[avr-gcc-list] custom signal prologue

[Ben Mann]

Hi all,

Is there any way to customise the prologue of an interrupt handler?

To explain...

I have a UDR interrupt handler in which I'd like to enable interrupts so
that another very time-critical interrupt can be handled promptly.

The usually accepted method does not work for UDR

 INTERRUPT(SIG_UART_DATA) {
     ...do something...
 }

because as soon as sei is enabled another UDR interrupt immediately triggers
and hangs the processor.

An inefficient but workable solution in C appears to be:

SIGNAL(SIG_UART_DATA) {
    //Disable UDRIE
    UCSRB &= _BV(UDRIE);
    sei();
    ...do something...
    UCSRB |= _BV(UDRIE);
}

The problem being that with complex replacements for "do something", the
prologue consists of pushing qutie a few registers. The trimmed .lst excerpt
below demonstrates the code the compiler generates for the first couple of
lines above in my application.

*********************
push __zero_reg__                               //UDRIE vector
push __tmp_reg__
in __tmp_reg__,__SREG__
push __tmp_reg__
clr __zero_reg__
push r18
push r19
push r20
push r21
push r22
push r23
push r24
push r25
push r26
push r27
push r30
push r31
/* prologue end (size=17) */
lds r24,154  /* disable UDRIE */
andi r24,lo8(-33)
sts 154,r24
sei   /* enable interrupts */
*********************

Is it possible to customise the prologue and somehow do the UDRIE disable
first, ie:

*********************
lds r24,154  /* disable UDRIE */        //UDRIE vector
andi r24,lo8(-33)
sts 154,r24
sei   /* enable interrupts */
push __zero_reg__
push __tmp_reg__
in __tmp_reg__,__SREG__
push __tmp_reg__
clr __zero_reg__
push r18
push r19
push r20
push r21
push r22
push r23
push r24
push r25
push r26
push r27
push r30
push r31
/* prologue end (size=21) */
*********************

For completeness I also looked at the promising and simple idea of:
SIGNAL(SIG_UART_DATA)
{
    //Disable UDRIE
    UCSRB &= _BV(UDRIE);
    sei();
    MyInterruptHandler();
}

Which I thought should require no prologue (MyInterruptHandler() pushing
what it needs) but the compiler still generates a similar looking prologue
for the interrupt handler - even with the interrupt and MyInterruptHandler()
in separate source files... (Which I found a little surprising)

Am I missing something obvious? Has anyone attempted to deal with this
before? (... sorry for the long post...I can't seem to keep it brief...)

Thanks

Ben Mann