[DebugInfo] Add support for constant debug values

Author: Snowy1803

docs/HowToUpdateDebugInfo.md

@@ -218,9 +218,6 @@ value.
 debug_value undef : $Int, let, name "x", expr op_consts:1:op_fragment:#Int._value
 ```
 
-> [!Caution]
-> This currently doesn't work, these variables are dropped by IRGen.
-
 ### Undef variables
 
 If the value of the variable cannot be recovered as the value is entirely

lib/IRGen/IRGenDebugInfo.cpp

@@ -3432,6 +3432,16 @@ void IRGenDebugInfoImpl::emitDbgIntrinsic(
   // /always/ emit an llvm.dbg.value of undef.
   // If we have undef, always emit a llvm.dbg.value in the current position.
   if (isa<llvm::UndefValue>(Storage)) {
+    if (Expr->getNumElements() &&
+        (Expr->getElement(0) == llvm::dwarf::DW_OP_consts
+         || Expr->getElement(0) == llvm::dwarf::DW_OP_constu)) {
+      /// Convert `undef, expr op_consts:N:...` to `N, expr ...`
+      Storage = llvm::ConstantInt::get(
+          llvm::IntegerType::getInt64Ty(Builder.getContext()),
+          Expr->getElement(1));
+      Expr = llvm::DIExpression::get(Builder.getContext(),
+                                     Expr->getElements().drop_front(2));
+    }
     DBuilder.insertDbgValueIntrinsic(Storage, Var, Expr, DL, ParentBlock);
     return;
   }

test/DebugInfo/irgen_undef.sil

@@ -16,16 +16,33 @@ return %0 : $Builtin.Int64
 sil @arithmetic : $@convention(thin) (Builtin.Int64) -> (Builtin.Int64) {
 bb0(%0 : $Builtin.Int64):
 // CHECK: call void @llvm.dbg.value(metadata i64 %0, metadata ![[CURRENT_VAR:[0-9]+]], metadata !DIExpression())
-debug_value %0 : $Builtin.Int64, var, name "current", type $Int, expr op_fragment:#Int._value
+debug_value %0 : $Builtin.Int64, var, name "current", type $Int64, expr op_fragment:#Int64._value
 // FIXME: It should work with the fragment, as it should be noop.
 // CHECK: call void @llvm.dbg.value(metadata i64 %0, metadata ![[PREVIOUS_VAR:[0-9]+]], metadata !DIExpression(DW_OP_consts, 1, DW_OP_minus, DW_OP_stack_value))
-debug_value %0 : $Builtin.Int64, var, name "previous", type $Int, expr op_consts:1:op_minus //:op_fragment:#Int._value
+debug_value %0 : $Builtin.Int64, var, name "previous", type $Int64, expr op_consts:1:op_minus //:op_fragment:#Int64._value
 // CHECK: call void @llvm.dbg.value(metadata i64 %0, metadata ![[NEXT_VAR:[0-9]+]], metadata !DIExpression(DW_OP_constu, 12, DW_OP_plus, DW_OP_stack_value))
-debug_value %0 : $Builtin.Int64, var, name "next", type $Int, expr op_constu:12:op_plus //:op_fragment:#Int._value
+debug_value %0 : $Builtin.Int64, var, name "next", type $Int64, expr op_constu:12:op_plus //:op_fragment:#Int64._value
 return %0 : $Builtin.Int64
 }
 
+// CHECK-LABEL: define {{.*}} @constant
+sil @constant : $@convention(thin) (Builtin.Int64) -> (Builtin.Int64) {
+bb0(%0 : $Builtin.Int64):
+// CHECK: call void @llvm.dbg.value(metadata i64 42, metadata ![[CONST_VAR:[0-9]+]], metadata !DIExpression(DW_OP_stack_value))
+debug_value undef : $Builtin.Int64, var, name "const", expr op_consts:42
+// CHECK: call void @llvm.dbg.value(metadata i64 3645, metadata ![[CONSTINT_VAR:[0-9]+]], metadata !DIExpression(DW_OP_stack_value))
+debug_value undef : $Builtin.Int64, var, name "constint", type $Int64, expr op_constu:3645:op_fragment:#Int64._value
+// CHECK: call void @llvm.dbg.value(metadata i64 6, metadata ![[CONSTUPLE_VAR:[0-9]+]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64))
+debug_value undef : $Builtin.Int64, var, name "constuple", type $(Int64, Int64), expr op_consts:6:op_tuple_fragment:$(Int64, Int64):0:op_fragment:#Int64._value
+
+return %0 : $Builtin.Int64
+}
+
+
 // CHECK: ![[JUST_UNDEF_VAR]] = !DILocalVariable(name: "optimizedout"
 // CHECK: ![[CURRENT_VAR]] = !DILocalVariable(name: "current"
 // CHECK: ![[PREVIOUS_VAR]] = !DILocalVariable(name: "previous"
 // CHECK: ![[NEXT_VAR]] = !DILocalVariable(name: "next"
+// CHECK: ![[CONST_VAR]] = !DILocalVariable(name: "const"
+// CHECK: ![[CONSTINT_VAR]] = !DILocalVariable(name: "constint"
+// CHECK: ![[CONSTUPLE_VAR]] = !DILocalVariable(name: "constuple"