Re: [PATCH PR62151]Fix uninitialized register issue caused by distribute_notes in combine pass

[Jeff Law <law at redhat dot com>]

On 09/01/14 05:38, Segher Boessenkool wrote:
> On Mon, Sep 01, 2014 at 11:36:07AM +0800, Bin.Cheng wrote:
> > > In the testcase (and comment in the proposed patch), why is combine
> > > combining four insns at all?  That means it rejected combining just the
> > > first three.  Why did it do that?
> > It is explicitly reject by below code in can_combine_p.
> >
> >    if (GET_CODE (PATTERN (i3)) == PARALLEL)
> >      for (i = XVECLEN (PATTERN (i3), 0) - 1; i >= 0; i--)
> >        if (GET_CODE (XVECEXP (PATTERN (i3), 0, i)) == CLOBBER)
> >      {
> >        /* Don't substitute for a register intended as a clobberable
> >           operand.  */
> >        rtx reg = XEXP (XVECEXP (PATTERN (i3), 0, i), 0);
> >        if (rtx_equal_p (reg, dest))
> >          return 0;
> >
> > Since insn i2 in the list of i0/i1/i2 as below contains parallel
> > clobber of dest_of_insn76/use_of_insn77.
> >     32: r84:SI=0
> >     76: flags:CC=cmp(r84:SI,0x1)
> >        REG_DEAD r84:SI
> >     77: {r84:SI=-ltu(flags:CC,0);clobber flags:CC;}
> >        REG_DEAD flags:CC
> >        REG_UNUSED flags:CC
>
> Archaeology suggests this check is because the clobber might be an
> earlyclobber.  Which seems silly: how can it be a valid insn at all
> in that case?  It seems to me the check can just be removed.  That
> will hide your issue, maybe even solve it (but I doubt it).
Silly for other reasons, namely that earlyclobber doesn't come into play 
until after combine (register allocation and later).


> Another question is why is r84 set twice in the first place?
Various transformations can set that kind of situation up.  One could 
argue that a local pass ssa-like-ize things like this would be good 
independent of this BZ.  The web code would probably help here, but 
seems awful heavyweight for an opportunity like this.

jeff