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

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.

Differential Revision: https://reviews.llvm.org/D147130

flang/include/flang/Tools/CLOptions.inc

@@ -99,7 +99,17 @@ void addNestedPassConditionally(
 
 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));
@@ -200,8 +210,8 @@ inline void createDefaultFIROptimizerPassPipeline(mlir::PassManager &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());
 
@@ -225,7 +235,7 @@ inline void createDefaultFIROptimizerPassPipeline(mlir::PassManager &pm,
 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());

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

@@ -0,0 +1,33 @@
+// 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