Re: [MUSL] microMIPS32R2 O32 port

[Andre McCurdy <armccurdy@gmail.com>]

On Wed, Apr 12, 2017 at 1:27 PM, Rich Felker <dalias@libc.org> wrote:
> On Wed, Apr 12, 2017 at 09:25:35PM +0200, Szabolcs Nagy wrote:
>> * Jaydeep Patil <Jaydeep.Patil@imgtec.com> [2017-04-12 11:54:10 +0000]:
>> > Hi Rich,
>> >
>> > We can reuse existing MIPS code for microMIPS. There are places where we read from $ra must be compiled for MIPS.
>> > Please refer to https://github.com/JaydeepIMG/musl-1/tree/micromips32r2_v2 for modifications.
>> >
>>
>> is micromips a different encoding for mips instructions
>> that works on some cpus but not others?
>
> Yes, it's something like thumb or thumb2 on arm, or the riscv
> compressed isa. What I'm not clear on is whether there are
> micromips-only cpu models that can't execute normal mips.

According to:

  https://imagination-technologies-cloudfront-assets.s3.amazonaws.com/documentation/MIPS_Architecture_microMIPS32_InstructionSet_AFP_P_MD00582_06.04.pdf

"microMIPS is also an alternative to the MIPS® instruction encoding
and can be implemented in parallel or stand-alone."

"If only one ISA mode exists (either MIPS or microMIPS) then this mode
switch mechanism does not exist"

> If so we probably need the ability to build musl as micromips, but as
> long as cpus which support both support interworking (calls between
> the two type of code in the same process) reasonably, I don't think
> there's any reason to consider it a different subarch.
>
> If not (that is, if all cpus that support micromips also support the
> normal mips isa) then I fail to see why there's any need to compile
> musl's asm files as micromips. They're not size or performance
> bottlenecks.
>
> Rich