http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38629
--- Comment #6 from Jan Hubicka <hubicka at gcc dot gnu.org> 2010-11-10
20:52:18 UTC ---
OK, at -Os the issue is that function is called once so inlining is a win.
Making multiple copies of it leads to GCC making clone:
delay_wait_us_ms.constprop.0:
.LFB3:
movl $136, %edi
jmp delay_wait_us
.LFE3:
and then calling it
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
call delay_wait_us_ms.constprop.0
at -Os,that is
With -O2 it is different story, we end up inlining everything. We get:
Analyzing function body size: delay_wait_us
freq: 1000 size: 1 time: 1 __asm__ __volatile__("wdr");
freq: 1000 size: 1 time: 1 MEM[(volatile unsigned char *)82B] ={v}
timeout_2(D);
freq: 1000 size: 1 time: 1 D.2719_5 ={v} MEM[(volatile unsigned char
*)88B];
freq: 1000 size: 1 time: 1 D.2720_6 = D.2719_5 | 1;
freq: 1000 size: 1 time: 1 MEM[(volatile unsigned char *)88B] ={v}
D.2720_6;
freq: 11111 size: 1 time: 1 D.2721_8 ={v} MEM[(volatile unsigned char
*)88B];
freq: 11111 size: 0 time: 0 D.2722_9 = (int) D.2721_8;
freq: 11111 size: 1 time: 1 D.2723_10 = D.2722_9 & 1;
freq: 11111 size: 2 time: 2 if (D.2723_10 == 0)
freq: 1000 size: 1 time: 2 return;
Likely eliminated
Overall function body time: 51-2 size: 10-1
With function call overhead time: 51-13 size: 10-3
that fits in early-inlining-insns. With --param early-inlining-insns=0 we get
it right. GCC inliner is guessing here that inlining such a small leaf
function will result in enough optimization so it pays back. I am not sure what
we can do here, early-inlining-insns is being pushed up by C++ code...
It is not terribly bad tradeoff even at -O2. I will try to get some data how
much early inlining insns cost us at -O2 and if it is too much, I will disable
the allowed growth for functions not declared inline.
