Hi!
Given code like:
DEFINE_STATIC_KEY_FALSE(sched_schedstats);
#define schedstat_enabled()
static_branch_unlikely(&sched_schedstats)
#define schedstat_set(var, val) do { if (schedstat_enabled()) { var =
(val); } } while (0)
#define __schedstat_set(var, val) do { var = (val); } while (0)
void foo(void)
{
struct task_struct *p = current;
schedstat_set(p->se.statistics.wait_start, 0);
schedstat_set(p->se.statistics.sleep_start, 0);
schedstat_set(p->se.statistics.block_start, 0);
}
Where the static_branch_unlikely() ends up being:
static __always_inline bool arch_static_branch(struct static_key * const key,
const bool branch)
{
asm_volatile_goto("1:"
".byte " __stringify(BYTES_NOP5) "\n\t"
".pushsection __jump_table, \"aw\" \n\t"
_ASM_ALIGN "\n\t"
".long 1b - ., %l[l_yes] - . \n\t"
_ASM_PTR "%c0 + %c1 - .\n\t"
".popsection \n\t"
: : "i" (key), "i" (branch) : : l_yes);
return false;
l_yes:
return true;
}
The compiler gives us code like:
000000000000a290 <foo>:
a290: 65 48 8b 04 25 00 00 00 00 mov %gs:0x0,%rax a295:
R_X86_64_32S current_task
a299: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
a29e: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
a2a3: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
a2a8: c3 retq
a2a9: 48 c7 80 28 01 00 00 00 00 00 00 movq $0x0,0x128(%rax)
a2b4: eb e8 jmp a29e <foo+0xe>
a2b6: 48 c7 80 58 01 00 00 00 00 00 00 movq $0x0,0x158(%rax)
a2c1: eb e0 jmp a2a3 <foo+0x13>
a2c3: 48 c7 80 70 01 00 00 00 00 00 00 movq $0x0,0x170(%rax)
a2ce: c3 retq
Now, in this case I can easily rewrite foo like:
void foo2(void)
{
struct task_struct *p = current;
if (schedstat_enabled()) {
__schedstat_set(p->se.statistics.wait_start, 0);
__schedstat_set(p->se.statistics.sleep_start, 0);
__schedstat_set(p->se.statistics.block_start, 0);
}
}
Which gives the far more reasonable:
000000000000a2d0 <foo2>:
a2d0: 65 48 8b 04 25 00 00 00 00 mov %gs:0x0,%rax a2d5:
R_X86_64_32S current_task
a2d9: 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
a2de: c3 retq
a2df: 48 c7 80 28 01 00 00 00 00 00 00 movq $0x0,0x128(%rax)
a2ea: 48 c7 80 58 01 00 00 00 00 00 00 movq $0x0,0x158(%rax)
a2f5: 48 c7 80 70 01 00 00 00 00 00 00 movq $0x0,0x170(%rax)
a300: c3 retq
But I've found a few sites where this isn't so trivial.
Is there *any* way in which we can have the compiler recognise that the
asm_goto only depends on its arguments and have it merge the branches
itself?
I do realize that asm-goto being volatile this is a fairly huge ask, but
I figured I should at least raise the issue, if only to raise awareness.
