R5900 short loop erratum

The R5900 short loop erratum is hardware bug of the R5900 processor of the PlayStation 2 that under certain conditions causes program loops to execute only once or twice, leading to undefined behaviour, including data corruption or crashes.

The GNU assembler (GAS) has the following note about it:

On the R5900 short loops need to be fixed by inserting a NOP in the branch delay slot.

The short loop bug under certain conditions causes loops to execute only once or twice. We must ensure that the assembler never generates loops that satisfy all of the following conditions:

• a loop consists of less than or equal to six instructions (including the branch delay slot);
• a loop contains only one conditional branch instruction at the end of the loop;
• a loop does not contain any other branch or jump instructions;
• a branch delay slot of the loop is not NOP (EE 2.9 or later).

We need to do this because of a hardware bug in the R5900 chip.

GAS handles the short loop bug in most cases. However, GAS is unable to adjust machine code having the noreorder directive, as used by the kernel on several occasions.

The short loop bug also affects user space programs, which is why generic MIPS code cannot execute unadjusted on the R5900. The GAS and GCC option -mfix-r5900 is recommended for such cases. This option is automatically enabled with a mipsr5900el-unknown-linux-gnu compiler.