Messages in this thread | | | Date | Fri, 5 Nov 2021 19:07:19 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage |
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On Fri, Nov 05, 2021 at 11:01:52AM -0700, Josh Poimboeuf wrote: > 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?
Can do I suppose.
> > 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:
I really don't know, this way we can get the bulk of the text out-of-line.
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