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Allow <undef> flags on def operands as well as uses.

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The <undef> flag says that a MachineOperand doesn't read its register,
or doesn't depend on the previous value of its register.

A full register def never depends on the previous register value.  A
partial register def may depend on the previous value if it is intended
to update part of a register.

For example:

  %vreg10:dsub_0<def,undef> = COPY %vreg1
  %vreg10:dsub_1<def> = COPY %vreg2

The first copy instruction defines the full %vreg10 register with the
bits not covered by dsub_0 defined as <undef>.  It is not considered a
read of %vreg10.

The second copy modifies part of %vreg10 while preserving the rest.  It
has an implicit read of %vreg10.

This patch adds a MachineOperand::readsReg() method to determine if an
operand reads its register.

Previously, this was modelled by adding a full-register <imp-def>
operand to the instruction.  This approach makes it possible to
determine directly from a MachineOperand if it reads its register.  No
scanning of MI operands is required.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141124 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jakob Stoklund Olesen
2011-10-04 21:49:33 +00:00
parent 0ebefdf834
commit 7016cf66ee
2 changed files with 34 additions and 3 deletions
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9
lib/CodeGen/LiveIntervalAnalysis.cpp
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@ -304,8 +304,15 @@ void LiveIntervals::handleVirtualRegisterDef(MachineBasicBlock *mbb,
// Make sure the first definition is not a partial redefinition. Add an
// <imp-def> of the full register.
if (MO.getSubReg())
if (MO.getSubReg()) {
mi->addRegisterDefined(interval.reg);
// Mark all defs of interval.reg on this instruction as reading <undef>.
for (unsigned i = MOIdx, e = mi->getNumOperands(); i != e; ++i) {
MachineOperand &MO2 = mi->getOperand(i);
if (MO2.isReg() && MO2.getReg() == interval.reg && MO2.getSubReg())
MO2.setIsUndef();
}
}
MachineInstr *CopyMI = NULL;
if (mi->isCopyLike()) {