In Clang 15, the detect_stack_use_after_return ASAN option was enabled by default. This option seems to make automatic storage duration objects not go on the stack. StackLocator assumes such objects are on the stack. See here - the object assumes this is on the stack and compares this to the bounds of the stack (which are determined in a platform-specific way) to estimate how much stack space remains. StackLocator also invokes UB by doing relational comparisons and subtractions on pointers that aren't from the same allocation.

One possible fix is to use __builtin_frame_address(0) to get the frame pointer instead of using the address of a local variable. However, StackLocator is only used in one place in mozjs here - it controls the amount of stack space available to javascript. Maybe instead of trying to fix StackLocator, we could do something different here like setting the quota to 50% of all stack space?

There's another minor test-only case here where we make the same assumption about an automatic storage duration variable being on the stack.