RE: [RFC] Optimize __arch_swab32 and __arch_swab16

"David Laight" [off-list ref] wrote on 2011/08/11 11:29:33:

quoted

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Which is a problem because the compiler could schedule
it be written back to real memory between the instructions.

It can? There is no memory here, just registers. Even if it
is written to memory, how would that affect the register?

Although the function argument is passed in a register, the
compiler could generate a store-load sequence before and
after each __asm__() line.

Ah, I see. Seems strange that the complier would do that for
the register in use(value). Other regs perhaps.

quoted

Assuming you are right, would rewriting it to
  __asm__("rlwimi %0,%0,16,0x00ff0000\n\t"
       "rlwinm %0,%0,24,0x0000ffff"
       : "+r"(value));
help?

Except that now you've stopped the compiler scheduling
another instruction between the two - probably forcing a
execution stall.

But this should be better than using 2 more insns and an
extra register.

 Jocke