[cfe-dev] RFC: Support x86 interrupt and exception handlers

[David Chisnall via cfe-dev]

On 22 Sep 2015, at 12:47, H.J. Lu <hjl.tools at gmail.com> wrote:
> 
> since __builtin_exception_error () is the same as
> __builtin_return_address (0) and __builtin_interrupt_data () is
> address of __builtin_exception_error () + size of register.

Except that they’re *not*.  __builtin_return_address(0) is guaranteed to be the same for the duration of the function.  __builtin_exception_error() needs to either:

1) Fetch the values early with interrupts disabled, store them in a well-known location, and load them from this place when the intrinsic is called, or

2) Force any function that calls the intrinsic (and wants a meaningful result) to run with interrupts disabled, which is something that the compiler can’t verify without knowing the full chain of code from the interrupt handler to the current point (and therefore prone to error).

It is trivial to write a little bit of inline assembly that reads these values from the CPU and expose that for C code.  There is a good reason why no one does this.

Without a better programmer model, you are not simplifying things, you are just providing more ways for introducing errors.

David