rewriter: add hardcoded call gate post condition call with no args

[kkysen]

For now, the name of the post condition checking function is ${target_name}_post_condition, and it's hardcoded to only emit for dav1d_get_picture.

• Part of Potential dav1d Call Gate Verification Discussion #465.

I'm not sure how to call the function with arguments, though. I was imagining the API be that the post condition function have the same arguments as the target function and a void return type.

It also takes 0 args for now. Later PRs will progressively add args.

[kkysen]

For the args, @ayrtonm and I decided we should push the caller args onto the stack at the beginning (only the 6 args in registers for now) and then pop them back into the 6 registers to call the post condition function. So for now it'll be limited to up to 6 args. Once we have that working, we'll work on also passing the target function return value as an argument as well, and more than 6 args.

[ayrtonm]

To add some other notes from our talk

• we want to check arguments in post-conditions to validate out parameters
• using the first 6 regs will clobber rdx which is fine if the return value is not 128 bits
• as a next step for proper argument handling we may want to use one of the purely stack-based calling conventions for the check functions

[kkysen]

@ayrtonm, can I merge this and split my stuff into smaller PRs? Next PR will do the 6 register args, which I'm working on now.

[ayrtonm]

can I merge this and split my stuff into smaller PRs

Maybe just do a single PR where the commits can be reviewed individually

[ayrtonm]

LGTM after adding an option to enable/disable the post-validation codegen.