Preventing scheduling of normal operations across calls to built-ins

[Ian Lance Taylor]

Ayonam Ray <ayonam@gmail.com> writes:

> Register r0 is set to zero as per software conventions.  Hence, it is
> made a fixed register.

If r0 is a fixed 0, then you should never set it to anything else in
RTL.  Doing so will inhibit some optimizations.  You should invent a new
register, one that does not exist in hardware, for this purpose.


> (define_insn "mvc_to_ccr"
>   [(unspec_volatile:SI [(match_operand:SI 0 "register_operand" "=Y")
>                         (match_operand:SI 1 "register_operand" "r")]
>                        UNSPEC_MVC_TO_CCR)
>    (set (reg:SI 0) (unspec_volatile [(reg:SI 0)]UNSPEC_CCR))]
>   ""


> (insn 15 14 16 g.c:980 (parallel [
>             (unspec_volatile:SI [
>                     (reg:SI 98 ccr8)
>                     (reg:SI 124)
>                 ] 28)
>             (set (reg:SI 1 r1)
>                 (const_int 0 [0x0]))
>         ]) -1 (nil))

The define_insn says (set (reg:SI 0) (unspec_volatile ...)).  The RTL
dump says (set (reg:SI 1) (const_int 0)).  Why the difference?

> P.S.:  Are attachments allowed on the GCC mailing lists?

I think so, but they can't be too large.

Ian