[TUHS] Re: Maximum Array Sizes in 16 bit C

On Sat, Sep 21, 2024 at 01:07:11AM +1000, Dave Horsfall wrote: Yes, but that was a stack variable. Let me see if I can say it more clearly. foo() { int a = 1, b = 2; int alias[5]; alias[-2] = 0; // try and set a to 0. } In v7 days, the stack would look like [stuff] [2 bytes for a] [2 bytes for b] [2 bytes for the alias address, which I think points forward] [10 bytes for alias contents] I'm hazy on how the space for alias[] is allocated, so I made that up. It's probably something like I said but Paul (or someone) will correct me. When using a negative index for alias[], the coder is assuming that the stack variables are placed in the order they were declared. Paul tried to explain that _might_ be true but is not always true. Modern compilers will look see which variables are used the most in the function, and place them next to each other so that if you have the cache line for one heavily used variable, the other one is right there next to it. Like so: int heavy1 = 1; int rarely1 = 2; int spacer[10]; int heavy2 = 3; int rarel2 = 4; The compiler might figure out that heavy{1,2} are used a lot and lay out the stack like so: [2 bytes (or 4 or 8 these days) for heavy1] [bytes for heavy2] [bytes for rarely1] [bytes for spacer[10]] [bytes for rarely2] Paul was saying that using a negative index in the array creates an alias, or another name, for the scalar integer on the stack (his description made me understand, for the first time in decades, why compiler writers hate aliases and I get it now). Aliases mess hard with optimizers. Optimizers may reorder the stack for better cache line usage and what you think array[-2] means doesn't work any more unless the optimizer catches that you made an alias and preserves it. Paul, how did I do? I'm not a compiler guy, just had to learn enough to walk the stack when the kernel panics.