[PATCH] D133386: [nfc] Refactor SlotIndex::getInstrDistance to better reflect actual functionality

[Aiden Grossman via Phabricator via llvm-commits]

aidengrossman updated this revision to Diff 458349.
aidengrossman added a comment.

Updated comment above getApproxInstrDistance to better encapsulate some nuance
in the spacing between SlotIndices.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D133386/new/

https://reviews.llvm.org/D133386

Files:
  llvm/include/llvm/CodeGen/SlotIndexes.h
  llvm/lib/CodeGen/RegAllocGreedy.cpp


Index: llvm/lib/CodeGen/RegAllocGreedy.cpp
===================================================================
--- llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -327,12 +327,12 @@
       // are singly defined, this produces optimal coloring in the absence of
       // global interference and other constraints.
       if (!ReverseLocalAssignment)
-        Prio = LI.beginIndex().getInstrDistance(Indexes->getLastIndex());
+        Prio = LI.beginIndex().getApproxInstrDistance(Indexes->getLastIndex());
       else {
         // Allocating bottom up may allow many short LRGs to be assigned first
         // to one of the cheap registers. This could be much faster for very
         // large blocks on targets with many physical registers.
-        Prio = Indexes->getZeroIndex().getInstrDistance(LI.endIndex());
+        Prio = Indexes->getZeroIndex().getApproxInstrDistance(LI.endIndex());
       }
     } else {
       // Allocate global and split ranges in long->short order. Long ranges that
Index: llvm/include/llvm/CodeGen/SlotIndexes.h
===================================================================
--- llvm/include/llvm/CodeGen/SlotIndexes.h
+++ llvm/include/llvm/CodeGen/SlotIndexes.h
@@ -215,8 +215,13 @@
     }
 
     /// Return the scaled distance from this index to the given one, where all
-    /// slots on the same instruction have zero distance.
-    int getInstrDistance(SlotIndex other) const {
+    /// slots on the same instruction have zero distance, assuming that the slot
+    /// indices are packed as densely as possible. This assumption is only true
+    /// for calculating the distance between two normally spaced instructions in
+    /// practice if (InstrDist / Slot_Count) - 1 instructions are inserted in
+    /// between them. Otherwise, this function will return a value greater than
+    /// the true instruction distance.
+    int getApproxInstrDistance(SlotIndex other) const {
       return (other.listEntry()->getIndex() - listEntry()->getIndex())
         / Slot_Count;
     }


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