[flang] Fixed computation of position of function's arg in AddDebugInfo. (#131672)

I am working on `-frepack-array` feature (#127147), which produces
non-trivial manipulations with arguments of `fir.declare`.
In this case, we end up with CFG computation of the `fir.declare`
argument, and AddDebugInfo pass incorrectly mapped two dummy arguments
to the same arg index in the debug attributes.
This patch makes sure that we assign the arg index only if we can prove
that we've traced the block argument to the function's entry block.
I believe this problem is not specific to `-frepack-arrays`, e.g.
it may appear due to MLIR inlining as well.

Author: vzakhari

flang/lib/Optimizer/Transforms/AddDebugInfo.cpp

@@ -206,8 +206,13 @@ void AddDebugInfoPass::handleDeclareOp(fir::cg::XDeclareOp declOp,
   // a dummy_scope operand).
   unsigned argNo = 0;
   if (declOp.getDummyScope()) {
-    if (auto arg = llvm::dyn_cast<mlir::BlockArgument>(declOp.getMemref()))
-      argNo = arg.getArgNumber() + 1;
+    if (auto arg = llvm::dyn_cast<mlir::BlockArgument>(declOp.getMemref())) {
+      // Check if it is the BlockArgument of the function's entry block.
+      if (auto funcLikeOp =
+              declOp->getParentOfType<mlir::FunctionOpInterface>())
+        if (arg.getOwner() == &funcLikeOp.front())
+          argNo = arg.getArgNumber() + 1;
+    }
   }
 
   auto tyAttr = typeGen.convertType(fir::unwrapRefType(declOp.getType()),

flang/test/Transforms/debug-dummy-argument.fir

@@ -0,0 +1,38 @@
+// Test the case when AddDebugInfo pass cannot easily compute
+// position of a dummy argument in the arguments list of the function.
+// RUN: fir-opt --add-debug-info --mlir-print-debuginfo %s | FileCheck %s
+
+// Only enabled on x86_64
+// REQUIRES: x86-registered-target
+
+// CHECK: #[[$ATTR_20:.+]] = #llvm.di_local_variable<scope = #di_subprogram, name = "expected", file = #di_file, line = 3, arg = 1, type = #di_basic_type>
+
+// 'x' is a block argument at the point of fircg.ext_declare,
+// but it is not the function's entry block's argument, so
+// 'arg' cannot be set currently.
+// CHECK: #[[$ATTR_21:.+]] = #llvm.di_local_variable<scope = #di_subprogram, name = "x", file = #di_file, line = 2, type = #di_composite_type>
+
+// Reference Fortran code (compiled with -frepack-arrays):
+// subroutine test(expected, x)
+//   integer :: x(:)
+//   integer :: expected
+// end subroutine test
+
+#loc1 = loc("debug-dummy-argument.f90":1:1)
+#loc4 = loc("debug-dummy-argument.f90":2:14)
+module attributes {dlti.dl_spec = #dlti.dl_spec<i128 = dense<128> : vector<2xi64>, f80 = dense<128> : vector<2xi64>, i1 = dense<8> : vector<2xi64>, !llvm.ptr<271> = dense<32> : vector<4xi64>, !llvm.ptr = dense<64> : vector<4xi64>, i64 = dense<64> : vector<2xi64>, !llvm.ptr<272> = dense<64> : vector<4xi64>, i32 = dense<32> : vector<2xi64>, i16 = dense<16> : vector<2xi64>, !llvm.ptr<270> = dense<32> : vector<4xi64>, i8 = dense<8> : vector<2xi64>, f128 = dense<128> : vector<2xi64>, f16 = dense<16> : vector<2xi64>, f64 = dense<64> : vector<2xi64>, "dlti.stack_alignment" = 128 : i64, "dlti.mangling_mode" = "e", "dlti.endianness" = "little">, fir.defaultkind = "a1c4d8i4l4r4", fir.kindmap = "", fir.target_cpu = "x86-64", llvm.data_layout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128", llvm.ident = "flang", llvm.target_triple = "x86_64-unknown-linux-gnu"} {
+  func.func @test_(%arg0: !fir.ref<i32> {fir.bindc_name = "expected"} loc("debug-dummy-argument.f90":1:1), %arg1: !fir.box<!fir.array<?xi32>> {fir.bindc_name = "x"} loc("debug-dummy-argument.f90":1:1)) attributes {fir.internal_name = "_QPtest"} {
+    %0 = fir.undefined !fir.dscope loc(#loc1)
+    %1 = fircg.ext_declare %arg0 dummy_scope %0 {uniq_name = "_QFtestEexpected"} : (!fir.ref<i32>, !fir.dscope) -> !fir.ref<i32> loc(#loc3)
+    %2 = fir.is_present %arg1 : (!fir.box<!fir.array<?xi32>>) -> i1 loc(#loc4)
+    cf.cond_br %2, ^bb5(%arg1 : !fir.box<!fir.array<?xi32>>), ^bb5(%arg1 : !fir.box<!fir.array<?xi32>>) loc(#loc4)
+  ^bb5(%17: !fir.box<!fir.array<?xi32>> loc("debug-dummy-argument.f90":2:14)):  // 2 preds: ^bb3, ^bb4
+    %18 = fircg.ext_declare %17 dummy_scope %0 {uniq_name = "_QFtestEx"} : (!fir.box<!fir.array<?xi32>>, !fir.dscope) -> !fir.box<!fir.array<?xi32>> loc(#loc4)
+    cf.br ^bb8 loc(#loc1)
+  ^bb8:  // 3 preds: ^bb5
+    return loc(#loc5)
+  } loc(#loc1)
+} loc(#loc)
+#loc = loc("debug-dummy-argument.f90":0:0)
+#loc3 = loc("debug-dummy-argument.f90":3:14)
+#loc5 = loc("debug-dummy-argument.f90":4:1)