Using Int (32 bits) over char (8 bits) to 'help' processor

Question

In C, often we use char for small number representations. However Processor always uses Int( or 32 bit) values for read from(or fetch from) registers. So every time we need to use a char or 8 bits in our program processor need to fetch 32 bits from regsiter and 'parse' 8 bits out of it. Hence does it sense to use Int more often in place of char if memory is not the limitation? Will it 'help' processor?

Answer 1

There's the compiler part and the cpu part.

If you tell the compiler you're using a char instead of an int, during static analysis it will know the bounds of the variable is between 0-255 instead of 0-(2^32-1). This will allow it to optimize your program better.

On the cpu side, your assumption isn't always correct. Take x86 as an example, it has registers eax and al for 32 bit and 8 bit register access. If you want to use chars only, using al is sufficient. There is no performance loss.

I did some simple benchmarks in response to below comments:

al:

format PE GUI 4.0

xor ecx, ecx
dec ecx
loop_start:
inc al
add al, al
dec al
dec al
loopd short loop_start
ret

eax:

format PE GUI 4.0

xor ecx, ecx
dec ecx
loop_start:
inc eax
add eax, eax
dec eax
dec eax
loopd short loop_start
ret

times:

$ time ./test_al.exe 
./test_al.exe  0.01s user 0.00s system 0% cpu 7.102 total
$ time ./test_eax.exe 
./test_eax.exe  0.01s user 0.01s system 0% cpu 7.120 total

So in this case, al is slightly faster, but sometimes eax came out faster. The difference is really negligible. But cpus aren't so simple, there might be code alignment issues, caches, and other things going on, so it's best to benchmark your own code to see if there's any performance improvement. But imo, if your code is not super tight, it's best to trust the compiler to optimize things.