[llvm] b817ea7 - [MachineFunction] Make comment for TracksLiveness more clearer

[Guozhi Wei via llvm-commits]

Author: Guozhi Wei
Date: 2021-04-30T12:10:36-07:00
New Revision: b817ea7b17b8ae68c2bd29834138ed84a8a7005d

URL: https://github.com/llvm/llvm-project/commit/b817ea7b17b8ae68c2bd29834138ed84a8a7005d
DIFF: https://github.com/llvm/llvm-project/commit/b817ea7b17b8ae68c2bd29834138ed84a8a7005d.diff

LOG: [MachineFunction] Make comment for TracksLiveness more clearer

As discussed in https://lists.llvm.org/pipermail/llvm-dev/2021-April/150225.html,
the current comments for TracksLiveness property and isKill flag are confusing.
This patch makes the comments more clearer.

Differential Revision: https://reviews.llvm.org/D101500

Added: 
    

Modified: 
    llvm/include/llvm/CodeGen/MachineFunction.h
    llvm/include/llvm/CodeGen/MachineOperand.h

Removed: 
    


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diff  --git a/llvm/include/llvm/CodeGen/MachineFunction.h b/llvm/include/llvm/CodeGen/MachineFunction.h
index ce78feef2823..e14b337098af 100644
--- a/llvm/include/llvm/CodeGen/MachineFunction.h
+++ b/llvm/include/llvm/CodeGen/MachineFunction.h
@@ -124,11 +124,14 @@ class MachineFunctionProperties {
   // NoPHIs: The machine function does not contain any PHI instruction.
   // TracksLiveness: True when tracking register liveness accurately.
   //  While this property is set, register liveness information in basic block
-  //  live-in lists and machine instruction operands (e.g. kill flags, implicit
-  //  defs) is accurate. This means it can be used to change the code in ways
-  //  that affect the values in registers, for example by the register
-  //  scavenger.
-  //  When this property is clear, liveness is no longer reliable.
+  //  live-in lists and machine instruction operands (e.g. implicit defs) is
+  //  accurate, kill flags are conservatively accurate (kill flag correctly
+  //  indicates the last use of a register, an operand without kill flag may or
+  //  may not be the last use of a register). This means it can be used to
+  //  change the code in ways that affect the values in registers, for example
+  //  by the register scavenger.
+  //  When this property is cleared at a very late time, liveness is no longer
+  //  reliable.
   // NoVRegs: The machine function does not use any virtual registers.
   // Legalized: In GlobalISel: the MachineLegalizer ran and all pre-isel generic
   //  instructions have been legalized; i.e., all instructions are now one of:

diff  --git a/llvm/include/llvm/CodeGen/MachineOperand.h b/llvm/include/llvm/CodeGen/MachineOperand.h
index b12351b8a702..f17904d54cdd 100644
--- a/llvm/include/llvm/CodeGen/MachineOperand.h
+++ b/llvm/include/llvm/CodeGen/MachineOperand.h
@@ -99,8 +99,11 @@ class MachineOperand {
   unsigned IsImp : 1;
 
   /// IsDeadOrKill
-  /// For uses: IsKill - True if this instruction is the last use of the
-  /// register on this path through the function.
+  /// For uses: IsKill - Conservatively indicates the last use of a register
+  /// on this path through the function. A register operand with true value of
+  /// this flag must be the last use of the register, a register operand with
+  /// false value may or may not be the last use of the register. After regalloc
+  /// we can use recomputeLivenessFlags to get precise kill flags.
   /// For defs: IsDead - True if this register is never used by a subsequent
   /// instruction.
   /// This is only valid on register operands.