On Mon, Apr 18, 2011 at 1:47 PM, Zak Elep <[email protected]> wrote: > I suppose the compiler optimized away > the empty for(), among other things...
yup.. gcc will remove those doing nothing codes for optimization... in this case the for() loop... im aware of this and thats the reason i didnt use the -O argument.. i really thought there was a bug on the openbsd scheduler or its gcc package and thats why i was eager to install openbsd this morning and look for it :-> to demonstrate between optimize versus non-optimize compilation... we can take a look at the machine code that gcc produced thru a gdb debugger... below.. compiled with no optimization.. you can see there when i disassemble the code.. between line 13 and 14 produced an assembly code for the for() loop... $ gcc -g -o waste waste.c $ gdb waste GNU gdb (GDB) Red Hat Enterprise Linux (7.0.1-23.el5) Copyright (C) 2009 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "i386-redhat-linux-gnu". For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>... Reading symbols from /home/rledesma/test/waste...done. (gdb) list 9,17 9 void WasteCPU(int waste, int percent) { 10 int i; 11 struct timeval sleep; 12 13 for(i = waste; i; i--); 14 sleep.tv_sec = 0; 15 sleep.tv_usec = (100 - percent) * 10000; 16 select(1, NULL, NULL, NULL, &sleep); 17 } (gdb) info line 9 Line 9 of "waste.c" starts at address 0x8048444 <WasteCPU> and ends at 0x804844a <WasteCPU+6>. (gdb) disassemble /m 0x804844a Dump of assembler code for function WasteCPU: 9 void WasteCPU(int waste, int percent) { 0x08048444 <WasteCPU+0>: push %ebp 0x08048445 <WasteCPU+1>: mov %esp,%ebp 0x08048447 <WasteCPU+3>: sub $0x28,%esp 10 int i; 11 struct timeval sleep; 12 13 for(i = waste; i; i--); 0x0804844a <WasteCPU+6>: mov 0x8(%ebp),%eax 0x0804844d <WasteCPU+9>: mov %eax,-0x4(%ebp) 0x08048450 <WasteCPU+12>: jmp 0x8048456 <WasteCPU+18> 0x08048452 <WasteCPU+14>: subl $0x1,-0x4(%ebp) 0x08048456 <WasteCPU+18>: cmpl $0x0,-0x4(%ebp) 0x0804845a <WasteCPU+22>: jne 0x8048452 <WasteCPU+14> 14 sleep.tv_sec = 0; 0x0804845c <WasteCPU+24>: movl $0x0,-0xc(%ebp) 15 sleep.tv_usec = (100 - percent) * 10000; 0x08048463 <WasteCPU+31>: mov $0x64,%eax 0x08048468 <WasteCPU+36>: sub 0xc(%ebp),%eax 0x0804846b <WasteCPU+39>: imul $0x2710,%eax,%eax 0x08048471 <WasteCPU+45>: mov %eax,-0x8(%ebp) 16 select(1, NULL, NULL, NULL, &sleep); 0x08048474 <WasteCPU+48>: lea -0xc(%ebp),%eax 0x08048477 <WasteCPU+51>: mov %eax,0x10(%esp) 0x0804847b <WasteCPU+55>: movl $0x0,0xc(%esp) 0x08048483 <WasteCPU+63>: movl $0x0,0x8(%esp) 0x0804848b <WasteCPU+71>: movl $0x0,0x4(%esp) 0x08048493 <WasteCPU+79>: movl $0x1,(%esp) 0x0804849a <WasteCPU+86>: call 0x8048348 <select@plt> 17 } 0x0804849f <WasteCPU+91>: leave 0x080484a0 <WasteCPU+92>: ret End of assembler dump. below.. compiled with optimization and no machine code produced between line 13 and 14... $ gcc -O2 -g -o waste waste.c $ gdb waste GNU gdb (GDB) Red Hat Enterprise Linux (7.0.1-23.el5) Copyright (C) 2009 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "i386-redhat-linux-gnu". For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>... Reading symbols from /home/rledesma/test/waste...done. (gdb) list 9,17 9 void WasteCPU(int waste, int percent) { 10 int i; 11 struct timeval sleep; 12 13 for(i = waste; i; i--); 14 sleep.tv_sec = 0; 15 sleep.tv_usec = (100 - percent) * 10000; 16 select(1, NULL, NULL, NULL, &sleep); 17 } (gdb) info line 9 Line 9 of "waste.c" starts at address 0x8048460 <WasteCPU> and ends at 0x8048461 <WasteCPU+1>. (gdb) disassemble /m 0x8048461 Dump of assembler code for function WasteCPU: 9 void WasteCPU(int waste, int percent) { 0x08048460 <WasteCPU+0>: push %ebp 0x08048466 <WasteCPU+6>: mov %esp,%ebp 0x08048468 <WasteCPU+8>: sub $0x28,%esp 10 int i; 11 struct timeval sleep; 12 13 for(i = waste; i; i--); 14 sleep.tv_sec = 0; 0x0804846e <WasteCPU+14>: movl $0x0,-0x8(%ebp) 15 sleep.tv_usec = (100 - percent) * 10000; 0x08048461 <WasteCPU+1>: mov $0x64,%eax 0x0804846b <WasteCPU+11>: sub 0xc(%ebp),%eax 0x0804847d <WasteCPU+29>: imul $0x2710,%eax,%eax 0x0804849a <WasteCPU+58>: mov %eax,-0x4(%ebp) 16 select(1, NULL, NULL, NULL, &sleep); 0x08048475 <WasteCPU+21>: movl $0x0,0xc(%esp) 0x08048483 <WasteCPU+35>: movl $0x0,0x8(%esp) 0x0804848b <WasteCPU+43>: movl $0x0,0x4(%esp) 0x08048493 <WasteCPU+51>: movl $0x1,(%esp) 0x0804849d <WasteCPU+61>: lea -0x8(%ebp),%eax 0x080484a0 <WasteCPU+64>: mov %eax,0x10(%esp) 0x080484a4 <WasteCPU+68>: call 0x8048354 <select@plt> 17 } 0x080484a9 <WasteCPU+73>: leave 0x080484aa <WasteCPU+74>: ret 0x080484ab: nop 0x080484ac: lea 0x0(%esi,%eiz,1),%esi End of assembler dump. fooler. _________________________________________________ Philippine Linux Users' Group (PLUG) Mailing List http://lists.linux.org.ph/mailman/listinfo/plug Searchable Archives: http://archives.free.net.ph
