> -----Original Message-----
> From: Karol Lewandowski [mailto:[email protected]]
> Subject: Re: [PATCH v6 4/4] libusbg: Add inline to
> gadget_write_string().
>
> On 01/23/2014 11:13 AM, Krzysztof Opasiak wrote:
> >> -----Original Message-----
> >> From: David Laight [mailto:[email protected]]>
> >> From: Of Stanislaw Wadas
> >>> Add inline to gadget_write_string().
> >> ...
> >>> -static void usbg_write_string(char *path, char *name, char
> >> *file, char *buf)
> >>> +static inline void usbg_write_string(char *path, char *name,
> >> char *file, char *buf)
> >>> {
> >>> usbg_write_buf(path, name, file, buf);
> >>> }
> >>
> >> Why?
> >> The compiler is very unlikely not to inline it anyway.
> >>
> >> If it doesn't inline it, then specifying 'inline' probably
> >> won't make any difference either.
> >>
> >
> > I agree that new version of gcc can do this without explicit
> inline, but
> > please think about some legacy systems and compilers. Older
> versions of
> > gcc or other compilers may not inline this function if there is
> no
> > keyword. We should write portable code and don't depend on any
> version
> > of compiler. So to do it good we should pass as much information
> to
> > compiler as possible.
>
> Given different optimization flags (-Os vs -O3) compiler can
> generate
> completely different code. Both will be "optimal" but by
> completely
> different criteria.
>
> We should not try to outsmart compiler, it's usually way smarter
> than
> us.
As far as I know , compiler is not smart but only has a good heuristic.
In my opinion this heuristic is based also on inline keyword. Let's take
an example 2 simple functions:
void func(int *a, int b)
{
int i =1;
for(i=1; i < b; ++a);
}
static inline void func2(int* a, int b)
{
func(a, b);
}
Then in main we have a set of 7 call to this functions. Using my gcc
4.6.3 with -01 flag I get different asm if I use inline keyword or not.
When using inline keyword I got func2 optimized out but if I don't use
inline keyword func2 is still present in code and called each time. Both
source and asm results you may found below this message.
I hope that you see now that using or not using inline may influent the
result of the compilation, so specifying inline makes a difference.
--
BR's
Krzysztof Opasiak
Samsung R&D Institute Poland
Samsung Electronics
---
test.c:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
void func(int *a, int b)
{
int i =1;
a++;
for(i=1; i < b; ++a);
}
static inline void func2(int* a, int b)
{
func(a, b);
}
int main(int argc, char** argv)
{
int a =(int) (long)argv[0];
int b =(int) (long)argv[1];
func2(&a, b);
func2(&a, b);
func2(&a, b);
func2(&a, b);
func2(&a, b);
func2(&a, b);
func2(&a, b);
a++;
return a;
}
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
gcc -S test.c -O1 without func2 inline
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
.file "test.c"
.text
.globl func
.type func, @function
func:
.LFB0:
.cfi_startproc
cmpl $1, %esi
jle .L4
.L3:
jmp .L3
.L4:
rep
ret
.cfi_endproc
.LFE0:
.size func, .-func
.type func2, @function
func2:
.LFB1:
.cfi_startproc
call func
rep
ret
.cfi_endproc
.LFE1:
.size func2, .-func2
.globl main
.type main, @function
main:
.LFB2:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
pushq %rbx
.cfi_def_cfa_offset 24
.cfi_offset 3, -24
subq $16, %rsp
.cfi_def_cfa_offset 40
movq %rsi, %rbx
movq 8(%rsi), %rbp
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func2
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func2
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func2
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func2
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func2
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func2
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func2
movq (%rbx), %rax
addl $1, %eax
addq $16, %rsp
.cfi_def_cfa_offset 24
popq %rbx
.cfi_def_cfa_offset 16
popq %rbp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE2:
.size main, .-main
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbit
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
gcc -S test.c -O1 with func2 inline
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
.file "test.c"
.text
.globl func
.type func, @function
func:
.LFB0:
.cfi_startproc
cmpl $1, %esi
jle .L4
.L3:
jmp .L3
.L4:
rep
ret
.cfi_endproc
.LFE0:
.size func, .-func
.globl main
.type main, @function
main:
.LFB2:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
pushq %rbx
.cfi_def_cfa_offset 24
.cfi_offset 3, -24
subq $16, %rsp
.cfi_def_cfa_offset 40
movq %rsi, %rbx
movq 8(%rsi), %rbp
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func
movl %ebp, %esi
leaq 12(%rsp), %rdi
call func
movq (%rbx), %rax
addl $1, %eax
addq $16, %rsp
.cfi_def_cfa_offset 24
popq %rbx
.cfi_def_cfa_offset 16
popq %rbp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE2:
.size main, .-main
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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