[mlir][Transforms][NFC] Dialect conversion: Eagerly build reverse mapping (#101476)

The "inverse mapping" is an inverse IRMapping that points from replaced
values to their original values. This inverse mapping is needed when
legalizing unresolved materializations, to figure out if a value has any
uses. (It is not sufficient to examine the IR, because some IR changes
have not been materialized yet.)

There was a check in `OperationConverter::finalize` that computed the
inverse mapping only when needed. This check is not needed.
`legalizeUnresolvedMaterializations` always computes the inverse
mapping, so we can just do that in `OperationConverter::finalize` before
calling `legalizeUnresolvedMaterializations`.

Depends on #98805

Author: matthias-springer

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -2352,7 +2352,7 @@ struct OperationConverter {
   LogicalResult legalizeUnresolvedMaterializations(
       ConversionPatternRewriter &rewriter,
       ConversionPatternRewriterImpl &rewriterImpl,
-      std::optional<DenseMap<Value, SmallVector<Value>>> &inverseMapping);
+      DenseMap<Value, SmallVector<Value>> &inverseMapping);
 
   /// Legalize an operation result that was marked as "erased".
   LogicalResult
@@ -2454,10 +2454,12 @@ LogicalResult OperationConverter::convertOperations(ArrayRef<Operation *> ops) {
 
 LogicalResult
 OperationConverter::finalize(ConversionPatternRewriter &rewriter) {
-  std::optional<DenseMap<Value, SmallVector<Value>>> inverseMapping;
   ConversionPatternRewriterImpl &rewriterImpl = rewriter.getImpl();
-  if (failed(legalizeConvertedArgumentTypes(rewriter, rewriterImpl)) ||
-      failed(legalizeUnresolvedMaterializations(rewriter, rewriterImpl,
+  if (failed(legalizeConvertedArgumentTypes(rewriter, rewriterImpl)))
+    return failure();
+  DenseMap<Value, SmallVector<Value>> inverseMapping =
+      rewriterImpl.mapping.getInverse();
+  if (failed(legalizeUnresolvedMaterializations(rewriter, rewriterImpl,
                                                 inverseMapping)))
     return failure();
 
@@ -2483,15 +2485,11 @@ OperationConverter::finalize(ConversionPatternRewriter &rewriter) {
       if (result.getType() == newValue.getType())
         continue;
 
-      // Compute the inverse mapping only if it is really needed.
-      if (!inverseMapping)
-        inverseMapping = rewriterImpl.mapping.getInverse();
-
       // Legalize this result.
       rewriter.setInsertionPoint(op);
       if (failed(legalizeChangedResultType(
               op, result, newValue, opReplacement->getConverter(), rewriter,
-              rewriterImpl, *inverseMapping)))
+              rewriterImpl, inverseMapping)))
         return failure();
     }
   }
@@ -2503,6 +2501,8 @@ LogicalResult OperationConverter::legalizeConvertedArgumentTypes(
     ConversionPatternRewriterImpl &rewriterImpl) {
   // Functor used to check if all users of a value will be dead after
   // conversion.
+  // TODO: This should probably query the inverse mapping, same as in
+  // `legalizeChangedResultType`.
   auto findLiveUser = [&](Value val) {
     auto liveUserIt = llvm::find_if_not(val.getUsers(), [&](Operation *user) {
       return rewriterImpl.isOpIgnored(user);
@@ -2796,20 +2796,18 @@ static LogicalResult legalizeUnresolvedMaterialization(
 LogicalResult OperationConverter::legalizeUnresolvedMaterializations(
     ConversionPatternRewriter &rewriter,
     ConversionPatternRewriterImpl &rewriterImpl,
-    std::optional<DenseMap<Value, SmallVector<Value>>> &inverseMapping) {
-  inverseMapping = rewriterImpl.mapping.getInverse();
-
+    DenseMap<Value, SmallVector<Value>> &inverseMapping) {
   // As an initial step, compute all of the inserted materializations that we
   // expect to persist beyond the conversion process.
   DenseMap<Operation *, UnresolvedMaterializationRewrite *> materializationOps;
   SetVector<UnresolvedMaterializationRewrite *> necessaryMaterializations;
   computeNecessaryMaterializations(materializationOps, rewriter, rewriterImpl,
-                                   *inverseMapping, necessaryMaterializations);
+                                   inverseMapping, necessaryMaterializations);
 
   // Once computed, legalize any necessary materializations.
   for (auto *mat : necessaryMaterializations) {
     if (failed(legalizeUnresolvedMaterialization(
-            *mat, materializationOps, rewriter, rewriterImpl, *inverseMapping)))
+            *mat, materializationOps, rewriter, rewriterImpl, inverseMapping)))
       return failure();
   }
   return success();