From a0c6ae02a58842e1dcb998e619d0284d62bf8f1d Mon Sep 17 00:00:00 2001
From: Michael Kuperstein <mkuper@google.com>
Date: Mon, 6 Jun 2016 23:38:33 +0000
Subject: [InstCombine] scalarizePHI should not assume the code it sees has
 been CSE'd

scalarizePHI only looked for phis that have exactly two uses - the "latch"
use, and an extract. Unfortunately, we can not assume all equivalent extracts
are CSE'd, since InstCombine itself may create an extract which is a duplicate
of an existing one. This extends it to handle several distinct extracts from
the same index.

This should fix at least some of the  performance regressions from PR27988.

Differential Revision: http://reviews.llvm.org/D20983

llvm-svn: 271961
---
 .../InstCombine/InstCombineVectorOps.cpp           | 38 +++++++++++++++-------
 1 file changed, 26 insertions(+), 12 deletions(-)

(limited to 'llvm/lib/Transforms')

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index d92806f08de..a7613875614 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -62,21 +62,31 @@ static bool cheapToScalarize(Value *V, bool isConstant) {
   return false;
 }
 
-// If we have a PHI node with a vector type that has only 2 uses: feed
+// If we have a PHI node with a vector type that is only used to feed
 // itself and be an operand of extractelement at a constant location,
 // try to replace the PHI of the vector type with a PHI of a scalar type.
 Instruction *InstCombiner::scalarizePHI(ExtractElementInst &EI, PHINode *PN) {
-  // Verify that the PHI node has exactly 2 uses. Otherwise return NULL.
-  if (!PN->hasNUses(2))
-    return nullptr;
+  SmallVector<Instruction *, 2> Extracts;
+  // The users we want the PHI to have are:
+  // 1) The EI ExtractElement (we already know this)
+  // 2) Possibly more ExtractElements with the same index.
+  // 3) Another operand, which will feed back into the PHI.
+  Instruction *PHIUser = nullptr;
+  for (auto U : PN->users()) {
+    if (ExtractElementInst *EU = dyn_cast<ExtractElementInst>(U)) {
+      if (EI.getIndexOperand() == EU->getIndexOperand())
+        Extracts.push_back(EU);
+      else
+        return nullptr;
+    } else if (!PHIUser) {
+      PHIUser = cast<Instruction>(U);
+    } else {
+      return nullptr;
+    }
+  }
 
-  // If so, it's known at this point that one operand is PHI and the other is
-  // an extractelement node. Find the PHI user that is not the extractelement
-  // node.
-  auto iu = PN->user_begin();
-  Instruction *PHIUser = dyn_cast<Instruction>(*iu);
-  if (PHIUser == cast<Instruction>(&EI))
-    PHIUser = cast<Instruction>(*(++iu));
+  if (!PHIUser)
+    return nullptr;
 
   // Verify that this PHI user has one use, which is the PHI itself,
   // and that it is a binary operation which is cheap to scalarize.
@@ -126,7 +136,11 @@ Instruction *InstCombiner::scalarizePHI(ExtractElementInst &EI, PHINode *PN) {
       scalarPHI->addIncoming(newEI, inBB);
     }
   }
-  return replaceInstUsesWith(EI, scalarPHI);
+
+  for (auto E : Extracts)
+    replaceInstUsesWith(*E, scalarPHI);
+
+  return &EI;
 }
 
 Instruction *InstCombiner::visitExtractElementInst(ExtractElementInst &EI) {
-- 
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