On Thu, 8 Apr 2021 at 18:53, Peter Zijlstra <pet...@infradead.org> wrote:
>
> 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.
>
Wouldn't that require the compiler to interpret the contents of the asm() block?
