Messages in this thread | | | Date | Fri, 5 Nov 2021 11:01:52 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage |
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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
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