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

[David Chisnall via cfe-dev]

On 21 Sep 2015, at 21:45, H.J. Lu via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> The main purpose of x86 interrupt attribute is to allow programmers
> to write x86 interrupt/exception handlers in C WITHOUT assembly
> stubs to avoid extra branch from assembly stubs to C functions.  I
> want to keep the number of new intrinsics to minimum without sacrificing
> handler performance. I leave faking error code in interrupt handler to
> the programmer.

The assembly stubs have to come from somewhere.  You either put them in an assembly file (most people doing embedded x86 stuff steal the ones from NetBSD), or you put them in the compiler where they can be inlined.  In terms of user interface, there’s not much difference in complexity.  Having written this kind of code in the past, I can honestly say that using the assembly stubs was the least difficult part of getting them right.  In terms of compiler complexity, there’s a big difference: in one case the compiler contains nothing, in the other it contains something special for a single use case.  In terms of performance, the compiler version has the potential to be faster, but if we’re going to pay for the complexity then I think that we’d need to see some strong evidence that someone else is getting a noticeable benefit.

David