[Bug target/96017] Powerpc suboptimal register spill in likely path

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96017

--- Comment #8 from Peter Bergner <bergner at gcc dot gnu.org> ---
Interesting, if I rewrite the test case so that foo is a parameter and not a
global var, then we get the code we want:

extern void slowpath(int *);

int
test (int *val, int foo)
{
  int ret = foo;

  if (__builtin_expect(*val != 0, 0))
    slowpath(val);

  return ret;
}

0:      addis 2,12,.TOC.-.LCF0@ha
        addi 2,2,.TOC.-.LCF0@l
        lwz 9,0(3)
        cmpwi 0,9,0
        bne 0,.L11
        mr 3,4
        blr
        .p2align 4,,15
.L11:
        mflr 0
        std 0,16(1)
        stdu 1,-48(1)
        std 4,32(1)
        bl slowpath
        nop
        ld 4,32(1)
        addi 1,1,48
        ld 0,16(1)
        mr 3,4
        mtlr 0
        blr

At first, I thought that split_live_ranges_for_shrink_wrap() split this nicely,
but what I found is that IRA assigned a volatile register to a pseudo that is
live across a call.  LRA then sees that is illegal and spills the pseudo which
ends up giving us what we want, but I don't like that.  IRA should never assign
a volatile register to a pseudo that is live across a call.  Yet another thing
to look into.