#167411 - Echo49 - Wed Mar 11, 2009 4:37 am
Not really to do with coding, but if I have a simple addition that does nothing:
| Code: |
for (int i=0;i<A_BIG_NUMBER;++i)
45+56;
|
Does the compiler exclude that line from the binary?
#167415 - Ruben - Wed Mar 11, 2009 7:22 am
Depends on the level of optimization.
-O0 will not. This level of optimization does absolutely no optimization and handles the code exactly as you type it.
-O1 may, depending on your compiler.
-O2 most likely will.
If you have -Wall or -Wextra, I can't remember, the compiler will warn you about a loop with no effect.
#167418 - Dwedit - Wed Mar 11, 2009 8:38 am
If you use -S, you can see what the compiler does.
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#167420 - Echo49 - Wed Mar 11, 2009 9:07 am
I have this code, compiled with -O0 as suggested.
| Code: |
for (i=0; i<LOOPS; ++i) {
54+56;
}
//...
for (i=0; i<LOOPS; ++i) {
34+57;87+21;
} |
Why does the first loop take more time to execute (when using the clock() function from time.h) than the second loop? LOOPS is 100,000,000
#167422 - Pete_Lockwood - Wed Mar 11, 2009 1:03 pm
What assembler does it produce? If it's really taking more time, the answer will be there.
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#167444 - Echo49 - Thu Mar 12, 2009 7:58 am
Curiously enough,
| Code: |
MOV EAX,DWORD PTR SS:[EBP-20]
MOV DWORD PTR SS:[EBP-20],EAX |
takes longer to execute than
| Code: |
MOV EAX,DWORD PTR SS:[EBP-20]
ADD EAX,1
MOV DWORD PTR SS:[EBP-20],EAX |
when both are executed one trillion times.
#167445 - Ruben - Thu Mar 12, 2009 8:33 am
I think it's got to do with the pipeline of the CPU, cos if they were constant timings, the second one should've taken longer.
#167456 - Miked0801 - Thu Mar 12, 2009 4:58 pm
Man I forgot how ugly x86 assembler is to ARM.
#167459 - Kyoufu Kawa - Thu Mar 12, 2009 6:53 pm
| Miked0801 wrote: |
| Man I forgot how ugly x86 assembler is to ARM. |
Amen to that.
#167462 - Echo49 - Thu Mar 12, 2009 8:39 pm
=.=;
#167484 - keldon - Fri Mar 13, 2009 6:39 pm
| Miked0801 wrote: |
| Man I forgot how ugly x86 assembler is to ARM. |
Beauty is in the eye of the beholder; although that is ugly syntax - RosAsm is nice to work with.
#169651 - Karatorian - Mon Jul 27, 2009 11:51 pm
While there's some truth to the statement regarding subjective beauty, x86's uglyness goes way beyond just the syntax. I've written asm for a few platforms and the x86 ISA is ugly clear to the bare metal.