On 02/25/16 17:22, Andrew Haley wrote:
On 02/25/2016 04:03 PM, Matthias Pfaller wrote:
On 02/25/2016 04:40 PM, Andrew Haley wrote:
On 02/25/2016 03:36 PM, Matthias Pfaller wrote:
When gcc decides to inline mrsbasepri it will again be free to CSE the
mrs instructions :-(. Is it really just me having run into this problem?
How do other people solve the problem that __asm__ without input is
handled like a __attribute((const)) function?
Either with a memoryclobber, or make it volatile, or both.
I have a memoryclobber, but that's output and doesn't help for input.
And marking it volatile will avoid deleting it in all cases. But I want
to get it deleted when I don't need the return value.
This works for me:
extern volatile int poo;
static __inline__ unsigned long long rdtsc(void)
{
unsigned hi, lo;
__asm__ ("rdtscp" : "=a"(lo), "=d"(hi) : "m"(poo) : "rcx");
return ( (unsigned long long)lo)|( ((unsigned long long)hi) <<32 );
}
long bar (int counts) {
long result = 0;
for (int i = 0; i < counts; i++)
result += rdtsc();
return result;
}
All you need is an externally visible memory effect.
The asm doesn't actually modify poo, but GCC doesn't know that.
.L3:
#APP
# 8 "z.c" 1
rdtscp
# 0 "" 2
#NO_APP
salq $32, %rdx
movl %eax, %eax
addl $1, %esi
orq %rdx, %rax
addq %rax, %r8
cmpl %esi, %edi
jne .L3
Andrew.
While that's nice with on the i386 it will force a useless register load
on load/store architectures:
@ 203 "/mnt/projekt/soft/crt/save/../at91/nvic.c" 1
mrs r8,primask/* [r5] avoid constant folding */
@ 0 "" 2
@ 203 "/mnt/projekt/soft/crt/save/../at91/nvic.c" 1
cpsid i
@ 0 "" 2
So that's not a solution I would like to use. I could use "i" (&poo)
(which would yeld an #poo to get discarded), but I does that count as a
memory reference?
regards, Matthias
