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

[David Chisnall]

On 22 Sep 2015, at 12:39, H.J. Lu via cfe-dev <cfe-dev@lists.llvm.org> wrote:
> 
> The center piece of my proposal is not to change how parameters
> are passed in compiler.  As for user experience, the feedbacks on
> my proposal from our users are very positive.

Implementing the intrinsics for getting the current interrupt requires a lot of support code for it to actually be useful.  For it to be useful, you are requiring all of the C code to be run with interrupts disabled (and even that doesn’t work if you get a NMI in the middle).  Most implementations use a small amount of assembly to capture the interrupt cause and the register state on entry to the handler, then reenable interrupts while the C code runs.  This means that any interrupts (e.g. page faults, illegal instruction traps, whatever) that happen while the C code is running do not mask the values.  Accessing these values from *existing* C code is simply a matter of loading a field from a structure.

I’m really unconvinced by something that something with such a narrow use case (and one that encourages writing bad code) belongs in the compiler.

David