[PATCH] D28930: [NewGVN] Optimize processing as instructions don't get more trivially dead over time

[Davide Italiano via Phabricator via llvm-commits]

davide created this revision.

Don't call `isTriviallyDeadInstructions()` once we discover that an instruction is dead. Instead, set DFS number zero (as suggested by Danny) and forget about it (this also speeds up things as we won't try to reprocess that block).


https://reviews.llvm.org/D28930

Files:
  lib/Transforms/Scalar/NewGVN.cpp


Index: lib/Transforms/Scalar/NewGVN.cpp
===================================================================
--- lib/Transforms/Scalar/NewGVN.cpp
+++ lib/Transforms/Scalar/NewGVN.cpp
@@ -238,7 +238,12 @@
 #endif
 
   // DFS info.
+  // This contains a mapping from Instructions to DFS numbers.
+  // The numbering starts at 1. An instruction with DFS number zero
+  // means that the instruction is dead.
   DenseMap<const Value *, unsigned> InstrDFS;
+
+  // This contains the mapping DFS numbers to instructions.
   SmallVector<Value *, 32> DFSToInstr;
 
   // Deletion info.
@@ -1501,7 +1506,12 @@
 // congruence finding, and updating mappings.
 void NewGVN::valueNumberInstruction(Instruction *I) {
   DEBUG(dbgs() << "Processing instruction " << *I << "\n");
-  if (isInstructionTriviallyDead(I, TLI)) {
+
+  // There's no need to call isInstructionTriviallyDead more than once on
+  // an instruction. Therefore, once we know that an instruction is dead
+  // we change its DFS number so that it doesn't get numbered again.
+  if (InstrDFS[I] != 0 && isInstructionTriviallyDead(I, TLI)) {
+    InstrDFS[I] = 0;
     DEBUG(dbgs() << "Skipping unused instruction\n");
     markInstructionForDeletion(I);
     return;
@@ -1702,8 +1712,14 @@
     // Walk through all the instructions in all the blocks in RPO.
     for (int InstrNum = TouchedInstructions.find_first(); InstrNum != -1;
          InstrNum = TouchedInstructions.find_next(InstrNum)) {
-      assert(InstrNum != 0 && "Bit 0 should never be set, something touched an "
-                              "instruction not in the lookup table");
+
+      // This instruction was found to be dead. We don't bother looking
+      // at it again.
+      if (InstrNum == 0) {
+        TouchedInstructions.reset(InstrNum);
+        continue;
+      }
+
       Value *V = DFSToInstr[InstrNum];
       BasicBlock *CurrBlock = nullptr;
 


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