From 0f64d71d9984a987e428a86a9895c89840ed0df3 Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Wed, 16 Sep 2009 20:25:11 +0000
Subject: Add a new pass for doing late hoisting of floating-point and vector
 constants out of loops. These aren't covered by the regular LICM pass,
 because in LLVM IR constants don't require separate instructions. They're not
 always covered by the MachineLICM pass either, because it doesn't know how to
 unfold folded constant-pool loads. This is somewhat experimental at this
 point, and off by default.

llvm-svn: 82076
---
 llvm/lib/Transforms/Scalar/CodeGenLICM.cpp | 117 +++++++++++++++++++++++++++++
 1 file changed, 117 insertions(+)
 create mode 100644 llvm/lib/Transforms/Scalar/CodeGenLICM.cpp

(limited to 'llvm/lib/Transforms')

diff --git a/llvm/lib/Transforms/Scalar/CodeGenLICM.cpp b/llvm/lib/Transforms/Scalar/CodeGenLICM.cpp
new file mode 100644
index 00000000000..ef9f81e266d
--- /dev/null
+++ b/llvm/lib/Transforms/Scalar/CodeGenLICM.cpp
@@ -0,0 +1,117 @@
+//===- CodeGenLICM.cpp - LICM a function for code generation --------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This function performs late LICM, hoisting constants out of loops that
+// are not valid immediates. It should not be followed by instcombine,
+// because instcombine would quickly stuff the constants back into the loop.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "codegen-licm"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Instructions.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/IVUsers.h"
+#include "llvm/ADT/DenseMap.h"
+using namespace llvm;
+
+namespace {
+  class CodeGenLICM : public LoopPass {
+    virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    explicit CodeGenLICM() : LoopPass(&ID) {}
+  };
+}
+
+char CodeGenLICM::ID = 0;
+static RegisterPass<CodeGenLICM> X("codegen-licm",
+                                   "hoist constants out of loops");
+
+Pass *llvm::createCodeGenLICMPass() {
+  return new CodeGenLICM();
+}
+
+bool CodeGenLICM::runOnLoop(Loop *L, LPPassManager &) {
+  bool Changed = false;
+
+  // Only visit outermost loops.
+  if (L->getParentLoop()) return Changed;
+
+  Instruction *PreheaderTerm = L->getLoopPreheader()->getTerminator();
+  DenseMap<Constant *, BitCastInst *> HoistedConstants;
+
+  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
+       I != E; ++I) {
+    BasicBlock *BB = *I;
+    for (BasicBlock::iterator BBI = BB->begin(), BBE = BB->end();
+         BBI != BBE; ++BBI) {
+      Instruction *I = BBI;
+      // Don't bother hoisting constants out of loop-header phi nodes.
+      if (BB == L->getHeader() && isa<PHINode>(I))
+        continue;
+      // TODO: For now, skip all intrinsic instructions, because some of them
+      // can require their operands to be constants, and we don't want to
+      // break that.
+      if (isa<IntrinsicInst>(I))
+        continue;
+      // LLVM represents fneg as -0.0-x; don't hoist the -0.0 out.
+      if (BinaryOperator::isFNeg(I) ||
+          BinaryOperator::isNeg(I) ||
+          BinaryOperator::isNot(I))
+        continue;
+      for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
+        // Don't hoist out switch case constants.
+        if (isa<SwitchInst>(I) && i == 1)
+          break;
+        // Don't hoist out shuffle masks.
+        if (isa<ShuffleVectorInst>(I) && i == 2)
+          break;
+        Value *Op = I->getOperand(i);
+        Constant *C = dyn_cast<Constant>(Op);
+        if (!C) continue;
+        // TODO: Ask the target which constants are legal. This would allow
+        // us to add support for hoisting ConstantInts and GlobalValues too.
+        if (isa<ConstantFP>(C) ||
+            isa<ConstantVector>(C) ||
+            isa<ConstantAggregateZero>(C)) {
+          BitCastInst *&BC = HoistedConstants[C];
+          if (!BC)
+            BC = new BitCastInst(C, C->getType(), "hoist", PreheaderTerm);
+          I->setOperand(i, BC);
+          Changed = true;
+        }
+      }
+    }
+  }
+
+  return Changed;
+}
+
+void CodeGenLICM::getAnalysisUsage(AnalysisUsage &AU) const {
+  // This pass preserves just about everything. List some popular things here.
+  AU.setPreservesCFG();
+  AU.addPreservedID(LoopSimplifyID);
+  AU.addPreserved<LoopInfo>();
+  AU.addPreserved<AliasAnalysis>();
+  AU.addPreserved<DominanceFrontier>();
+  AU.addPreserved<DominatorTree>();
+  AU.addPreserved<ScalarEvolution>();
+  AU.addPreserved<IVUsers>();
+
+  // Hoisting requires a loop preheader.
+  AU.addRequiredID(LoopSimplifyID);
+}
-- 
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