Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage

Josh Poimboeuf
2021-11-05T18:02:23Z

On Fri, Nov 05, 2021 at 06:10:43PM +0100, Peter Zijlstra wrote:
> +#ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
> +
> +static inline unsigned long load_unaligned_zeropad(const void *addr)
> +{
> + unsigned long offset, data;
> + unsigned long ret;
> +
> + asm_volatile_goto(
> + "1: mov %[mem], %[ret]\n"
> +
> + _ASM_EXTABLE(1b, %l[do_exception])
> +
> + : [ret] "=&r" (ret)
> + : [mem] "m" (*(unsigned long *)addr)
> + : : do_exception);
> +
> +out:
> + return ret;
> +
> +do_exception: __cold;
> +
> + offset = (unsigned long)addr & (sizeof(long) - 1);
> + addr = (void *)((unsigned long)addr & ~(sizeof(long) - 1));
> + data = *(unsigned long *)addr;
> + ret = data >> offset * 8;
> + goto out;

Superfluous goto, can just return here?

> static inline unsigned long load_unaligned_zeropad(const void *addr)
> {
> - unsigned long ret, dummy;
> + unsigned long offset, data;
> + unsigned long ret, err = 0;
>
> - asm(
> - "1:\tmov %2,%0\n"
> + asm( "1: mov %[mem], %[ret]\n"
> "2:\n"
> - ".section .fixup,\"ax\"\n"
> - "3:\t"
> - "lea %2,%1\n\t"
> - "and %3,%1\n\t"
> - "mov (%1),%0\n\t"
> - "leal %2,%%ecx\n\t"
> - "andl %4,%%ecx\n\t"
> - "shll $3,%%ecx\n\t"
> - "shr %%cl,%0\n\t"
> - "jmp 2b\n"
> - ".previous\n"
> - _ASM_EXTABLE(1b, 3b)
> - :"=&r" (ret),"=&c" (dummy)
> - :"m" (*(unsigned long *)addr),
> - "i" (-sizeof(unsigned long)),
> - "i" (sizeof(unsigned long)-1));
> +
> + _ASM_EXTABLE_FAULT(1b, 2b)
> +
> + : [ret] "=&r" (ret), "+a" (err)
> + : [mem] "m" (*(unsigned long *)addr));
> +
> + if (unlikely(err)) {
> + offset = (unsigned long)addr & (sizeof(long) - 1);
> + addr = (void *)((unsigned long)addr & ~(sizeof(long) - 1));
> + data = *(unsigned long *)addr;
> + ret = data >> offset * 8;
> + }
> +
> return ret;

This adds a (normally not taken) conditional jump, would a straight jmp
over the fixup not be better?

i.e.

1: mov %[mem], %[ret]
jmp 2
... fixup code ...
2:

--
Josh