[llvm-commits] [llvm] r45799 - /llvm/trunk/lib/CodeGen/StrongPHIElimination.cpp

[Evan Cheng]

Geeze. I asked for comments and was ignored. Tanya asks for it and  
she gets it right away. :-)

Evan
On Jan 9, 2008, at 5:15 PM, Tanya Lattner wrote:

> Nice!
>
> On Jan 9, 2008, at 4:47 PM, Owen Anderson wrote:
>
>> Author: resistor
>> Date: Wed Jan  9 18:47:01 2008
>> New Revision: 45799
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=45799&view=rev
>> Log:
>> Add more comments explaining the basics of how the decision of when
>> to rename and when to insert
>> copies is made.
>>
>> Modified:
>>     llvm/trunk/lib/CodeGen/StrongPHIElimination.cpp
>>
>> Modified: llvm/trunk/lib/CodeGen/StrongPHIElimination.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/
>> StrongPHIElimination.cpp?rev=45799&r1=45798&r2=45799&view=diff
>>
>> ===================================================================== 
>> =
>> ========
>> --- llvm/trunk/lib/CodeGen/StrongPHIElimination.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/StrongPHIElimination.cpp Wed Jan  9
>> 18:47:01 2008
>> @@ -390,7 +390,10 @@
>>    return interference;
>>  }
>>
>> -/// processBlock - Eliminate PHIs in the given block
>> +/// processBlock - Determine how to break up PHIs in the current
>> block.  Each
>> +/// PHI is broken up by some combination of renaming its operands
>> and inserting
>> +/// copies.  This method is responsible for determining which
>> operands receive
>> +/// which treatment.
>>  void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
>>    LiveVariables& LV = getAnalysis<LiveVariables>();
>>
>> @@ -398,21 +401,37 @@
>>    // before the current one.
>>    std::set<unsigned> ProcessedNames;
>>
>> +  // Iterate over all the PHI nodes in this block
>>    MachineBasicBlock::iterator P = MBB->begin();
>>    while (P != MBB->end() && P->getOpcode() ==  
>> TargetInstrInfo::PHI) {
>>      LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand
>> (0).getReg());
>>
>>      unsigned DestReg = P->getOperand(0).getReg();
>>
>> -    // Hold the names that are currently in the candidate set.
>> +    // PHIUnion is the set of incoming registers to the PHI node  
>> that
>> +    // are going to be renames rather than having copies
>> inserted.  This set
>> +    // is refinded over the course of this function.
>> UnionedBlocks is the set
>> +    // of corresponding MBBs.
>>      std::set<unsigned> PHIUnion;
>>      std::set<MachineBasicBlock*> UnionedBlocks;
>>
>> +    // Iterate over the operands of the PHI node
>>      for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
>>        unsigned SrcReg = P->getOperand(i-1).getReg();
>>        LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);
>>
>> -      // Check for trivial interferences
>> +      // Check for trivial interferences via liveness information,
>> allowing us
>> +      // to avoid extra work later.  Any registers that interfere
>> cannot both
>> +      // be in the renaming set, so choose one and add copies for
>> it instead.
>> +      // The conditions are:
>> +      //   1) if the operand is live into the PHI node's block OR
>> +      //   2) if the PHI node is live out of the operand's
>> defining block OR
>> +      //   3) if the operand is itself a PHI node and the original
>> PHI is
>> +      //      live into the operand's defining block OR
>> +      //   4) if the operand is already being renamed for another
>> PHI node
>> +      //      in this block OR
>> +      //   5) if any two operands are defined in the same block,
>> insert copies
>> +      //      for one of them
>>        if (isLiveIn(SrcInfo, P->getParent()) ||
>>            isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
>>            ( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
>> @@ -420,24 +439,32 @@
>>            ProcessedNames.count(SrcReg) ||
>>            UnionedBlocks.count(SrcInfo.DefInst->getParent())) {
>>
>> -        // add a copy from a_i to p in Waiting[From[a_i]]
>> +        // Add a copy for the selected register
>>          MachineBasicBlock* From = P->getOperand(i).getMBB();
>>          Waiting[From].insert(std::make_pair(SrcReg, DestReg));
>>          UsedByAnother.insert(SrcReg);
>>        } else {
>> +        // Otherwise, add it to the renaming set
>>          PHIUnion.insert(SrcReg);
>>          UnionedBlocks.insert(SrcInfo.DefInst->getParent());
>>        }
>>      }
>>
>> +    // Compute the dominator forest for the renaming set.  This is
>> a forest
>> +    // where the nodes are the registers and the edges represent
>> dominance
>> +    // relations between the defining blocks of the registers
>>      std::vector<StrongPHIElimination::DomForestNode*> DF =
>>
>> computeDomForest(PHIUnion);
>>
>> -    // Walk DomForest to resolve interferences
>> +    // Walk DomForest to resolve interferences at an inter-block
>> level.  This
>> +    // will remove registers from the renaming set (and insert
>> copies for them)
>> +    // if interferences are found.
>>      std::vector<std::pair<unsigned, unsigned> > localInterferences;
>>      processPHIUnion(P, PHIUnion, DF, localInterferences);
>>
>> -    // Check for local interferences
>> +    // The dominator forest walk may have returned some register
>> pairs whose
>> +    // interference cannot be determines from dominator analysis.
>> We now
>> +    // examine these pairs for local interferences.
>>      for (std::vector<std::pair<unsigned, unsigned> >::iterator I =
>>          localInterferences.begin(), E = localInterferences.end();
>> I != E; ++I) {
>>        std::pair<unsigned, unsigned> p = *I;
>> @@ -481,10 +508,13 @@
>>        }
>>      }
>>
>> -    // Cache renaming information
>> +    // Add the renaming set for this PHI node to our overal
>> renaming information
>>      RenameSets.insert(std::make_pair(P->getOperand(0).getReg(),
>> PHIUnion));
>>
>> +    // Remember which registers are already renamed, so that we
>> don't try to
>> +    // rename them for another PHI node in this block
>>      ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
>> +
>>      ++P;
>>    }
>>  }
>>
>>
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