[flang][NFC] speed-up external name conversion pass (#86814)

The ExternalNameConversion pass can be surprisingly slow on big
programs. On an example with a 50kloc Fortran file with about 10000
calls to external procedures, the pass alone took 25s on my machine.
This patch reduces this to 0.16s.

The root cause is that using `replaceAllSymbolUses` on each modified
FuncOp is very expensive: it is walking all operations and attribute
every time.

An alternative would be to use mlir::SymbolUserMap to avoid walking the
module again and again, but this is still much more expensive than what
is needed because it is essentially caching all symbol uses of the
module, and there is no need to such caching here.

Instead:
- Do a shallow walk of the module (only top level operation) to detect
FuncOp/GlobalOp that needs to be updated. Update them and place the name
remapping in a DenseMap.
- If any remapping were done, do a single deep walk of the module
operation, and update any SymbolRefAttr that matches a name that was
remapped.

1 files changed, 34 insertions, 121 deletions

diff --git a/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp b/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp
index 3a96864..b265c74 100644
--- a/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp
+++ b/flang/lib/Optimizer/Transforms/ExternalNameConversion.cpp
@@ -12,13 +12,9 @@
 #include "flang/Optimizer/Dialect/FIROpsSupport.h"
 #include "flang/Optimizer/Support/InternalNames.h"
 #include "flang/Optimizer/Transforms/Passes.h"
-#include "mlir/Dialect/LLVMIR/LLVMDialect.h"
-#include "mlir/Dialect/OpenACC/OpenACC.h"
-#include "mlir/Dialect/OpenMP/OpenMPDialect.h"
 #include "mlir/IR/Attributes.h"
 #include "mlir/IR/SymbolTable.h"
 #include "mlir/Pass/Pass.h"
-#include "mlir/Transforms/DialectConversion.h"
 
 namespace fir {
 #define GEN_PASS_DEF_EXTERNALNAMECONVERSION
@@ -44,102 +40,8 @@ mangleExternalName(const std::pair<fir::NameUniquer::NameKind,
                                                   appendUnderscore);
 }
 
-//===----------------------------------------------------------------------===//
-// Rewrite patterns
-//===----------------------------------------------------------------------===//
-
 namespace {
 
-struct MangleNameOnFuncOp : public mlir::OpRewritePattern<mlir::func::FuncOp> {
-public:
-  using OpRewritePattern::OpRewritePattern;
-
-  MangleNameOnFuncOp(mlir::MLIRContext *ctx, bool appendUnderscore)
-      : mlir::OpRewritePattern<mlir::func::FuncOp>(ctx),
-        appendUnderscore(appendUnderscore) {}
-
-  mlir::LogicalResult
-  matchAndRewrite(mlir::func::FuncOp op,
-                  mlir::PatternRewriter &rewriter) const override {
-    mlir::LogicalResult ret = success();
-    rewriter.startOpModification(op);
-    llvm::StringRef oldName = op.getSymName();
-    auto result = fir::NameUniquer::deconstruct(oldName);
-    if (fir::NameUniquer::isExternalFacingUniquedName(result)) {
-      auto newSymbol =
-          rewriter.getStringAttr(mangleExternalName(result, appendUnderscore));
-
-      // Try to update all SymbolRef's in the module that match the current op
-      if (mlir::ModuleOp mod = op->getParentOfType<mlir::ModuleOp>())
-        ret = op.replaceAllSymbolUses(newSymbol, mod);
-
-      op.setSymNameAttr(newSymbol);
-      mlir::SymbolTable::setSymbolName(op, newSymbol);
-
-      op->setAttr(fir::getInternalFuncNameAttrName(),
-                  mlir::StringAttr::get(op->getContext(), oldName));
-    }
-    rewriter.finalizeOpModification(op);
-    return ret;
-  }
-
-private:
-  bool appendUnderscore;
-};
-
-struct MangleNameForCommonBlock : public mlir::OpRewritePattern<fir::GlobalOp> {
-public:
-  using OpRewritePattern::OpRewritePattern;
-
-  MangleNameForCommonBlock(mlir::MLIRContext *ctx, bool appendUnderscore)
-      : mlir::OpRewritePattern<fir::GlobalOp>(ctx),
-        appendUnderscore(appendUnderscore) {}
-
-  mlir::LogicalResult
-  matchAndRewrite(fir::GlobalOp op,
-                  mlir::PatternRewriter &rewriter) const override {
-    rewriter.startOpModification(op);
-    auto result = fir::NameUniquer::deconstruct(
-        op.getSymref().getRootReference().getValue());
-    if (fir::NameUniquer::isExternalFacingUniquedName(result)) {
-      auto newName = mangleExternalName(result, appendUnderscore);
-      op.setSymrefAttr(mlir::SymbolRefAttr::get(op.getContext(), newName));
-      SymbolTable::setSymbolName(op, newName);
-    }
-    rewriter.finalizeOpModification(op);
-    return success();
-  }
-
-private:
-  bool appendUnderscore;
-};
-
-struct MangleNameOnAddrOfOp : public mlir::OpRewritePattern<fir::AddrOfOp> {
-public:
-  using OpRewritePattern::OpRewritePattern;
-
-  MangleNameOnAddrOfOp(mlir::MLIRContext *ctx, bool appendUnderscore)
-      : mlir::OpRewritePattern<fir::AddrOfOp>(ctx),
-        appendUnderscore(appendUnderscore) {}
-
-  mlir::LogicalResult
-  matchAndRewrite(fir::AddrOfOp op,
-                  mlir::PatternRewriter &rewriter) const override {
-    auto result = fir::NameUniquer::deconstruct(
-        op.getSymbol().getRootReference().getValue());
-    if (fir::NameUniquer::isExternalFacingUniquedName(result)) {
-      auto newName = SymbolRefAttr::get(
-          op.getContext(), mangleExternalName(result, appendUnderscore));
-      rewriter.replaceOpWithNewOp<fir::AddrOfOp>(op, op.getResTy().getType(),
-                                                 newName);
-    }
-    return success();
-  }
-
-private:
-  bool appendUnderscore;
-};
-
 class ExternalNameConversionPass
     : public fir::impl::ExternalNameConversionBase<ExternalNameConversionPass> {
 public:
@@ -162,31 +64,42 @@ void ExternalNameConversionPass::runOnOperation() {
   auto *context = &getContext();
 
   appendUnderscores = (usePassOpt) ? appendUnderscoreOpt : appendUnderscores;
+  llvm::DenseMap<mlir::StringAttr, mlir::FlatSymbolRefAttr> remappings;
+  // Update names of external Fortran functions and names of Common Block
+  // globals.
+  for (auto &funcOrGlobal : op->getRegion(0).front()) {
+    if (llvm::isa<mlir::func::FuncOp>(funcOrGlobal) ||
+        llvm::isa<fir::GlobalOp>(funcOrGlobal)) {
+      auto symName = funcOrGlobal.getAttrOfType<mlir::StringAttr>(
+          mlir::SymbolTable::getSymbolAttrName());
+      auto deconstructedName = fir::NameUniquer::deconstruct(symName);
+      if (fir::NameUniquer::isExternalFacingUniquedName(deconstructedName)) {
+        auto newName = mangleExternalName(deconstructedName, appendUnderscores);
+        auto newAttr = mlir::StringAttr::get(context, newName);
+        mlir::SymbolTable::setSymbolName(&funcOrGlobal, newAttr);
+        auto newSymRef = mlir::FlatSymbolRefAttr::get(newAttr);
+        remappings.try_emplace(symName, newSymRef);
+        if (llvm::isa<mlir::func::FuncOp>(funcOrGlobal))
+          funcOrGlobal.setAttr(fir::getInternalFuncNameAttrName(), symName);
+      }
+    }
+  }
 
-  mlir::RewritePatternSet patterns(context);
-  patterns.insert<MangleNameOnFuncOp, MangleNameForCommonBlock,
-                  MangleNameOnAddrOfOp>(context, appendUnderscores);
-
-  ConversionTarget target(*context);
-  target.addLegalDialect<fir::FIROpsDialect, LLVM::LLVMDialect,
-                         acc::OpenACCDialect, omp::OpenMPDialect>();
-
-  target.addDynamicallyLegalOp<mlir::func::FuncOp>([](mlir::func::FuncOp op) {
-    return !fir::NameUniquer::needExternalNameMangling(op.getSymName());
-  });
-
-  target.addDynamicallyLegalOp<fir::GlobalOp>([](fir::GlobalOp op) {
-    return !fir::NameUniquer::needExternalNameMangling(
-        op.getSymref().getRootReference().getValue());
-  });
-
-  target.addDynamicallyLegalOp<fir::AddrOfOp>([](fir::AddrOfOp op) {
-    return !fir::NameUniquer::needExternalNameMangling(
-        op.getSymbol().getRootReference().getValue());
+  if (remappings.empty())
+    return;
+
+  // Update all uses of the functions and globals that have been renamed.
+  op.walk([&remappings](mlir::Operation *nestedOp) {
+    llvm::SmallVector<std::pair<mlir::StringAttr, mlir::SymbolRefAttr>> updates;
+    for (const mlir::NamedAttribute &attr : nestedOp->getAttrDictionary())
+      if (auto symRef = llvm::dyn_cast<mlir::SymbolRefAttr>(attr.getValue()))
+        if (auto remap = remappings.find(symRef.getRootReference());
+            remap != remappings.end())
+          updates.emplace_back(std::pair<mlir::StringAttr, mlir::SymbolRefAttr>{
+              attr.getName(), mlir::SymbolRefAttr(remap->second)});
+    for (auto update : updates)
+      nestedOp->setAttr(update.first, update.second);
   });
-
-  if (failed(applyPartialConversion(op, target, std::move(patterns))))
-    signalPassFailure();
 }
 
 std::unique_ptr<mlir::Pass> fir::createExternalNameConversionPass() {