C Comparison Question

I had the following code on an old ARM compiler:

unsigned char *a;
unsigned char *b;

....

if (*a != (~(*b)))

and when I looked at the assembly-language, it was putting *b into a
32-bit register, complementing the entire register, then comparing
against *a which was in a second 32-bit register.

The upper 24 bits of *b were always set to 1 because they started as
0.

This wouldn't work as I intended it, because even if *a and *b were
complements, the test would fail.

Question: in the comparison above, is the compiler allowed to extend
*b to 32 bits, complement the 32 bits, extend *a to 32 bits, then
compare the two 32-bit values?

Or should the compiler be comparing only 8 bits?

Thanks, DTA
.

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