Re: [m32c-elf] losing track of register lifetime in combine?

[Ian Lance Taylor <iant at google dot com>]

"Dave Korn" <dave.korn@artimi.com> writes:

> On 10 November 2006 07:13, Ian Lance Taylor wrote:
> 
> > DJ Delorie <dj@redhat.com> writes:
> > 
> >> I compared the generated code with an equivalent explicit test,
> >> and discovered that gcc uses a separate rtx for the intermediate:
> >> 
> >> i = 0xfffff;
> >> if (j >= 16)
> >>   {
> >>     int i2;
> >>     i2 = i >> 8;
> >>     i = i2 >> 8;
> >>     j -= 16;
> >>   }
> >> 
> >> This seems to avoid the combiner problem, becuase you don't have the
> >> same register being set and being used in one insn.  Does this explain
> >> why combine was having a problem, or was this a legitimate thing to do
> >> and the combiner is still wrong?  Using a temp in the expander works
> >> around the problem.
> > 
> > Interesting.  Using a temporary is the natural way to implement this
> > code.  But not using a temporary should be valid.  So I think there is
> > a bug in combine.
> 
> 
>   Doesn't this just suggest that there's a '+' constraint modifier missing
> from an operand in a pattern in the md file somewhere, such as the one that
> expands the conditional in the first place?

Not necessarily.  I would guess that it's a define_expand which
generates a pseudo-register and uses it as
    (set (reg) (ashiftrt (reg) (const_int 8)))
That is OK.

In any case a '+' constraint doesn't make any difference this early in
the RTL passes.  combine doesn't look at constraints.

Ian