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add a comment that I should have written a long time ago

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25084 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2006-01-04 07:29:33 +00:00
parent 2adfa7e932
commit 51d6e76ff4
1 changed files with 28 additions and 0 deletions
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include/llvm/CodeGen/LiveVariables.h
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@ -38,6 +38,34 @@ class MRegisterInfo;
class LiveVariables : public MachineFunctionPass {
public:
/// VarInfo - This represents the regions where a virtual register is live in
/// the program. We represent this with three difference pieces of
/// information: the instruction that uniquely defines the value, the set of
/// blocks the instruction is live into and live out of, and the set of
/// non-phi instructions that are the last users of the value.
///
/// In the common case where a value is defined and killed in the same block,
/// DefInst is the defining inst, there is one killing instruction, and
/// AliveBlocks is empty.
///
/// Otherwise, the value is live out of the block. If the value is live
/// across any blocks, these blocks are listed in AliveBlocks. Blocks where
/// the liveness range ends are not included in AliveBlocks, instead being
/// captured by the Kills set. In these blocks, the value is live into the
/// block (unless the value is defined and killed in the same block) and lives
/// until the specified instruction. Note that there cannot ever be a value
/// whose Kills set contains two instructions from the same basic block.
///
/// PHI nodes complicate things a bit. If a PHI node is the last user of a
/// value in one of its predecessor blocks, it is not listed in the kills set,
/// but does include the predecessor block in the AliveBlocks set (unless that
/// block also defines the value). This leads to the (perfectly sensical)
/// situation where a value is defined in a block, and the last use is a phi
/// node in the successor. In this case, DefInst will be the defining
/// instruction, AliveBlocks is empty (the value is not live across any
/// blocks) and Kills is empty (phi nodes are not included). This is sensical
/// because the value must be live to the end of the block, but is not live in
/// any successor blocks.
struct VarInfo {
/// DefInst - The machine instruction that defines this register.
///