[SCEV] SCEVExpander::isHighCostExpansionHelper(): begin cost modelling - model cast cost

Summary:
This is not a NFC, although it does not change any of the existing tests.
I'm not really sure if we should have specific tests for the cost modelling itself.

This is the first patch that actually makes `SCEVExpander::isHighCostExpansionHelper()`
account for the cost of the SCEV expression, and consider the budget available,
by modelling cast expressions.

I believe the logic itself is "pretty obviously correct" - from budget,
we need to subtract the cost of the cast expression from inner type `Op->getType()`
to the `S->getType()` type, and recurse into the expression we are casting.

Reviewers: reames, mkazantsev, wmi, sanjoy

Reviewed By: mkazantsev

Subscribers: xbolva00, hiraditya, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73716

llvm/include/llvm/Analysis/ScalarEvolutionExpander.h

@@ -173,8 +173,8 @@ namespace llvm {
       ChainedPhis.clear();
     }
 
-    /// Return true for expressions that may incur non-trivial cost to evaluate
-    /// at runtime.
+    /// Return true for expressions that can't be evaluated at runtime
+    /// within given \b Budget.
     ///
     /// At is an optional parameter which specifies point in code where user is
     /// going to expand this expression. Sometimes this knowledge can lead to a

llvm/lib/Analysis/ScalarEvolutionExpander.cpp

@@ -2138,6 +2138,9 @@ SCEVExpander::getRelatedExistingExpansion(const SCEV *S, const Instruction *At,
 bool SCEVExpander::isHighCostExpansionHelper(
     const SCEV *S, Loop *L, const Instruction *At, int &BudgetRemaining,
     const TargetTransformInfo &TTI, SmallPtrSetImpl<const SCEV *> &Processed) {
+  if (BudgetRemaining < 0)
+    return true; // Already run out of budget, give up.
+
   // Was the cost of expansion of this expression already accounted for?
   if (!Processed.insert(S).second)
     return false; // We have already accounted for this expression.
@@ -2153,17 +2156,26 @@ bool SCEVExpander::isHighCostExpansionHelper(
     return false; // Assume to be zero-cost.
   }
 
-  // Zero/One operand expressions
-  switch (S->getSCEVType()) {
-  case scTruncate:
-    return isHighCostExpansionHelper(cast<SCEVTruncateExpr>(S)->getOperand(), L,
-                                     At, BudgetRemaining, TTI, Processed);
-  case scZeroExtend:
-    return isHighCostExpansionHelper(cast<SCEVZeroExtendExpr>(S)->getOperand(),
-                                     L, At, BudgetRemaining, TTI, Processed);
-  case scSignExtend:
-    return isHighCostExpansionHelper(cast<SCEVSignExtendExpr>(S)->getOperand(),
-                                     L, At, BudgetRemaining, TTI, Processed);
+  if (auto *CastExpr = dyn_cast<SCEVCastExpr>(S)) {
+    unsigned Opcode;
+    switch (S->getSCEVType()) {
+    case scTruncate:
+      Opcode = Instruction::Trunc;
+      break;
+    case scZeroExtend:
+      Opcode = Instruction::ZExt;
+      break;
+    case scSignExtend:
+      Opcode = Instruction::SExt;
+      break;
+    default:
+      llvm_unreachable("There are no other cast types.");
+    }
+    const SCEV *Op = CastExpr->getOperand();
+    BudgetRemaining -=
+        TTI.getOperationCost(Opcode, S->getType(), Op->getType());
+    return isHighCostExpansionHelper(Op, L, At, BudgetRemaining, TTI,
+                                     Processed);
   }