On Tue, Sep 20, 2016 at 12:12 PM, Gustavo Sverzut Barbieri
<barbi...@gmail.com> wrote:
> On Tue, Sep 20, 2016 at 11:29 AM, Carsten Haitzler <ras...@rasterman.com>
> wrote:
>> On Tue, 20 Sep 2016 08:27:25 -0300 Gustavo Sverzut Barbieri
>> <barbi...@gmail.com> said:
>>
>>> > static inline void *
>>> > @@ -104,12 +101,11 @@ eina_array_foreach(Eina_Array *array, Eina_Each_Cb
>>> > cb, void *fdata) Eina_Bool ret = EINA_TRUE;
>>> >
>>> > EINA_ARRAY_ITER_NEXT(array, i, data, iterator)
>>> > - if (cb(array, data, fdata) != EINA_TRUE)
>>> > - {
>>> > - ret = EINA_FALSE;
>>> > - break;
>>> > - }
>>> > -
>>> > + {
>>> > + if (cb(array, data, fdata) == EINA_TRUE) continue;
>>> > + ret = EINA_FALSE;
>>> > + break;
>>> > + }
>>> > return ret;
>>> > }
>>>
>>>
>>> for these wouldn't EINA_LIKELY/UNLIKELY be more clear on what is the
>>> expected/preference to the user AND the compiler. What you did is
>>> likely to work for one compiler but not the other, if branch
>>> prediction preference is difference.
>>
>> no. likely/unlikely dont do anything useful since 486 architecture days. back
>> then the cmp would always predict either true or false (i dont remember) so
>> the
>> compiler SHOULD adjust the compare so the more likely outcome is what the cpu
>> will then just blindly run while waiting for the result. its is not true
>> anymore
>> and these really do nothing useful.
>
> it should have the same impact, but admittedly I did not test to guarantee.
Okay, I had to test:
#include <stdio.h>
int main(int argc, char *argv[]) {
if (argc < 1)
puts("less");
else
fprintf(stderr, "more: %d", argc);
return 0;
}
$ gcc -O2 -S /tmp/a.c -o /tmp/original-order.s
$ cat /tmp/original-order.s
.file "a.c"
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
.string "less"
.LC1:
.string "more: %d"
.section .text.startup,"ax",@progbits
.p2align 4,,15
.globl main
.type main, @function
main:
.LFB11:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
testl %edi, %edi
jle .L6
movl %edi, %edx
movq stderr(%rip), %rdi
movl $.LC1, %esi
xorl %eax, %eax
call fprintf
.L3:
xorl %eax, %eax
addq $8, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
.L6:
.cfi_restore_state
movl $.LC0, %edi
call puts
jmp .L3
.cfi_endproc
.LFE11:
.size main, .-main
.ident "GCC: (GNU) 6.2.1 20160830"
.section .note.GNU-stack,"",@progbits
that is, fprintf() is prefered over puts()... It doesn't read like our
input code, since -O1 will imply -freorder-blocks.
GCC -O2 prefers "else" condition, which can be confirmed by using
__builtin_expect((argc < 1), 0) -> EINA_UNLIKELY()
$ gcc -O2 -S /tmp/a.c -o /tmp/unlikely.s
$ diff -u /tmp/original-order.s /tmp/unlikely.s
no changes.
now force different prediction by using __builtin_expect((argc < 1),
1) -> EINA_LIKELY()
$ gcc -O2 -S /tmp/a.c -o /tmp/likely.s
$ diff -u /tmp/original-order.s /tmp/likely.s
--- /tmp/original-order.s 2016-09-20 12:31:35.747803857 -0300
+++ /tmp/likely.s 2016-09-20 12:34:02.599250150 -0300
@@ -14,22 +14,22 @@
subq $8, %rsp
.cfi_def_cfa_offset 16
testl %edi, %edi
- jle .L6
- movl %edi, %edx
- movq stderr(%rip), %rdi
- movl $.LC1, %esi
- xorl %eax, %eax
- call fprintf
+ jg .L2
+ movl $.LC0, %edi
+ call puts
.L3:
xorl %eax, %eax
addq $8, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
-.L6:
+.L2:
.cfi_restore_state
- movl $.LC0, %edi
- call puts
+ movl %edi, %edx
+ movq stderr(%rip), %rdi
+ movl $.LC1, %esi
+ xorl %eax, %eax
+ call fprintf
jmp .L3
.cfi_endproc
.LFE11:
that is, puts is now before fprintf. Inverting the branch ourselves,
no __builtin_expect():
$ gcc -O2 -S /tmp/a.c -o /tmp/
$ diff -u /tmp/original-order.s /tmp/unlikely.s
--- /tmp/original-order.s 2016-09-20 12:31:35.747803857 -0300
+++ /tmp/inverted-order.s 2016-09-20 12:35:56.710377570 -0300
@@ -1,9 +1,9 @@
.file "a.c"
.section .rodata.str1.1,"aMS",@progbits,1
.LC0:
- .string "less"
-.LC1:
.string "more: %d"
+.LC1:
+ .string "less"
.section .text.startup,"ax",@progbits
.p2align 4,,15
.globl main
@@ -14,10 +14,10 @@
subq $8, %rsp
.cfi_def_cfa_offset 16
testl %edi, %edi
- jle .L6
+ jle .L2
movl %edi, %edx
movq stderr(%rip), %rdi
- movl $.LC1, %esi
+ movl $.LC0, %esi
xorl %eax, %eax
call fprintf
.L3:
@@ -26,9 +26,9 @@
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
-.L6:
+.L2:
.cfi_restore_state
- movl $.LC0, %edi
+ movl $.LC1, %edi
call puts
jmp .L3
.cfi_endproc
That is, only the stings changed order... the code remained the SAME :-/
Maybe it was pure luck on your side, something we can't trust. IF you
want to be sure, do use the __builtin_expect().
--
Gustavo Sverzut Barbieri
--------------------------------------
Mobile: +55 (16) 99354-9890
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