[flang] do not rely on existing fir.convert in TargetRewrite (#157413)

TargetRewrite is doing a shallow rewrite of function signatures. It is
only rewriting function definitions (FuncOp), calls (CallOp) and
AddressOfOp. It is not trying to visit each operations that may have an
operand with a function type.
It therefore needs function signature casts around the operations it is
rewriting.

Currently, these casts were not inserted after AddressOfOp rewrites
because lowering tends to always insert function cast after generating
AddressOfOp to the void type so the pass relied on implicitly updating
this cast operand type to get the required cast. This is brittle because
there is no guarantee such convert must be here and canonicalization and
passes may remove them.

Insert a cast after on the result of rewritten operations. If it is
redundant, it will be canonicalized away later.

Author: jeanPerier

flang/lib/Optimizer/CodeGen/TargetRewrite.cpp

@@ -1336,7 +1336,15 @@ class TargetRewrite : public fir::impl::TargetRewritePassBase<TargetRewrite> {
 private:
   // Replace `op` and remove it.
   void replaceOp(mlir::Operation *op, mlir::ValueRange newValues) {
-    op->replaceAllUsesWith(newValues);
+    llvm::SmallVector<mlir::Value> casts;
+    for (auto [oldValue, newValue] : llvm::zip(op->getResults(), newValues)) {
+      if (oldValue.getType() == newValue.getType())
+        casts.push_back(newValue);
+      else
+        casts.push_back(fir::ConvertOp::create(*rewriter, op->getLoc(),
+                                               oldValue.getType(), newValue));
+    }
+    op->replaceAllUsesWith(casts);
     op->dropAllReferences();
     op->erase();
   }

flang/test/Fir/struct-return-x86-64.fir

@@ -17,6 +17,10 @@ module attributes {fir.defaultkind = "a1c4d8i4l4r4", fir.kindmap = "", llvm.data
     %1 = fir.convert %0 : (() -> !fits_in_reg) -> (() -> ())
     return %1 : () -> ()
   }
+  func.func @test_addr_of_inreg_2() -> (() -> !fits_in_reg) {
+    %0 = fir.address_of(@test_inreg) : () -> !fits_in_reg
+    return %0 : () -> !fits_in_reg
+  }
   func.func @test_dispatch_inreg(%arg0: !fir.ref<!fits_in_reg>, %arg1: !fir.class<!fir.type<somet>>) {
     %0 = fir.dispatch "bar"(%arg1 : !fir.class<!fir.type<somet>>) (%arg1 : !fir.class<!fir.type<somet>>) -> !fits_in_reg {pass_arg_pos = 0 : i32}
     fir.store %0 to %arg0 : !fir.ref<!fits_in_reg>
@@ -62,8 +66,15 @@ module attributes {fir.defaultkind = "a1c4d8i4l4r4", fir.kindmap = "", llvm.data
 
 // CHECK-LABEL:   func.func @test_addr_of_inreg() -> (() -> ()) {
 // CHECK:           %[[VAL_0:.*]] = fir.address_of(@test_inreg) : () -> tuple<i64, f32>
-// CHECK:           %[[VAL_1:.*]] = fir.convert %[[VAL_0]] : (() -> tuple<i64, f32>) -> (() -> ())
-// CHECK:           return %[[VAL_1]] : () -> ()
+// CHECK:           %[[VAL_1:.*]] = fir.convert %[[VAL_0]] : (() -> tuple<i64, f32>) -> (() -> !fir.type<t1{i:f32,j:i32,k:f32}>)
+// CHECK:           %[[VAL_2:.*]] = fir.convert %[[VAL_1]] : (() -> !fir.type<t1{i:f32,j:i32,k:f32}>) -> (() -> ())
+// CHECK:           return %[[VAL_2]] : () -> ()
+// CHECK:         }
+
+// CHECK-LABEL:   func.func @test_addr_of_inreg_2() -> (() -> !fir.type<t1{i:f32,j:i32,k:f32}>) {
+// CHECK:           %[[VAL_0:.*]] = fir.address_of(@test_inreg) : () -> tuple<i64, f32>
+// CHECK:           %[[VAL_1:.*]] = fir.convert %[[VAL_0]] : (() -> tuple<i64, f32>) -> (() -> !fir.type<t1{i:f32,j:i32,k:f32}>)
+// CHECK:           return %[[VAL_1]] : () -> !fir.type<t1{i:f32,j:i32,k:f32}>
 // CHECK:         }
 
 // CHECK-LABEL:   func.func @test_dispatch_inreg(
@@ -95,8 +106,9 @@ module attributes {fir.defaultkind = "a1c4d8i4l4r4", fir.kindmap = "", llvm.data
 
 // CHECK-LABEL:   func.func @test_addr_of_sret() -> (() -> ()) {
 // CHECK:           %[[VAL_0:.*]] = fir.address_of(@test_sret) : (!fir.ref<!fir.type<t2{i:!fir.array<5xf32>}>>) -> ()
-// CHECK:           %[[VAL_1:.*]] = fir.convert %[[VAL_0]] : ((!fir.ref<!fir.type<t2{i:!fir.array<5xf32>}>>) -> ()) -> (() -> ())
-// CHECK:           return %[[VAL_1]] : () -> ()
+// CHECK:           %[[VAL_1:.*]] = fir.convert %[[VAL_0]] : ((!fir.ref<!fir.type<t2{i:!fir.array<5xf32>}>>) -> ()) -> (() -> !fir.type<t2{i:!fir.array<5xf32>}>)
+// CHECK:           %[[VAL_2:.*]] = fir.convert %[[VAL_1]] : (() -> !fir.type<t2{i:!fir.array<5xf32>}>) -> (() -> ())
+// CHECK:           return %[[VAL_2]] : () -> ()
 // CHECK:         }
 
 // CHECK-LABEL:   func.func @test_dispatch_sret(