On Thu, May 28, 2015 at 05:46:49PM +0100, Maciej W. Rozycki wrote: > Correct a regression introduced with 8453eebd [MIPS: Fix strnlen_user() > return value in case of overlong strings.] causing assembler warnings > and broken code generated in __strnlen_kernel_nocheck_asm: > > arch/mips/lib/strnlen_user.S: Assembler messages: > arch/mips/lib/strnlen_user.S:64: Warning: Macro instruction expanded into multiple instructions in a branch delay slot > > with the CPU_DADDI_WORKAROUNDS option set, resulting in the function > looping indefinitely upon mounting NFS root. > > Use conditional assembly to avoid a microMIPS code size regression. > Using $at unconditionally would cause such a regression as there are no > 16-bit instruction encodings available for ALU operations using this > register. Using $v1 unconditionally would produce short microMIPS > encodings, but would prevent this register from being used across calls > to this function. > > The extra LI operation introduced is free, replacing a NOP originally > scheduled into the delay slot of the branch that follows. > > Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx> > --- > Ralf, > > The jump to the delay slot combined with the unusual register usage > convention taken here made it trickier than it would normally be to make a > fix that does not regress -- in terms of code size -- unaffected microMIPS > systems. I tried several versions and eventually I came up with this one > that I believe produces the best code in all cases, at the cost of these > #ifdefs. I hope they are acceptable. I think it's all a hint to rewrite the thing in a language that transparently handles the DADDIU issue. Such as C. Which would also make using a better algorithm easier. Ralf
