[DebugInfo] Fix undef debug values being removed

Author: Snowy1803

docs/HowToUpdateDebugInfo.md

@@ -218,7 +218,7 @@ debug_value undef : $Int, let, name "x", expr op_consts:1:op_fragment:#Int._valu
 ```
 
 > [!Caution]
-> This currently doesn't work, such debug values are dropped.
+> This currently doesn't work, an implicit op_deref is added.
 
 ### Undef variables
 
@@ -229,9 +229,6 @@ optimized away, an undef debug value should still be kept:
 debug_value undef : $Int, let, name "x"
 ```
 
-> [!Caution]
-> This currently doesn't work, such debug values are dropped.
-
 Additionally, if a previous `debug_value` exists for the variable, a debug value
 of undef invalidates the previous value, in case the value of the variable isn't
 known anymore:

lib/IRGen/IRGenSIL.cpp

@@ -5598,11 +5598,10 @@ void IRGenSILFunction::visitDebugValueInst(DebugValueInst *i) {
 
   auto VarInfo = i->getVarInfo();
   assert(VarInfo && "debug_value without debug info");
-  if (isa<SILUndef>(SILVal)) {
+  if (isa<SILUndef>(SILVal) && VarInfo->Name == "$error") {
     // We cannot track the location of inlined error arguments because it has no
     // representation in SIL.
-    if (!IsAddrVal &&
-        !i->getDebugScope()->InlinedCallSite && VarInfo->Name == "$error") {
+    if (!IsAddrVal && !i->getDebugScope()->InlinedCallSite) {
       auto funcTy = CurSILFn->getLoweredFunctionType();
       emitErrorResultVar(funcTy, funcTy->getErrorResult(), i);
     }

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -77,8 +77,10 @@ static bool seemsUseful(SILInstruction *I) {
   }
 
   // Is useful if it's associating with a function argument
+  // If undef, it is useful and it doesn't cost anything.
   if (isa<DebugValueInst>(I))
-    return isa<SILFunctionArgument>(I->getOperand(0));
+    return isa<SILFunctionArgument>(I->getOperand(0))
+      || isa<SILUndef>(I->getOperand(0));
 
   return false;
 }

test/DebugInfo/irgen_undef.sil

@@ -0,0 +1,24 @@
+// RUN: %target-swiftc_driver -Xfrontend -disable-debugger-shadow-copies -O -g -emit-ir %s | %FileCheck %s
+sil_stage canonical
+
+import Swift
+import Builtin
+
+// CHECK-LABEL: define {{.*}} @just_undef
+sil @just_undef : $@convention(thin) (Builtin.Int64) -> (Builtin.Int64) {
+bb0(%0 : $Builtin.Int64):
+// CHECK: call void @llvm.dbg.value(metadata i64 undef, metadata ![[JUST_UNDEF_VAR:[0-9]+]], metadata !DIExpression())
+debug_value undef : $Builtin.Int64, var, name "optimizedout"
+return %0 : $Builtin.Int64
+}
+
+// CHECK: ![[JUST_UNDEF_VAR]] = !DILocalVariable(name: "optimizedout"
+
+
+// TO ADD AS TESTS:
+//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 "current", type $Int, expr op_fragment:#Int._value
+//debug_value %0 : $Builtin.Int64, var, name "offseta", type $Int, expr op_consts:1:op_fragment:#Int._value
+//debug_value undef : $Builtin.Int64, var, name "offsetb", type $Int, expr op_consts:1:op_fragment:#Int._value
+//debug_value undef : $Builtin.Int64, var, name "offsetc", expr op_consts:1
+// debug_value %0 : $Builtin.Int64, var, name "offsetd", expr op_consts:1