[llvm-commits] [PATCH] Fix PR15031

[Bill Schmidt]

On Thu, 2013-01-24 at 18:10 -0200, Adhemerval Zanella wrote:
> On 01/24/2013 03:59 PM, Bill Schmidt wrote:
> > This patch addresses bug 15031.
> >
> > The common code in the post-RA scheduler to break anti-dependencies on the
> > critical path contained a flaw.  In the reported case, an anti-dependency
> > between the overlapping registers %X4 and %R4 exists:
> >
> > 	%X29<def> = OR8 %X4, %X4
> > 	%R4<def>, %X3<def,dead,tied3> = LBZU 1, %X3<kill,tied1>
> >
> > The unpatched code breaks the dependency by replacing %R4 and its uses
> > with %R3, the first register on the available list.  However, %R3 and
> > %X3 overlap, so this creates two overlapping definitions on the same
> > instruction.
> >
> > The fix is straightforward, preventing selection of a register that
> > overlaps any other defined register on the same instruction.
> >
> > The test case is reduced from the bug report, and verifies that we no
> > longer produce "lbzu 3, 1(3)" when breaking this anti-dependency.
> >
> > Is this OK to commit?
> >
> > Thanks!
> 
> The patch looks ok, just two small suggestions:
> 
> 
>        continue;
> +    // If NewReg overlaps any of the forbidden registers, we can't use it.
> +    bool Forbidden = false;
> +    for (std::vector<unsigned>::iterator it = Forbid.begin();
> +         it != Forbid.end(); ++it)
> +      if (TRI->regsOverlap(NewReg, *it)) {
> +        Forbidden = true;
> +        break;
> +      }
> +    if (Forbidden) continue;
>      return NewReg;
>    }
> 
> Avoid to evaluate end() at very time, use something like:
> 
> for (std::vector<unsigned>::iterator it = Forbid.begin(), ite = Forbid.end();
>      it != ite; ++it)
> 
> Also, since the std::vector is for unsigned and has a small size I think SmallVector
> could be a better joice.

Revised patch attached.  I think this should be ready to commit, unless
anyone has remaining concerns.

Thanks,
Bill

> 
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