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

[Reid Kleckner via cfe-dev]

-gcc-dev, not because I'm exclusionary, I'm just not sure if cross-posting
to it is OK...

For what it's worth, there's precedent for this in clang. We already
support __attribute__((interrupt)) on ARM and MSP430. Neither of them
introduce a new calling convention at the Clang level. I think MSP430 adds
one at the LLVM level, but I don't think it matters.

On Thu, Sep 3, 2015 at 10:37 AM, H.J. Lu via cfe-dev <cfe-dev at lists.llvm.org
> wrote:

> The interrupt and exception handlers are called by x86 processors.  X86
> hardware puts information on stack and calls the handler.  The
> requirements are
>
> 1. Both interrupt and exception handlers must use the 'IRET' instruction,
> instead of the 'RET' instruction, to return from the handlers.
> 2. All registers are callee-saved in interrupt and exception handlers.
> 3. The difference between interrupt and exception handlers is the
> exception handler must pop 'ERROR_CODE' off the stack before the 'IRET'
> instruction.
>
> The design goals of interrupt and exception handlers for x86 processors
> are:
>
> 1. No new calling convention in compiler.
> 2. Support both 32-bit and 64-bit modes.
> 3. Flexible for compilers to optimize.
> 4. Easy to use by programmers.
>
> To implement interrupt and exception handlers for x86 processors, a
> compiler should support:
>
> 1. void * __builtin_ia32_interrupt_data (void)
>
> This function returns a pointer to interrupt or exception data pushed
> onto the stack by processor.
>
> The __builtin_frame_address builtin isn't suitable for interrupt and
> exception handlers since it returns the stack frame address on the
> callee side and compiler may generate a new stack frame for stack
> alignment.
>
> 2. 'interrupt' attribute
>
> Use this attribute to indicate that the specified void function without
> arguments is an interrupt handler.  The compiler generates function entry
> and exit sequences suitable for use in an interrupt handler when this
> attribute is present.  The 'IRET' instruction, instead of the
> 'RET' instruction, is used to return from interrupt handlers.  All
> registers, except for the EFLAGS register which is restored by the
> 'IRET' instruction, are preserved by the compiler.  The red zone
> isn't supported in an interrupt handler; that is an interrupt
> handler can't access stack beyond the current stack pointer.
>
> You can use the builtin '__builtin_ia32_interrupt_data' function to access
> data pushed onto the stack by processor:
>
> void
> f () __attribute__ ((interrupt))
> {
>   void *p = __builtin_ia32_interrupt_data ();
>   ...
> }
>
> 3. 'exception' attribute
>
> Use 'exception' instead of 'interrupt' for handlers intended to be
> used for 'exception' (i.e. those that must pop 'ERROR_CODE' off the
> stack before the 'IRET' instruction):
>
> void
> f () __attribute__ ((exception))
> {
>   void *p = __builtin_ia32_interrupt_data ();
>   ...
> }
>
> Any comments, suggestions?
>
> Thanks.
>
>
> --
> H.J.
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>
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