Saturday, March 24, 2012

I played around with the compiler trying to reduce the test program output and found a few things.

There is a note attached to the add instruction that the R3 regisgter in HI mode is unused after that instruction, so we could skip the "jnc, inc" sequence.

The check commonly used for the other machines, "find_reg_note(insn, REG_UNUSED, operands[0])", will not work here. There are two reasons for this. The first is that the operand is in SI mode, but the note is for HI mode. The second is that the function only checks for pointer equality, and does not compare the RTX expressions to which the pointers refer. I'd have to make a new find_reg_note function to do note checking.

If I just change the order of typecasting, all this goes away:

char test(long a) {return((char)a+'0');}


mov r2, r1
swpb r1
ai r1, >3000
b *r11

This is the ideal sequence I was looking for originally, so another best practice seems to be to to late promotion and early demotion of datatypes. That seems to result in better code.

So for now, I'll put compiler changes on hold, and just go back to printf.

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