[llvm] [PredicateInfo] Clean up DFS sorting (NFC) (PR #144943)

[Nikita Popov via llvm-commits]

================
@@ -116,28 +103,34 @@ struct ValueDFS_Compare {
   bool operator()(const ValueDFS &A, const ValueDFS &B) const {
     if (&A == &B)
       return false;
-    // The only case we can't directly compare them is when they in the same
-    // block, and both have localnum == middle.  In that case, we have to use
-    // comesbefore to see what the real ordering is, because they are in the
-    // same basic block.
+    assert(!A.Def && !B.Def && "Should not have Def during comparison");
 
-    assert((A.DFSIn != B.DFSIn || A.DFSOut == B.DFSOut) &&
+    // Order by block first.
+    if (A.DFSIn != B.DFSIn)
+      return A.DFSIn < B.DFSIn;
+    assert(A.DFSOut == B.DFSOut &&
            "Equal DFS-in numbers imply equal out numbers");
-    bool SameBlock = A.DFSIn == B.DFSIn;
+
+    // Then order by first/middle/last.
+    if (A.LocalNum != B.LocalNum)
+      return A.LocalNum < B.LocalNum;
 
     // We want to put the def that will get used for a given set of phi uses,
     // before those phi uses.
     // So we sort by edge, then by def.
     // Note that only phi nodes uses and defs can come last.
-    if (SameBlock && A.LocalNum == LN_Last && B.LocalNum == LN_Last)
+    if (A.LocalNum == LN_Last)
       return comparePHIRelated(A, B);
 
-    bool isADef = A.Def;
-    bool isBDef = B.Def;
-    if (!SameBlock || A.LocalNum != LN_Middle || B.LocalNum != LN_Middle)
-      return std::tie(A.DFSIn, A.LocalNum, isADef) <
-             std::tie(B.DFSIn, B.LocalNum, isBDef);
-    return localComesBefore(A, B);
+    // Use block-local ordering for instructions in the middle.
+    if (A.LocalNum == LN_Middle)
+      return localComesBefore(A, B);
+
+    // The order of PredicateInfo definitions at the start of the block does not
+    // matter.
+    assert(A.LocalNum == LN_First);
+    assert(A.PInfo && B.PInfo && "Must be predicate info def");
+    return false;
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nikic wrote:

This was not obvious in the old implementation, but we don't define an order for defs at LN_First -- this relies on the fact that we use stable_sort.

https://github.com/llvm/llvm-project/pull/144943