On Thu, Feb 04, 2016 at 04:18:34PM +0100, Ralf Baechle wrote:
> On Thu, Feb 04, 2016 at 02:31:57PM +0000, Paul Burton wrote:
>
> > CC arch/mips/mm/c-r4k.o
> > {standard input}: Assembler messages:
> > {standard input}:4105: Warning: dla used to load 32-bit register;
> > recommend using la instead
> > {standard input}:4129: Warning: dla used to load 32-bit register;
> > recommend using la instead
>
> Sigh. Another new binutils warning?
>
> > Avoid this by instead making use of the PTR_LA macro which defines the
> > appropriate variant of the "la" instruction to use.
> >
> > Tested with Codescape GNU Tools 2015.06-05 for MIPS IMG Linux, which
> > includes binutils 2.24.90 & gcc 4.9.2.
>
> > @@ -54,22 +55,16 @@
> >
> > /*
> > * gcc has a tradition of misscompiling the previous construct using the
> > - * address of a label as argument to inline assembler. Gas otoh has the
> > - * annoying difference between la and dla which are only usable for 32-bit
> > - * rsp. 64-bit code, so can't be used without conditional compilation.
> > - * The alterantive is switching the assembler to 64-bit code which happens
> > - * to work right even for 32-bit code ...
> > + * address of a label as argument to inline assembler.
> > */
> > #define instruction_hazard() \
> > do { \
> > unsigned long tmp; \
> > \
> > __asm__ __volatile__( \
> > - " .set "MIPS_ISA_LEVEL" \n" \
> > - " dla %0, 1f \n" \
> > - " jr.hb %0 \n" \
> > - " .set mips0 \n" \
> > - "1: \n" \
> > + __stringify(PTR_LA) " %0, 1f\n\t" \
> > + "jr.hb %0\n\t" \
> > + "1:" \
> > : "=r" (tmp)); \
> > } while (0)
>
>
> The .set will need to stay or this will fail up on older processors
> with
>
> /tmp/ccKNXiPT.s:21: Error: opcode not supported on this processor: mips1 (mips1) `jr.hb '
>
> The opcode of JR.HB will by older processors be treated as just a JR afair.
Ok, I'll put the .set back.
> Or with less inline assembler obscurities something like:
>
> void foo(void)
> {
> void *jr = &&jr;
>
> __asm__ __volatile__(
> " .set "MIPS_ISA_LEVEL" \n"
> " jr.hb \n"
> " .set mips0 \n"
> : /* no outputs */
> : "r" (jr));
> jr: ;
> }
>
> Now GCC can even schedule loading the address or do other clever things.
Yes, but judging from the comment preceeding instruction_hazard() some
versions of gcc can also do clever things like miscompile that. Maybe we
don't care any more - the comment doesn't say which versions were
affected...
Thanks,
Paul
> Ralf
