Re: [RFC] gcc feature request: Moving blocks into sections

Mathieu Desnoyers
2013-08-05T21:28:59Z

* Linus Torvalds (torvalds@linux-foundation.org) wrote:
> On Mon, Aug 5, 2013 at 12:54 PM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
> >
> > I remember that choosing between 2 and 5 bytes nop in the asm goto was
> > tricky: it had something to do with the fact that gcc doesn't know the
> > exact size of each instructions until further down within compilation
>
> Oh, you can't do it in the coompiler, no. But you don't need to. The
> assembler will pick the right version if you just do "jmp target".

Yep.

Another thing that bothers me with Steven's approach is that decoding
jumps generated by the compiler seems fragile IMHO.

x86 decoding proposed by https://lkml.org/lkml/2012/3/8/464 :

+static int make_nop_x86(void *map, size_t const offset)
+{
+ unsigned char *op;
+ unsigned char *nop;
+ int size;
+
+ /* Determine which type of jmp this is 2 byte or 5. */
+ op = map + offset;
+ switch (*op) {
+ case 0xeb: /* 2 byte */
+ size = 2;
+ nop = ideal_nop2_x86;
+ break;
+ case 0xe9: /* 5 byte */
+ size = 5;
+ nop = ideal_nop;
+ break;
+ default:
+ die(NULL, "Bad jump label section (bad op %x)\n", *op);
+ __builtin_unreachable();
+ }

My though is that the code above does not cover all jump encodings that
can be generated by past, current and future x86 assemblers.

Another way around this issue might be to keep the instruction size
within a non-allocated section:

static __always_inline bool arch_static_branch(struct static_key *key)
{
asm goto("1:"
"jmp %l[l_yes]\n\t"
"2:"

".pushsection __jump_table, \"aw\" \n\t"
_ASM_ALIGN "\n\t"
_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
".popsection \n\t"

".pushsection __jump_table_ilen \n\t"
_ASM_PTR "1b \n\t" /* Address of the jmp */
".byte 2b - 1b \n\t" /* Size of the jmp instruction */
".popsection \n\t"

: : "i" (key) : : l_yes);
return false;
l_yes:
return true;
}

And use (2b - 1b) to know what size of no-op should be used rather than
to rely on instruction decoding.

Thoughts ?

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.