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SubjectRe: [PATCH v7 2/2] tty: add rpmsg driver
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On 26. 03. 20, 1:01, Joe Perches wrote:
> On Wed, 2020-03-25 at 14:31 +0100, Jiri Slaby wrote:
>> The question was exactly about that: can a compiler optimize it to a
>> bare number or will strlen call remain there?
>
> $ cat str.c
> #include <string.h>
>
> int foo(void)
> {
> return strlen("abc");
> }
>
> $ gcc -c -O2 str.c
> $ objdump -d str.o
> str.o: file format elf64-x86-64
>
>
> Disassembly of section .text:
>
> 0000000000000000 <foo>:
> 0: f3 0f 1e fa endbr64
> 4: b8 03 00 00 00 mov $0x3,%eax
> 9: c3 retq

Perfect, but:
* that is userspace (different strlen).
* what about other archs -- AFAIR, i386 implements strlen in asm in the
kernel

Maybe compilers use intrinsics and only after optimizations, they put a
call to strlen?

/me digging

Yeah, even gimple already contains:
int D.2094;

D.2094 = 3;
return D.2094;


That means, even -O0 should generate 3 instead of the call:
$ echo -e '#include <string.h>\nint f() { return strlen("abc"); }' | gcc
-S -x c - -o - -O0
...
f:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl $3, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc


So fine, use strlen :).

thanks,
--
js
suse labs

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