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Notes on the Mips target (3/4/2012)
The initial implementation of Mips support in the compiler is untested on
actual hardware, and as such should be expected to have many bugs. However,
the vast majority of code for Mips support is either shared with other
tested targets, or was taken from the functional Mips JIT compiler. The
expectation is that when it is first tried out on actual hardware lots of
small bugs will be flushed out, but it should not take long to get it
solidly running. The following areas are considered most likely to have
problems that need to be addressed:
o Endianness. Focus was on little-endian support, and if a big-endian
target is desired, you should pay particular attention to the
code generation for switch tables, fill array data, 64-bit
data handling and the register usage conventions.
o The memory model. Verify that GenMemoryBarrier() generates the
appropriate flavor of sync.
Register promotion
The resource masks in the LIR structure are 64-bits wide, which is enough
room to fully describe def/use info for Arm and x86 instructions. However,
the larger number of MIPS core and float registers render this too small.
Currently, the workaround for this limitation is to avoid using floating
point registers 16-31. These are the callee-save registers, which therefore
means that no floating point promotion is allowed. Among the solution are:
o Expand the def/use mask (which, unfortunately, is a significant change)
o The Arm target uses 52 of the 64 bits, so we could support float
registers 16-27 without much effort.
o We could likely assign the 4 non-register bits (kDalvikReg, kLiteral,
kHeapRef & kMustNotAlias) to positions occuped by MIPS registers that
don't need def/use bits because they are never modified by code
subject to scheduling: r_K0, r_K1, r_SP, r_ZERO, r_S1 (rSELF).
Branch delay slots
Little to no attempt was made to fill branch delay slots. Branch
instructions in the encoding map are given a length of 8 bytes to include
an implicit NOP. It should not be too difficult to provide a slot-filling
pass following successful assembly, but thought should be given to the
design. Branches are currently treated as scheduling barriers. One
simple solution would be to copy the instruction at branch targets to the
slot and adjust the displacement. However, given that code expansion is
already a problem it would be preferable to use a more sophisticated
scheduling solution.
Code expansion
Code expansion for the MIPS target is significantly higher than we see
for Arm and x86. It might make sense to replace the inline code generation
for some of the more verbose Dalik byte codes with subroutine calls to
shared helper functions.