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RegisterCoalescer: Improve a comment.

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Explain the relation of the example to the variables in the code,
explain what bad behaviour the code avoids in this case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237706 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Matthias Braun 2015-05-19 17:52:32 +00:00
parent 038aeda7cf
commit f7ad33bd8a
1 changed files with 5 additions and 6 deletions
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11
lib/CodeGen/RegisterCoalescer.cpp
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@ -955,12 +955,11 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
TII->reMaterialize(*MBB, MII, DstReg, SrcIdx, DefMI, *TRI);
MachineInstr *NewMI = std::prev(MII);
// A situation like the following:
// %vreg0:subX = instr ; DefMI
// %vregY = copy %vreg:subX ; CopyMI
// does not need subregisters/regclass widening after rematerialization, just
// do:
// %vregY = instr
// In a situation like the following:
// %vreg0:subreg = instr ; DefMI, subreg = DstIdx
// %vreg1 = copy %vreg0:subreg ; CopyMI, SrcIdx = 0
// instead of widening %vreg1 to the register class of %vreg0 simply do:
// %vreg1 = instr
const TargetRegisterClass *NewRC = CP.getNewRC();
if (DstIdx != 0) {
MachineOperand &DefMO = NewMI->getOperand(0);