[flang][hlfir] disable region simplification in HLFIR pipeline

[jeanPerier]

Summary

Block merging is disabled with FIR: some FIR value should not be promoted
to block arguments, and the region simplification block merging is
promoting all SSA value types to block argument when two blocks are
similar except for the usage these values.

[clementval]

LGTM

flang/include/flang/Tools/CLOptions.inc

void addNestedPassConditionally(
   if (!disabled)
     pm.addNestedPass<OP>(ctor());
 }
 
 } // namespace
 
 namespace fir {
 
-static void defaultFlangInlinerOptPipeline(mlir::OpPassManager &pm) {
+/// Add MLIR Canonicalizer pass with region simplification disabled.
+/// FIR does not support the promotion of some SSA value to block arguments (or
+/// into arith.select operands) that may be done by mlir block merging in the
+/// region simplification (e.g., !fir.shape<> SSA values are not supported as
+/// block arguments).
+/// Aside from the fir.shape issue, moving some abstract SSA value into block
+/// arguments may have a heavy cost since it forces their code generation that
+/// may be expensive (array temporary). The MLIR pass does not take these
+/// extra costs into account when doing block merging.
+static void addCanonicalizerPassWithoutRegionSimplification(
+    mlir::OpPassManager &pm) {
   mlir::GreedyRewriteConfig config;
   config.enableRegionSimplification = false;
   pm.addPass(mlir::createCanonicalizerPass(config));
 }
 
 inline void addCfgConversionPass(mlir::PassManager &pm) {
   addNestedPassConditionally<mlir::func::FuncOp>(
       pm, disableCfgConversion, fir::createFirToCfgPass);
inline void createDefaultFIROptimizerPassPipeline(mlir::PassManager &pm,
   if (stackArrays)
     pm.addPass(fir::createStackArraysPass());
   else
     fir::addMemoryAllocationOpt(pm);
 
   // The default inliner pass adds the canonicalizer pass with the default
   // configuration. Create the inliner pass with tco config.
   llvm::StringMap<mlir::OpPassManager> pipelines;
-  pm.addPass(
-      mlir::createInlinerPass(pipelines, defaultFlangInlinerOptPipeline));
+  pm.addPass(mlir::createInlinerPass(
+      pipelines, addCanonicalizerPassWithoutRegionSimplification));
   pm.addPass(fir::createSimplifyRegionLitePass());
   pm.addPass(mlir::createCSEPass());
 
   // Polymorphic types
   pm.addPass(fir::createPolymorphicOpConversionPass());
 
   // convert control flow to CFG form
   fir::addCfgConversionPass(pm);
   pm.addPass(mlir::createConvertSCFToCFPass());
 
   pm.addPass(mlir::createCanonicalizerPass(config));
   pm.addPass(fir::createSimplifyRegionLitePass());
   pm.addPass(mlir::createCSEPass());
 }
 
 /// Create a pass pipeline for lowering from HLFIR to FIR
 ///
 /// \param pm - MLIR pass manager that will hold the pipeline definition
 /// \param optLevel - optimization level used for creating FIR optimization
 ///   passes pipeline
 inline void createHLFIRToFIRPassPipeline(
     mlir::PassManager &pm, llvm::OptimizationLevel optLevel = defaultOptLevel) {
   if (optLevel.isOptimizingForSpeed())
-    pm.addPass(mlir::createCanonicalizerPass());
+    addCanonicalizerPassWithoutRegionSimplification(pm);
   pm.addPass(hlfir::createLowerHLFIRIntrinsicsPass());
   pm.addPass(hlfir::createBufferizeHLFIRPass());
   pm.addPass(hlfir::createConvertHLFIRtoFIRPass());
 }
 
 #if !defined(FLANG_EXCLUDE_CODEGEN)
 inline void createDebugPasses(
     mlir::PassManager &pm, llvm::codegenoptions::DebugInfoKind debugLevel) {

flang/test/HLFIR/no-block-merging.fir

+// Test that the HLFIR pipeline does not call MLIR canonicalizer with block
+// merging enabled (moving fir.shape to block argument would cause failures
+// when translating the FIR to LLVM).
+// RUN: %flang_fc1 %s -flang-experimental-hlfir -emit-llvm -O2 -o - | FileCheck %s
+
+func.func @no_shape_merge(%cdt: i1, %from: !fir.ref<!fir.array<?xf64>>, %to : !fir.ref<f64>) {
+  %c10 = arith.constant 10 : index
+  %c20 = arith.constant 20 : index
+  %c5 = arith.constant 5 : index
+  %shape1 = fir.shape %c10 : (index) -> !fir.shape<1>
+  %shape2 = fir.shape %c20 : (index) -> !fir.shape<1>
+  cf.cond_br %cdt, ^bb1, ^bb2
+^bb1:  // pred: ^bb0
+  %coor1 = fir.array_coor %from(%shape1) %c5 : (!fir.ref<!fir.array<?xf64>>, !fir.shape<1>, index) -> !fir.ref<f64>
+  %load1 = fir.load %coor1 : !fir.ref<f64>
+  fir.store %load1 to %to : !fir.ref<f64>
+  cf.br ^bb3
+^bb2:  // pred: ^bb0
+  %coor2 = fir.array_coor %from(%shape2) %c5 : (!fir.ref<!fir.array<?xf64>>, !fir.shape<1>, index) -> !fir.ref<f64>
+  %load2 = fir.load %coor2 : !fir.ref<f64>
+  fir.store %load2 to %to : !fir.ref<f64>
+  cf.br ^bb3
+^bb3:  // pred: ^bb1, ^bb2
+  return
+}
+
+// Note: block merging happens in the output below, but after FIR codegen.
+
+// CHECK-LABEL:  define void @no_shape_merge(
+// CHECK:  %[[GEP:.*]] = getelementptr double, ptr %{{.*}}
+// CHECK:  %[[LOAD:.*]] = load double, ptr %[[GEP]]
+// CHECK:  store double %[[LOAD]], ptr %{{.*}}
+// CHECK:  ret void