[avr-llvm-devel] Fwd: New backend implementation

[Borja F. <bor...@gm...>]

I've just finished fixing the register calling convention. I had a huge TODO
in my code to handle passing chars in even registers and returning a char in
R24. I couldnt use the typical approach of using the Tablegen code to define
the calling convention because wide arguments are splitted into i8 pieces
making LLVM treat all args as chars so there was no easy way of knowing the
real size of each arg. I had to write it down in C++ code to check the real
argument size and allocate the regs using AVR's ABI spec.

A piece of code to show it:

typedef int t;
t foo(t a, t b, char c, int d)
{
    return a+b+d;
}

foo:                                    # @foo
    .ent foo
# BB#0:                                 # %entry
    add    r22, r18
    adc    r23, r19
    adc    r24, r20
    adc    r25, r21
    add    r22, r12
    adc    r23, r13
    adc    r24, r14
    adc    r25, r15
    ret

GCC generated this:

120:    cf 92           push    r12
 122:    df 92           push    r13
 124:    ef 92           push    r14
 126:    ff 92           push    r15
    return (a + b +d);
 128:    26 0f           add    r18, r22
 12a:    37 1f           adc    r19, r23
 12c:    48 1f           adc    r20, r24
 12e:    59 1f           adc    r21, r25
 130:    2c 0d           add    r18, r12
 132:    3d 1d           adc    r19, r13
 134:    4e 1d           adc    r20, r14
 136:    5f 1d           adc    r21, r15
 138:    b9 01           movw    r22, r18
 13a:    ca 01           movw    r24, r20
 13c:    ff 90           pop    r15
 13e:    ef 90           pop    r14
 140:    df 90           pop    r13
 142:    cf 90           pop    r12
 144:    08 95           ret

Forget the fact that our code isnt saving regs into the stack, that's
currently unimplemented, but notice those two movws are totally redundant,
pretty funny.
LLVM's site states that the new version will get released tomorrow, so i'll
start porting my code once that is done.