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

H. Peter Anvin via cfe-dev cfe-dev at lists.llvm.org
Tue Sep 22 12:21:46 PDT 2015

On 09/22/15 04:52, David Chisnall wrote:
> 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.

This is why it makes no sense for the intrinsics to be callable from
anywhere except inside the interrupt handler.  It is really nothing
other than a way to pass arguments -- whether or not it is simpler for
the compilers to implement than supporting a different function
signature is beyond my scope of expertise.


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