Re: -fuse-caller-save - Enable for MIPS

[Richard Sandiford <rdsandiford at googlemail dot com>]

Tom de Vries <Tom_deVries@mentor.com> writes:
> diff --git a/gcc/config/mips/mips.c b/gcc/config/mips/mips.c
> index 45256e9..b61cd44 100644
> --- a/gcc/config/mips/mips.c
> +++ b/gcc/config/mips/mips.c
> @@ -7027,11 +7027,17 @@ mips_expand_call (enum mips_call_type type, rtx result, rtx addr,
>  {
>    rtx orig_addr, pattern, insn;
>    int fp_code;
> +  rtx post_call_tmp_reg = gen_rtx_REG (word_mode, POST_CALL_TMP_REG);
>  
>    fp_code = aux == 0 ? 0 : (int) GET_MODE (aux);
>    insn = mips16_build_call_stub (result, &addr, args_size, fp_code);
>    if (insn)
>      {
> +      if (TARGET_EXPLICIT_RELOCS
> +	  && TARGET_CALL_CLOBBERED_GP
> +	  && !find_reg_note (insn, REG_NORETURN, 0))
> +	clobber_reg (&CALL_INSN_FUNCTION_USAGE (insn), post_call_tmp_reg);
> +

I think this condition should go in mips_emit_call_insn instead,
so that we don't have 4 instances of it.  untyped_call could then
use mips_expand_call as well.

Until now there was no real downside to using $6 for non-MIPS16 code.
Now that there is, it would probably be worth making it:

+#define POST_CALL_TMP_REG \
  (TARGET_MIPS16 ? GP_ARG_FIRST + 2 : PIC_OFFSET_TABLE_REGNUM)

and only adding the clobber for MIPS16.

> diff --git a/gcc/testsuite/gcc.target/mips/fuse-caller-save.c b/gcc/testsuite/gcc.target/mips/fuse-caller-save.c
> new file mode 100644
> index 0000000..1fd6c7d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/mips/fuse-caller-save.c
> @@ -0,0 +1,30 @@
> +/* { dg-do compile } */
> +/* { dg-options "-fuse-caller-save" } */
> +/* { dg-skip-if "" { *-*-* }  { "*" } { "-Os" } } */

I might have asked this before, sorry, but why this skip?  Please add a brief
comment (in the string, if short enough).

> +/* Testing -fuse-caller-save optimization option.  */
> +
> +static int __attribute__((noinline)) NOCOMPRESSION
> +bar (int x)
> +{
> +  return x + 3;
> +}
> +
> +int __attribute__((noinline)) NOCOMPRESSION
> +foo (int y)
> +{
> +  return y + bar (y);
> +}
> +
> +int NOCOMPRESSION
> +main (void)
> +{
> +  return !(foo (5) == 13);
> +}
> +
> +/* Check that there are only 2 stack-saves: r31 in main and foo.  */
> +
> +/* Check that there only 2 sw/sd.  */
> +/* { dg-final { scan-assembler-times "(?n)s\[wd\]\t\\\$.*,.*\\(\\\$sp\\)" 2 } } */
> +
> +/* Check that the first caller-save register is unused.  */
> +/* { dg-final { scan-assembler-not "\\\$16" } } */

It'd be good to avoid NOCOMPRESSION because the only case that really
needs the temporary register is MIPS16.  Please try putting the test
in a header file and reusing it for three tests, one each of MIPS16,
microMIPS and uncompressed.

Thanks,
Richard