[llvm] r200941 - Track register pressure a bit more carefully (weird corner case).

[Sean Silva]

+    }
+    else {

Someone isn't using clang-format... :}


On Thu, Feb 6, 2014 at 2:20 PM, Andrew Trick <atrick at apple.com> wrote:

> Author: atrick
> Date: Thu Feb  6 13:20:41 2014
> New Revision: 200941
>
> URL: http://llvm.org/viewvc/llvm-project?rev=200941&view=rev
> Log:
> Track register pressure a bit more carefully (weird corner case).
>
> This solves a problem where a def machine operand has no uses but has
> not been marked dead. In this case, the initial RP analysis was being
> extra precise and determining from LiveIntervals the the register was
> actually dead. This caused us to omit the register from the RP
> tracker's block live out. That's all good, but the per-instruction
> summary still accounted for it as a valid def. This could cause an
> assertion in the tracker later when we underflow pressure.
>
> This is from a bug report on an out-of-tree target. It is not
> reproducible on well-behaved targets. I'm just making an obvious fix
> without unit test.
>
> Modified:
>     llvm/trunk/lib/CodeGen/RegisterPressure.cpp
>
> Modified: llvm/trunk/lib/CodeGen/RegisterPressure.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegisterPressure.cpp?rev=200941&r1=200940&r2=200941&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/RegisterPressure.cpp (original)
> +++ llvm/trunk/lib/CodeGen/RegisterPressure.cpp Thu Feb  6 13:20:41 2014
> @@ -506,7 +506,14 @@ bool RegPressureTracker::recede(SmallVec
>          DeadDef = LRQ.isDeadDef();
>        }
>      }
> -    if (!DeadDef) {
> +    if (DeadDef) {
> +      // LiveIntervals knows this is a dead even though it's
> MachineOperand is
> +      // not flagged as such. Since this register will not be recorded as
> +      // live-out, increase its PDiff value to avoid underflowing
> pressure.
> +      if (PDiff)
> +        PDiff->addPressureChange(Reg, false, MRI);
> +    }
> +    else {
>        if (LiveRegs.erase(Reg))
>          decreaseRegPressure(Reg);
>        else
>
>
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