[PATCH] D99147: Never sinking if none operand is used in or after loop exit block

[zhengping.hu via Phabricator via llvm-commits]

xjtuhu created this revision.
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IndVarSimplify::sinkUnusedInvariants would sink a instruction from a loop's preheader block to exit block to reduce register pressure. But the optimization is not conservative, it could increase register pressure conversely if none operand is used in or after loop exit block, because sinking could extend operands' liveness range hence increase register pressure.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D99147

Files:
  llvm/lib/Transforms/Scalar/IndVarSimplify.cpp


Index: llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
+++ llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
@@ -1265,6 +1265,32 @@
     if (UsedInLoop)
       continue;
 
+    // If none operand is used in or after the loop exit block, sinking the
+    // instruction from pre-header to exit block will extend operands'
+    // live range, hence could increase register pressure. The instruction
+    // should not be sinked for conservativeness.
+    bool OperandsUsedAfterLoop = false;
+    for (Value *Op : I->operands()) {
+      if (isa<Constant>(Op)) {
+        continue;
+      }
+      for (Use &U : Op->uses()) {
+        Instruction *User = cast<Instruction>(U.getUser());
+        BasicBlock *UseBB = User->getParent();
+        if (PHINode *P = dyn_cast<PHINode>(User)) {
+          unsigned i =
+            PHINode::getIncomingValueNumForOperand(U.getOperandNo());
+          UseBB = P->getIncomingBlock(i);
+        }
+        if (DT->dominates(ExitBlock, UseBB)) {
+          OperandsUsedAfterLoop = true;
+          break;
+        }
+      }
+    }
+    if (!OperandsUsedAfterLoop)
+      continue;
+
     // Otherwise, sink it to the exit block.
     Instruction *ToMove = &*I;
     bool Done = false;


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