Add createDebugFragments utility API

Create debug_value fragment for a new partial value based on a
projection of an old debug_value.

Author: atrick

include/swift/SILOptimizer/Utils/DebugOptUtils.h

@@ -19,6 +19,7 @@
 #define SWIFT_SILOPTIMIZER_DEBUGOPTUTILS_H
 
 #include "swift/SIL/DebugUtils.h"
+#include "swift/SIL/Projection.h"
 #include "swift/SIL/SILValue.h"
 #include "swift/SILOptimizer/Utils/InstOptUtils.h"
 
@@ -47,6 +48,13 @@ inline void deleteAllDebugUses(SILInstruction *inst,
 /// new `debug_value` instruction before \p I is deleted.
 void salvageDebugInfo(SILInstruction *I);
 
+/// Create debug_value fragment for a new partial value.
+///
+/// Precondition: \p oldValue is a struct or class aggregate. \p proj projects a
+/// field from the aggregate into \p newValue corresponding to struct_extract.
+void createDebugFragments(SILValue oldValue, Projection proj,
+                          SILValue newValue);
+
 /// Erases the instruction \p I from it's parent block and deletes it, including
 /// all debug instructions which use \p I.
 /// Precondition: The instruction may only have debug instructions as uses.

lib/SILOptimizer/Utils/InstOptUtils.cpp

@@ -1792,6 +1792,7 @@ void swift::endLifetimeAtLeakingBlocks(SILValue value,
       });
 }
 
+// TODO: this currently fails to notify the pass with notifyNewInstruction.
 void swift::salvageDebugInfo(SILInstruction *I) {
   if (!I)
     return;
@@ -1872,6 +1873,38 @@ void swift::salvageDebugInfo(SILInstruction *I) {
   }
 }
 
+// TODO: this currently fails to notify the pass with notifyNewInstruction.
+void swift::createDebugFragments(SILValue oldValue, Projection proj,
+                                 SILValue newValue) {
+  if (proj.getKind() != ProjectionKind::Struct)
+    return;
+
+  for (auto *use : getDebugUses(oldValue)) {
+    auto debugVal = dyn_cast<DebugValueInst>(use->getUser());
+    if (!debugVal)
+      continue;
+
+    // Can't create a fragment of a fragment.
+    auto varInfo = debugVal->getVarInfo();
+    if (!varInfo || varInfo->DIExpr.hasFragment())
+      continue;
+
+    SILType baseType = oldValue->getType();
+
+    // Copy VarInfo and add the corresponding fragment DIExpression.
+    SILDebugVariable newVarInfo = *varInfo;
+    newVarInfo.DIExpr.append(
+        SILDebugInfoExpression::createFragment(proj.getVarDecl(baseType)));
+
+    if (!newVarInfo.Type)
+      newVarInfo.Type = baseType;
+
+    // Create a new debug_value
+    SILBuilder(debugVal, debugVal->getDebugScope())
+        .createDebugValue(debugVal->getLoc(), newValue, newVarInfo);
+  }
+}
+
 IntegerLiteralInst *swift::optimizeBuiltinCanBeObjCClass(BuiltinInst *bi,
                                                          SILBuilder &builder) {
   assert(bi->getBuiltinInfo().ID == BuiltinValueKind::CanBeObjCClass);