[llvm-commits] [llvm] r144527 - in /llvm/trunk: include/llvm/CodeGen/MachineBranchProbabilityInfo.h lib/CodeGen/MachineBranchProbabilityInfo.cpp

[Benjamin Kramer]

On 14.11.2011, at 10:00, Chandler Carruth <chandlerc at gmail.com> wrote:

> Author: chandlerc
> Date: Mon Nov 14 02:55:59 2011
> New Revision: 144527
>
> URL: http://llvm.org/viewvc/llvm-project?rev=144527&view=rev
> Log:
> Reuse the logic in getEdgeProbability within getHotSucc in order to
> correctly handle blocks whose successor weights sum to more than
> UINT32_MAX. This is slightly less efficient, but the entire thing is
> already linear on the number of successors. Calling it within any hot
> routine is a mistake, and indeed no one is calling it. It also
> simplifies the code.

Does this problem also exist in the non-machine BPI version of getHotSucc?

>
> Modified:
>    llvm/trunk/include/llvm/CodeGen/MachineBranchProbabilityInfo.h
>    llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp
>
> Modified: llvm/trunk/include/llvm/CodeGen/MachineBranchProbabilityInfo.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/CodeGen/MachineBranchProbabilityInfo.h?rev=144527&r1=144526&r2=144527&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/CodeGen/MachineBranchProbabilityInfo.h (original)
> +++ llvm/trunk/include/llvm/CodeGen/MachineBranchProbabilityInfo.h Mon Nov 14 02:55:59 2011
> @@ -59,6 +59,7 @@
>   bool isEdgeHot(MachineBasicBlock *Src, MachineBasicBlock *Dst) const;
>
>   // Return a hot successor for the block BB or null if there isn't one.
> +  // NB: This routine's complexity is linear on the number of successors.
>   MachineBasicBlock *getHotSucc(MachineBasicBlock *MBB) const;
>
>   // Return a probability as a fraction between 0 (0% probability) and
>
> Modified: llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp?rev=144527&r1=144526&r2=144527&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp (original)
> +++ llvm/trunk/lib/CodeGen/MachineBranchProbabilityInfo.cpp Mon Nov 14 02:55:59 2011
> @@ -76,26 +76,18 @@
>
> MachineBasicBlock *
> MachineBranchProbabilityInfo::getHotSucc(MachineBasicBlock *MBB) const {
> -  uint32_t Sum = 0;
>   uint32_t MaxWeight = 0;
>   MachineBasicBlock *MaxSucc = 0;
> -
>   for (MachineBasicBlock::const_succ_iterator I = MBB->succ_begin(),
>        E = MBB->succ_end(); I != E; ++I) {
> -    MachineBasicBlock *Succ = *I;
> -    uint32_t Weight = getEdgeWeight(MBB, Succ);
> -    uint32_t PrevSum = Sum;
> -
> -    Sum += Weight;
> -    assert(Sum > PrevSum); (void) PrevSum;
> -
> +    uint32_t Weight = getEdgeWeight(MBB, *I);
>     if (Weight > MaxWeight) {
>       MaxWeight = Weight;
> -      MaxSucc = Succ;
> +      MaxSucc = *I;
>     }
>   }
>
> -  if (BranchProbability(MaxWeight, Sum) >= BranchProbability(4, 5))
> +  if (getEdgeProbability(MBB, MaxSucc) >= BranchProbability(4, 5))
>     return MaxSucc;
>
>   return 0;
>
>
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