[llvm] r313988 - [SCEV] Generalize folding of trunc(x)+n*trunc(y) into folding m*trunc(x)+n*trunc(y)

[Daniel Neilson via llvm-commits]

Author: dneilson
Date: Fri Sep 22 08:47:57 2017
New Revision: 313988

URL: http://llvm.org/viewvc/llvm-project?rev=313988&view=rev
Log:
[SCEV] Generalize folding of trunc(x)+n*trunc(y) into folding m*trunc(x)+n*trunc(y)

Summary:
A SCEV such as:
 {%v2,+,((-1 * (trunc i64 (-1 * %v1) to i32)) + (-1 * (trunc i64 %v1 to i32)))}<%loop>

can be folded into, simply, {%v2,+,0}. However, the current code in ::getAddExpr()
will not try to apply the simplification m*trunc(x)+n*trunc(y) -> trunc(trunc(m)*x+trunc(n)*y)
because it only keys off having a non-multiplied trunc as the first term in the simplification.

This patch generalizes this code to try to do a more generic fold of these trunc
expressions.

Reviewers: sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37888

Modified:
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp
    llvm/trunk/unittests/Analysis/ScalarEvolutionTest.cpp

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=313988&r1=313987&r2=313988&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Fri Sep 22 08:47:57 2017
@@ -2338,12 +2338,23 @@ const SCEV *ScalarEvolution::getAddExpr(
 
   // Check for truncates. If all the operands are truncated from the same
   // type, see if factoring out the truncate would permit the result to be
-  // folded. eg., trunc(x) + m*trunc(n) --> trunc(x + trunc(m)*n)
+  // folded. eg., n*trunc(x) + m*trunc(y) --> trunc(trunc(m)*x + trunc(n)*y)
   // if the contents of the resulting outer trunc fold to something simple.
-  for (; Idx < Ops.size() && isa<SCEVTruncateExpr>(Ops[Idx]); ++Idx) {
-    const SCEVTruncateExpr *Trunc = cast<SCEVTruncateExpr>(Ops[Idx]);
-    Type *DstType = Trunc->getType();
-    Type *SrcType = Trunc->getOperand()->getType();
+  auto FindTruncSrcType = [&]() -> Type * {
+    // We're ultimately looking to fold an addrec of truncs and muls of only
+    // constants and truncs, so if we find any other types of SCEV
+    // as operands of the addrec then we bail and return nullptr here.
+    // Otherwise, we return the type of the operand of a trunc that we find.
+    if (auto *T = dyn_cast<SCEVTruncateExpr>(Ops[Idx]))
+      return T->getOperand()->getType();
+    if (const auto *Mul = dyn_cast<SCEVMulExpr>(Ops[Idx])) {
+      const auto *LastOp = Mul->getOperand(Mul->getNumOperands() - 1);
+      if (const auto *T = dyn_cast<SCEVTruncateExpr>(LastOp))
+        return T->getOperand()->getType();
+    }
+    return nullptr;
+  };
+  if (auto *SrcType = FindTruncSrcType()) {
     SmallVector<const SCEV *, 8> LargeOps;
     bool Ok = true;
     // Check all the operands to see if they can be represented in the
@@ -2386,7 +2397,7 @@ const SCEV *ScalarEvolution::getAddExpr(
       const SCEV *Fold = getAddExpr(LargeOps, Flags, Depth + 1);
       // If it folds to something simple, use it. Otherwise, don't.
       if (isa<SCEVConstant>(Fold) || isa<SCEVUnknown>(Fold))
-        return getTruncateExpr(Fold, DstType);
+        return getTruncateExpr(Fold, Ty);
     }
   }
 

Modified: llvm/trunk/unittests/Analysis/ScalarEvolutionTest.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Analysis/ScalarEvolutionTest.cpp?rev=313988&r1=313987&r2=313988&view=diff
==============================================================================
--- llvm/trunk/unittests/Analysis/ScalarEvolutionTest.cpp (original)
+++ llvm/trunk/unittests/Analysis/ScalarEvolutionTest.cpp Fri Sep 22 08:47:57 2017
@@ -1009,5 +1009,37 @@ TEST_F(ScalarEvolutionsTest, SCEVAddRecF
   auto Result = SE.createAddRecFromPHIWithCasts(cast<SCEVUnknown>(Expr));
 }
 
+TEST_F(ScalarEvolutionsTest, SCEVFoldSumOfTruncs) {
+  // Verify that the following SCEV gets folded to a zero:
+  //  (-1 * (trunc i64 (-1 * %0) to i32)) + (-1 * (trunc i64 %0 to i32)
+  Type *ArgTy = Type::getInt64Ty(Context);
+  Type *Int32Ty = Type::getInt32Ty(Context);
+  SmallVector<Type *, 1> Types;
+  Types.push_back(ArgTy);
+  FunctionType *FTy = FunctionType::get(Type::getVoidTy(Context), Types, false);
+  Function *F = cast<Function>(M.getOrInsertFunction("f", FTy));
+  BasicBlock *BB = BasicBlock::Create(Context, "entry", F);
+  ReturnInst::Create(Context, nullptr, BB);
+
+  ScalarEvolution SE = buildSE(*F);
+
+  auto *Arg = &*(F->arg_begin());
+  const auto *ArgSCEV = SE.getSCEV(Arg);
+
+  // Build the SCEV
+  const auto *A0 = SE.getNegativeSCEV(ArgSCEV);
+  const auto *A1 = SE.getTruncateExpr(A0, Int32Ty);
+  const auto *A = SE.getNegativeSCEV(A1);
+
+  const auto *B0 = SE.getTruncateExpr(ArgSCEV, Int32Ty);
+  const auto *B = SE.getNegativeSCEV(B0);
+
+  const auto *Expr = SE.getAddExpr(A, B);
+  dbgs() << "DDN\nExpr: " << *Expr << "\n";
+  // Verify that the SCEV was folded to 0
+  const auto *ZeroConst = SE.getConstant(Int32Ty, 0);
+  EXPECT_EQ(Expr, ZeroConst);
+}
+
 }  // end anonymous namespace
 }  // end namespace llvm