[llvm-dev] [MIPS] MIPS 1 EI/DI instruction regression since 3.9

[Daniel Sanders via llvm-dev]

> On Oct 27, 2018, at 06:42, Dan Ravensloft via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> So, I have been working on Clang for the PS2, and recently decided a different approach of pretending the very quirky EE is a MIPS II CPU.
> 
> During this process, I found that Clang refused to compile some assembly code with the MIPS instructions ei and di (enable/disable interrupts):
> 
> src/glue.c:32:17: error: instruction requires a CPU feature not currently enabled
>                 asm volatile ("di");
>                               ^
> 
> Though I haven't bisected it in full, judging by this Godbolt (https://godbolt.org/z/iRVpzd <https://godbolt.org/z/iRVpzd>), these instructions have been feature-locked (incorrectly, as it is a MIPS 1 instruction) ever since Clang 3.9 (3.8.x works okay). It's worth noting that the instructions work fine with MIPS32r2 and above.

It's been a while but that's probably around the point I went through the instructions and made sure MIPS II - V worked correctly in the assembler and verified the behaviour against GNU AS using the available documentation and llvm-mc-disassembler-fuzzer to generate test cases. Before that, there was no MIPS II - V support in LLVM. MIPS I was excluded since there's no known usage of that ISA.

The architecture docs (https://www.mips.com/products/architectures/mips32-2/) say it was added in MIPS32r2. If it was defined before then then it's odd that that document doesn't define it for MIPS32r1. MIPS didn't have any of pre MIPS32 docs available so the earliest docs I could find at the time was for MIPS II - IV (https://www.cs.cmu.edu/afs/cs/academic/class/15740-f97/public/doc/mips-isa.pdf) which doesn't mention ei/di. Additionally, they're COP0 instructions and it's mentioned at https://en.wikipedia.org/wiki/MIPS_architecture that COP0 is implementation defined for MIPS I - V. That being the case, ei/di for EE may be different from the MIPS32r2 one and should probably enabled for EE specifically rather than for MIPS II - V.

> I will try to bisect and possibly fix this bug.
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