69 5b 78 mr r9,r11 38: 80 1f 00 18 lwz r0,24(r31) 3c: 90 09 00 00 stw r0,0(r9) 40: 7d 63 5b 78 mr r3,r11 44: 3d 20 00 00 lis r9,0 46: R_PPC_ADDR16_HA _ZTIi 48: 38 89 00 00 addi r4,r9,0 4a: R_PPC_ADDR16_LO _ZTIi 4c: 38 a0 00 00 li r5,0 50: 48 00 00 01 bl 50 <_Z3fooi+0x50> 50: R_PPC_REL24 __cxa_throw 54: 90 7f 00 1c stw r3,28(r31) 58: 7c 80 23 78 mr r0,r4 5c: 2f 80 00 01 cmpwi cr7,r0,1 60: 41 9e 00 0c beq- cr7,6c <_Z3fooi+0x6c> 64: 80 7f 00 1c lwz r3,28(r31) 68: 48 00 00 01 bl 68 <_Z3fooi+0x68> 68: R_PPC_REL24 _Unwind_Resume catch (int e) { i = -e; } 6c: 80 7f 00 1c lwz r3,28(r31) 70: 48 00 00 01 bl 70 <_Z3fooi+0x70> 70: R_PPC_REL24 __cxa_begin_catch 74: 7c 60 1b 78 mr r0,r3 78: 7c 09 03 78 mr r9,r0 7c: 80 09 00 00 lwz r0,0(r9) 80: 90 1f 00 08 stw r0,8(r31) 84: 80 1f 00 08 lwz r0,8(r31) 88: 7c 00 00 d0 neg r0,r0 8c: 90 1f 00 18 stw r0,24(r31) 90: 48 00 00 01 bl 90 <_Z3fooi+0x90> 90: R_PPC_REL24 __cxa_end_catch return (i); 94: 80 1f 00 18 lwz r0,24(r31) } 98: 7c 03 03 78 mr r3,r0 9c: 81 61 00 00 lwz r11,0(r1) a0: 80 0b 00 04 lwz r0,4(r11) a4: 7c 08 03 a6 mtlr r0 a8: 83 eb ff fc lwz r31,-4(r11) ac: 7d 61 5b 78 mr r1,r11 b0: 4e 80 00 20 blr $ g++ppc -v 2>&1 | tail -1 gcc version 4.3.3 (Wind River VxWorks G++ 4.3-315) $