Add a variation on CopyPropagation to handle debug_value correctly.

MandatoryCopyPropagation must be a separate pass in order to preserve
all debug_value instructions. CopyPropagation cannot preserve
debug_value because, as a rule, debug information cannot affect -O
behavior.

Author: atrick

include/swift/SILOptimizer/PassManager/Passes.def

@@ -123,7 +123,9 @@ PASS(ConstantEvaluableSubsetChecker, "test-constant-evaluable-subset",
 PASS(CopyForwarding, "copy-forwarding",
      "Copy Forwarding to Remove Redundant Copies")
 PASS(CopyPropagation, "copy-propagation",
-     "Copy propagation to Remove Redundant SSA Copies")
+     "Copy propagation to Remove Redundant SSA Copies, pruning debug info")
+PASS(MandatoryCopyPropagation, "mandatory-copy-propagation",
+     "Copy propagation to Remove Redundant SSA Copies, preserving debug info")
 PASS(COWOpts, "cow-opts",
      "Optimize COW operations")
 PASS(Differentiation, "differentiation",

lib/SILOptimizer/Transforms/CopyPropagation.cpp

@@ -38,6 +38,12 @@ using namespace swift;
 
 namespace {
 class CopyPropagation : public SILFunctionTransform {
+  /// True if debug_value instructions should be pruned.
+  bool pruneDebug;
+
+public:
+  CopyPropagation(bool pruneDebug): pruneDebug(pruneDebug) {}
+
   /// The entry point to this function transformation.
   void run() override;
 };
@@ -64,7 +70,7 @@ void CopyPropagation::run() {
     }
   }
   // Perform copy propgation for each copied value.
-  CanonicalizeOSSALifetime canonicalizer(/*pruneDebug*/ true);
+  CanonicalizeOSSALifetime canonicalizer(pruneDebug);
   for (auto &def : copiedDefs) {
     canonicalizer.canonicalizeValueLifetime(def);
     if (SILValue outerCopy = canonicalizer.createdOuterCopy()) {
@@ -81,4 +87,10 @@ void CopyPropagation::run() {
   }
 }
 
-SILTransform *swift::createCopyPropagation() { return new CopyPropagation(); }
+SILTransform *swift::createCopyPropagation() {
+  return new CopyPropagation(/*pruneDebug*/ true);
+}
+
+SILTransform *swift::createMandatoryCopyPropagation() {
+  return new CopyPropagation(/*pruneDebug*/ false);
+}