Tuesday, January 1, 2013

I'm about halfway through removing the fake PC register. Right now, there's a new "*rt" instruction using the UNSPEC framework to emit "b *r11".

The UNSPEC functions are intended to handle instructions which are otherwise undescribable using normal RTX expressions. This is perfect for the TMS9900 return sequence, since we can't use the normal return instruction. The reason for that is if a return instruction is defined, we cannot apply a function epilogue since that is implemented in the place where a normal return instruction would live. Also, we need a specific label for any type of explicit jump, which won't work for a return.

The fake PC register has been a worrying point for a long time, and we've been burned by it before. Removing it will eliminate all possibility for further unexpected messes.

All this is great, but now I have to remove all the hackery I used earlier to insert the fake PC register. That involves register definitions, location of the first pseudoregister, a special case for the MOV instruction as well as other special cases in the register allocation directives.

That's quite a bit of work, and it's late. We'll try that tomorrow.

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