Context

We are porting C code that was originally compiled using an 8-bit C compiler for the PIC microcontroller. A common idiom that was used in order to prevent unsigned global variables (for example, error counters) from rolling over back to zero is the following:

if(~counter) counter++;

The bitwise operator here inverts all the bits and the statement is only true if counter is less than the maximum value. Importantly, this works regardless of the variable size.

Problem

We are now targeting a 32-bit ARM processor using GCC. We've noticed that the same code produces different results. So far as we can tell, it looks like the bitwise complement operation returns a value that is a different size than we would expect. To reproduce this, we compile, in GCC:

uint8_t i = 0;
int sz;

sz = sizeof(i);
printf("Size of variable: %d\n", sz); // Size of variable: 1

sz = sizeof(~i);
printf("Size of result: %d\n", sz); // Size of result: 4

In the first line of output, we get what we would expect: i is 1 byte. However, the bitwise complement of i is actually four bytes which causes a problem because comparisons with this now will not give the expected results. For example, if doing (where i is a properly-initialized uint8_t):

if(~i) i++;

we will see i "wrap around" from 0xFF back to 0x00. This behaviour is different in GCC compared with when it used to work as we intended in the previous compiler and 8-bit PIC microcontroller.

We are aware that we can resolve this by casting like so:

if((uint8_t)~i) i++;

or, by

if(i < 0xFF) i++;

however in both of these workarounds, the size of the variable must be known and is error-prone for the software developer. These kinds of upper bounds checks occur throughout the codebase. There are multiple sizes of variables (eg., uint16_t and unsigned char etc.) and changing these in an otherwise working codebase is not something we're looking forward to.

Question

Is our understanding of the problem correct, and are there options available to resolving this that do not require re-visiting each case where we've used this idiom? Is our assumption correct, that an operation like bitwise complement should return a result that is the same size as the operand? It seems like this would break, depending on processor architectures. I feel like I'm taking crazy pills and that C should be a bit more portable than this. Again, our understanding of this could be wrong.

On the surface this might not seem like a huge issue but this previously-working idiom is used in hundreds of locations and we're eager to understand this before proceeding with expensive changes.

Note: There is a seemingly similar but not exact duplicate question here: Bitwise operation on char gives 32 bit result

I didn't see the actual crux of the issue discussed there, namely, the result size of a bitwise complement being different than what's passed into the operator.