I thought that might be the case - thanks Nikolay. And I meant to say
lower bits of a REGISTER, not an instruction!
Admittedly I'm cycle-counting and byte-counting again! I was looking
for ways to reduce 13 bytes of padding in one of my pure assembly
language routines and realised I could make a saving there. The only
thing I can think of that I have to watch out for logically is if I
change, say, TEST EAX, $80 to TEST AL, $80, the latter will set the sign
flag if the most-significant bit is 1 after the 'and' operation) while
the former always clears the sign flag.
I have used such subregisters before in the FPC RTL, in fpc_int_real and
fpc_frac_real in rtl/x86_64/math.inc, where I read AX instead of the
larger RAX, but that's only after a call to "SHR RAX, 48" that
guarantees that everything above the 16th bit is zero, and after testing
other implementation candidates a kind of informal competition.
(Surprisingly, I think "shr $48, %rax; and $0x7ff0,%ax; cmp $0x4330,%ax"
runs faster than moving 64-bit constants into temporary registers (since
64-bit immediates aren't supported outside of MOV) and using 'and' and
'cmp' on %rax directly)
I think you always get a read penalty when using the high-byte registers
because the processor has to do an implicit shift operation.
Thanks again for the answer.
Gareth aka. Kit
On 01/10/2020 19:43, Nikolay Nikolov via fpc-devel wrote:
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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