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
