[Bug target/66960] Add interrupt attribute to x86 backend

["hjl.tools at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66960

--- Comment #4 from H.J. Lu <hjl.tools at gmail dot com> ---
The current specification:

The interrupt and exception handlers are called by x86 processors.  X86
hardware pushes information onto 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. Support both 32-bit and 64-bit modes.
2. Flexible for compilers to optimize.
3. Easy to use by programmers.

To implement interrupt and exception handlers for x86 processors, a
compiler should support:

'interrupt' attribute

Use this attribute to indicate that the specified function with
mandatory arguments is an interrupt or exception 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 or
exception handlers.  All registers, except for the EFLAGS register which
is restored by the 'IRET' instruction, are preserved by the compiler.

Since the direction flag in the FLAGS register in interrupt (exception)
handlers is undetermined, cld instruction must be emitted in function
prologue if rep string instructions are used in interrupt (exception)
handler or interrupt (exception) handler isn't a leaf function.

Any interruptible-without-stack-switch code must be compiled with
-mno-red-zone since interrupt handlers can and will, because of the
hardware design, touch the red zone.

1. interrupt handler must be declared with a mandatory pointer argument:

struct interrupt_frame;

__attribute__ ((interrupt))
void
f (struct interrupt_frame *frame)
{
...
}

and user must properly define the structure the pointer pointing to.

2. exception handler:

The exception handler is very similar to the interrupt handler with
a different mandatory function signature:

typedef unsigned int uword_t __attribute__ ((mode (__word__)));

struct interrupt_frame;

__attribute__ ((interrupt))
void
f (struct interrupt_frame *frame, uword_t error_code)
{
...
}

and compiler pops the error code off stack before the 'IRET' instruction.

The exception handler should only be used for exceptions which push an
error code and all other exceptions must use the interrupt handler.
The system will crash if the wrong handler is used.

'no_caller_saved_registers' attribute

Use this attribute to indicate that the specified function has no
caller-saved registers.  That is, all registers are callee-saved.
The compiler generates proper function entry and exit sequences to
save and restore any modified registers.

The user can call functions specified with 'no_caller_saved_registers'
attribute from an interrupt handler without saving and restoring all
call clobbered registers.

On x86, interrupt handlers are only called by processors which push
interrupt data onto stack at the address where the normal return address
is.  Interrupt handlers must access interrupt data via pointers so that
they can update interrupt data.