Sign Extension

Moxie zero-extends all 8 and 16-bit loads from memory. Until recently, however, the GCC port didn't understand how loads worked, and would always shift loaded values back and forth to either empty out the upper bits or sign-extend the loaded value. While correct, it was overly bloated. If we're loading an unsigned char into a register, there's no need to force the upper bits to clear. The hardware does this for us.

For instance, this simple C code....

..would compile to...

Thanks to help from hackers on the GCC mailing list, I was finally able to teach the compiler how to treat memory loads correctly. This led to two changes...

1. The introduction of 8 and 16-bit sign extension instructions (sex.b and sex.s). Sometimes we really do need to sign-extend values, and logical shift left followed by arithmetic shift right is a pretty expensive way to do this on moxie.
2. The char type is now unsigned by default. If you have zero-extending 8-bit loads then you had better make your char type unsigned, otherwise your compiler output will be littered with sign extension instructions.

Now for the C code above, we get this nice output....

I believe that this was the last major code quality issue from the GCC port, and the compiler output should be pretty good now

I've updated the upstream GCC, binutils and gdb (sim) repositories, my QEMU fork in github, as well as the MoxieLite VHDL core in the moxie-cores git repo.