In the interview, I asked which one would be faster, while(1) or while(2)
Whether it's (1) or (2), it's true
I said I think they'd be at the same speed
while(1) is faster. Why?
while(1) {
//What code?
}
or
while(2) {
//What code?
}
To see how two codes actually run, The assemblies of the two codes must be compared in gcc.
int main(void)
{
while(1)
{
}
return 0;
}
int main(void)
{
while(2)
{
}
return 0;
}
Based on the results, The assembly of the two codes is the same with or without the optimization option (-O0). So there's no speed difference between the two repetitive doors.
For any optimization (gcc main.c-S-masm=intel+option) both always produced the same results.
At -O0:
.file "main.c"
.intel_syntax noprefix
.def __main; .scl 2; .type 32; .endef
.text
.globl main
.def main; .scl 2; .type 32; .endef
.seh_proc main
main:
push rbp
.seh_pushreg rbp
mov rbp, rsp
.seh_setframe rbp, 0
sub rsp, 32
.seh_stackalloc 32
.seh_endprologue
call __main
.L2:
jmp .L2
.seh_endproc
.ident "GCC: (tdm64-2) 4.8.1"
From -O1:
.file "main.c"
.intel_syntax noprefix
.def __main; .scl 2; .type 32; .endef
.text
.globl main
.def main; .scl 2; .type 32; .endef
.seh_proc main
main:
sub rsp, 40
.seh_stackalloc 40
.seh_endprologue
call __main
.L2:
jmp .L2
.seh_endproc
.ident "GCC: (tdm64-2) 4.8.1"
At -O2/-O3:
.file "main.c"
.intel_syntax noprefix
.def __main; .scl 2; .type 32; .endef
.section .text.startup,"x"
.p2align 4,,15
.globl main
.def main; .scl 2; .type 32; .endef
.seh_proc main
main:
sub rsp, 40
.seh_stackalloc 40
.seh_endprologue
call __main
.L2:
jmp .L2
.seh_endproc
.ident "GCC: (tdm64-2) 4.8.1"
Can you see that the repetition sentence is the following form using any optimization method?
.L2:
jmp .L2
.seh_endproc
.ident "GCC: (tdm64-2) 4.8.1"
The important thing is here,
.L2:
jmp .L2
I'm not good at assembly language, but this is an unconditional jump to L2 without comparing it with an unconditional loop. If you change this to c code,
L2:
goto L2;
This is.
To sum up, When a repeat statement is compiled into the assembly language, if it has a constant value, the compiler preprocesses it.
Therefore, while(1), while(2), while(1==1), while(3==3&&4==4), while(0.1+0.2) do not differ in assembly speed when compiled.
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