[llvm] 3800b15 - [SimplifyCFG] Share code to compute switch density between ShouldBuildLookupTable() and ReduceSwitchRange()

[Hans Wennborg via llvm-commits]

Author: Hans Wennborg
Date: 2022-06-10T15:29:36+02:00
New Revision: 3800b157d77b80fba204afff8cc8d545198899d5

URL: https://github.com/llvm/llvm-project/commit/3800b157d77b80fba204afff8cc8d545198899d5
DIFF: https://github.com/llvm/llvm-project/commit/3800b157d77b80fba204afff8cc8d545198899d5.diff

LOG: [SimplifyCFG] Share code to compute switch density between ShouldBuildLookupTable() and ReduceSwitchRange()

They're computing the same thing. No functionality change.

Differential revision: https://reviews.llvm.org/D127482

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index c0f3707b7874..067d432cd19b 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6107,6 +6107,27 @@ static bool isTypeLegalForLookupTable(Type *Ty, const TargetTransformInfo &TTI,
          DL.fitsInLegalInteger(IT->getBitWidth());
 }
 
+static bool isSwitchDense(uint64_t NumCases, uint64_t CaseRange) {
+  // 40% is the default density for building a jump table in optsize/minsize
+  // mode. See also TargetLoweringBase::isSuitableForJumpTable(), which this
+  // function was based on.
+  const uint64_t MinDensity = 40;
+
+  if (CaseRange >= UINT64_MAX / 100)
+    return false; // Avoid multiplication overflows below.
+
+  return NumCases * 100 >= CaseRange * MinDensity;
+}
+
+static bool isSwitchDense(ArrayRef<int64_t> Values) {
+  uint64_t Diff = (uint64_t)Values.back() - (uint64_t)Values.front();
+  uint64_t Range = Diff + 1;
+  if (Range < Diff)
+    return false; // Overflow.
+
+  return isSwitchDense(Values.size(), Range);
+}
+
 /// Determine whether a lookup table should be built for this switch, based on
 /// the number of cases, size of the table, and the types of the results.
 // TODO: We could support larger than legal types by limiting based on the
@@ -6116,8 +6137,8 @@ static bool
 ShouldBuildLookupTable(SwitchInst *SI, uint64_t TableSize,
                        const TargetTransformInfo &TTI, const DataLayout &DL,
                        const SmallDenseMap<PHINode *, Type *> &ResultTypes) {
-  if (SI->getNumCases() > TableSize || TableSize >= UINT64_MAX / 10)
-    return false; // TableSize overflowed, or mul below might overflow.
+  if (SI->getNumCases() > TableSize)
+    return false; // TableSize overflowed.
 
   bool AllTablesFitInRegister = true;
   bool HasIllegalType = false;
@@ -6147,10 +6168,7 @@ ShouldBuildLookupTable(SwitchInst *SI, uint64_t TableSize,
   if (HasIllegalType)
     return false;
 
-  // The table density should be at least 40%. This is the same criterion as for
-  // jump tables, see SelectionDAGBuilder::handleJTSwitchCase.
-  // FIXME: Find the best cut-off.
-  return SI->getNumCases() * 10 >= TableSize * 4;
+  return isSwitchDense(SI->getNumCases(), TableSize);
 }
 
 /// Try to reuse the switch table index compare. Following pattern:
@@ -6486,17 +6504,6 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
   return true;
 }
 
-static bool isSwitchDense(ArrayRef<int64_t> Values) {
-  // See also SelectionDAGBuilder::isDense(), which this function was based on.
-  uint64_t Diff = (uint64_t)Values.back() - (uint64_t)Values.front();
-  uint64_t Range = Diff + 1;
-  uint64_t NumCases = Values.size();
-  // 40% is the default density for building a jump table in optsize/minsize mode.
-  uint64_t MinDensity = 40;
-
-  return NumCases * 100 >= Range * MinDensity;
-}
-
 /// Try to transform a switch that has "holes" in it to a contiguous sequence
 /// of cases.
 ///