On 10/1/20 8:17 PM, J. Gareth Moreton via fpc-devel wrote:
Hi everyone,
I have a small question with assembler size optimisation that maybe
one of you guys can give me a second opinion on:
If you are using the "test" instruction to test some of the lower bits
of an instruction, e.g. TEST RCX, $2, is there a penalty with calling
TEST CL, $2 instead? The instruction size is a lot smaller on account
of the immediate only being 1 byte long instead of 4 bytes, and are
mathematically equivalent. I know you have to be careful with partial
write penalties, but partial reads seem to be a bit more nebulous (the
register is not modified with TEST).
Yes, I think the shorter TEST CL, $2 is preferred over TEST RCX, $2 on
every x86_64 CPU. AFAIK, there's no penalty for using 8-bit subregisters
(except perhaps AH, BH, CH and DH, but the FPC code generator doesn't
use them). Others can correct me if I'm wrong.
Nikolay
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