[MLIR][LLVM] Infer export location scope from location, if possible (#70465)

This commit changes the debug location exporter to try to infer the
locations scope from the MLIR location, if possible. This is necessary
when the function containing the operation does not have a DISubprogram
attached to it.

We observed a roundtrip crash with a case where the the subprogram was
missing on a function, but a debug intrinsic referenced a subprogram
non-the-less. This lead to a successful import, but the export silently
dropped the location, which results in invalid IR.

Author: Dinistro

mlir/lib/Target/LLVMIR/DebugTranslation.cpp

@@ -286,7 +286,7 @@ llvm::DILocation *DebugTranslation::translateLoc(Location loc,
                                                  llvm::DILocalScope *scope,
                                                  llvm::DILocation *inlinedAt) {
   // LLVM doesn't have a representation for unknown.
-  if (!scope || isa<UnknownLoc>(loc))
+  if (isa<UnknownLoc>(loc))
     return nullptr;
 
   // Check for a cached instance.
@@ -301,17 +301,12 @@ llvm::DILocation *DebugTranslation::translateLoc(Location loc,
     llvmLoc = translateLoc(callLoc.getCallee(), scope, callerLoc);
 
   } else if (auto fileLoc = dyn_cast<FileLineColLoc>(loc)) {
-    llvm::DILocalScope *locationScope = scope;
-    // Only construct a new DIFile when no local scope is present. This
-    // prioritizes existing DI information when it's present.
-    if (!locationScope) {
-      auto *file = translateFile(fileLoc.getFilename());
-      locationScope = llvm::DILexicalBlockFile::get(llvmCtx, scope, file,
-                                                    /*Discriminator=*/0);
-    }
-    llvmLoc = llvm::DILocation::get(llvmCtx, fileLoc.getLine(),
-                                    fileLoc.getColumn(), locationScope,
-                                    const_cast<llvm::DILocation *>(inlinedAt));
+    // A scope of a DILocation cannot be null.
+    if (!scope)
+      return nullptr;
+    llvmLoc =
+        llvm::DILocation::get(llvmCtx, fileLoc.getLine(), fileLoc.getColumn(),
+                              scope, const_cast<llvm::DILocation *>(inlinedAt));
 
   } else if (auto fusedLoc = dyn_cast<FusedLoc>(loc)) {
     ArrayRef<Location> locations = fusedLoc.getLocations();

mlir/test/Target/LLVMIR/llvmir-debug.mlir

@@ -205,3 +205,30 @@ llvm.func @func_with_inlined_dbg_value(%arg0: i32) -> (i32) {
 // CHECK-DAG: ![[VAR_LOC0]] = !DILocalVariable(name: "a", scope: ![[OUTER_FUNC]], file: ![[FILE]]
 // CHECK-DAG: ![[VAR_LOC1]] = !DILocalVariable(name: "b", scope: ![[LEXICAL_BLOCK_FILE]], file: ![[FILE]]
 // CHECK-DAG  ![[LABEL]] = !DILabel(scope: ![[LEXICAL_BLOCK_FILE]], name: "label", file: ![[FILE]], line: 42)
+
+// -----
+
+#di_basic_type = #llvm.di_basic_type<tag = DW_TAG_base_type, name = "int", sizeInBits = 32, encoding = DW_ATE_signed>
+#di_file = #llvm.di_file<"foo.mlir" in "/test/">
+#di_compile_unit = #llvm.di_compile_unit<
+  sourceLanguage = DW_LANG_C, file = #di_file, producer = "MLIR",
+  isOptimized = true, emissionKind = Full
+>
+#di_subprogram = #llvm.di_subprogram<
+  compileUnit = #di_compile_unit, scope = #di_file, name = "func",
+  file = #di_file, subprogramFlags = Definition>
+#di_local_variable = #llvm.di_local_variable<scope = #di_subprogram, name = "a", file = #di_file, type = #di_basic_type>
+
+#loc = loc("foo.mlir":0:0)
+
+// CHECK-LABEL: define void @func_without_subprogram(
+// CHECK-SAME: i32 %[[ARG:.*]])
+llvm.func @func_without_subprogram(%0 : i32) {
+  // CHECK: call void @llvm.dbg.value(metadata i32 %[[ARG]], metadata ![[VAR_LOC:[0-9]+]], metadata !DIExpression()), !dbg ![[DBG_LOC0:.*]]
+  llvm.intr.dbg.value #di_local_variable = %0 : i32 loc(fused<#di_subprogram>[#loc])
+  llvm.return
+}
+
+// CHECK: ![[FILE:.*]] = !DIFile(filename: "foo.mlir", directory: "/test/")
+// CHECK-DAG: ![[FUNC:.*]] = distinct !DISubprogram(name: "func", scope: ![[FILE]]
+// CHECK-DAG: ![[VAR_LOC]] = !DILocalVariable(name: "a", scope: ![[FUNC]], file: ![[FILE]]