MIPS: Add `-mfix-r5900' option for the R5900 short loop erratum

MIPS: Add `-mfix-r5900' option for the R5900 short loop erratum

The short loop bug under certain conditions causes loops to
execute only once or twice, due to a hardware bug in the R5900 chip.

`-march=r5900' already enables the R5900 short loop workaround.
However, the R5900 ISA and most other MIPS ISAs are mutually
exclusive since R5900-specific instructions are generated as well.

The `-mfix-r5900' option can be used in combination with e.g.
`-mips2' or `-mips3' to generate generic MIPS binaries that also
work with the R5900 target.  The workaround is implemented by GAS
rather than by GCC.

The following small `shortloop.c' file has been used as a test
with GCC 8.2.0:

void shortloop(void)
{
    __asm__ __volatile__ (
	"	li $3, 300\n"
	"loop:\n"
	"	addi $3, -1\n"
	"	addi $4, -1\n"
	"	bne $3, $0, loop\n"
	"	li $4, 3\n"
	::);
}

The following six combinations have been tested:

% mipsr5900el-unknown-linux-gnu-gcc -O1 -c shortloop.c
% mipsr5900el-unknown-linux-gnu-gcc -O1 -c shortloop.c -mfix-r5900
% mipsr5900el-unknown-linux-gnu-gcc -O1 -c shortloop.c -mno-fix-r5900

% mipsr4000el-unknown-linux-gnu-gcc -O1 -c shortloop.c
% mipsr4000el-unknown-linux-gnu-gcc -O1 -c shortloop.c -mfix-r5900
% mipsr4000el-unknown-linux-gnu-gcc -O1 -c shortloop.c -mno-fix-r5900

The R5900 short loop erratum is corrected in exactly three cases:

1. for the target `mipsr5900el' by default;

2. for the target `mipsr5900el' with `-mfix-r5900';

3. for any other MIPS target (e.g. `mipsr4000el') with `-mfix-r5900'.

In all other cases the correction is not made.

2018-11-27  Fredrik Noring  <noring@nocrew.org>

	gcc/
	* config/mips/mips.c (mips_reorg_process_insns)
	(mips_option_override): Handle `-mfix-r5900'.
	* config/mips/mips.h (ASM_SPEC): Add `mfix-r5900' and
	`mno-fix-r5900'.
	* config/mips/mips.opt (mfix-r5900): New option.
	* doc/invoke.texi: Document the `r5900' processor name, and
	`-mfix-r5900' and `-mno-fix-r5900' options.

From-SVN: r266519