Add frontend flags for developers to easily control copy propagation:

-enable-copy-propagation: enables whatever form of copy propagation
 the current pipeline runs (mandatory-copy-propagation at -Onone,
 regular copy-propation at -O).

-disable-copy-propagation: similarly disables any form of copy
 propagation in the current pipelien.

include/swift/AST/SILOptions.h

@@ -44,6 +44,16 @@ public:
   /// Remove all runtime assertions during optimizations.
   bool RemoveRuntimeAsserts = false;
 
+  /// Force-run SIL copy propagation to shorten object lifetime in whatever
+  /// optimization pipeline is currently used.
+  /// When this is 'false' the pipeline has default behavior.
+  bool EnableCopyPropagation = false;
+
+  /// Disable SIL copy propagation to preserve object lifetime in whatever
+  /// optimization pipeline is currently used.
+  /// When this is 'false' the pipeline has default behavior.
+  bool DisableCopyPropagation = false;
+
   /// Controls whether the SIL ARC optimizations are run.
   bool EnableARCOptimizations = true;
 

include/swift/Option/FrontendOptions.td

@@ -188,6 +188,11 @@ def batch_scan_input_file
 def import_prescan : Flag<["-"], "import-prescan">,
    HelpText<"When performing a dependency scan, only dentify all imports of the main Swift module sources">;
 
+def enable_copy_propagation : Flag<["-"], "enable-copy-propagation">,
+  HelpText<"Run SIL copy propagation to shorten object lifetime.">;
+def disable_copy_propagation : Flag<["-"], "disable-copy-propagation">,
+  HelpText<"Don't run SIL copy propagation to preserve object lifetime.">;
+
 } // end let Flags = [FrontendOption, NoDriverOption]
 
 def debug_crash_Group : OptionGroup<"<automatic crashing options>">;

lib/Frontend/CompilerInvocation.cpp

@@ -1185,6 +1185,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   // -Ounchecked might also set removal of runtime asserts (cond_fail).
   Opts.RemoveRuntimeAsserts |= Args.hasArg(OPT_RemoveRuntimeAsserts);
 
+  Opts.EnableCopyPropagation |= Args.hasArg(OPT_enable_copy_propagation);
+  Opts.DisableCopyPropagation |= Args.hasArg(OPT_disable_copy_propagation);
   Opts.EnableARCOptimizations &= !Args.hasArg(OPT_disable_arc_opts);
   Opts.EnableOSSAModules |= Args.hasArg(OPT_enable_ossa_modules);
   Opts.EnableOSSAOptimizations &= !Args.hasArg(OPT_disable_ossa_opts);

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -350,7 +350,9 @@ void addFunctionPasses(SILPassPipelinePlan &P,
   if (P.getOptions().EnableOSSAModules) {
     // We earlier eliminated ownership if we are not compiling the stdlib. Now
     // handle the stdlib functions, re-simplifying, eliminating ARC as we do.
-    P.addCopyPropagation();
+    if (!P.getOptions().DisableCopyPropagation) {
+      P.addCopyPropagation();
+    }
     P.addSemanticARCOpts();
   }
 
@@ -372,7 +374,9 @@ void addFunctionPasses(SILPassPipelinePlan &P,
 
   // Clean up Semantic ARC before we perform additional post-inliner opts.
   if (P.getOptions().EnableOSSAModules) {
-    P.addCopyPropagation();
+    if (!P.getOptions().DisableCopyPropagation) {
+      P.addCopyPropagation();
+    }
     P.addSemanticARCOpts();
   }
 
@@ -437,7 +441,9 @@ void addFunctionPasses(SILPassPipelinePlan &P,
 
   // Run a final round of ARC opts when ownership is enabled.
   if (P.getOptions().EnableOSSAModules) {
-    P.addCopyPropagation();
+    if (!P.getOptions().DisableCopyPropagation) {
+      P.addCopyPropagation();
+    }
     P.addSemanticARCOpts();
   }
 }
@@ -471,7 +477,9 @@ static void addPerfEarlyModulePassPipeline(SILPassPipelinePlan &P) {
   // Cleanup after SILGen: remove trivial copies to temporaries.
   P.addTempRValueOpt();
   // Cleanup after SILGen: remove unneeded borrows/copies.
-  P.addCopyPropagation();
+  if (!P.getOptions().DisableCopyPropagation) {
+    P.addCopyPropagation();
+  }
   P.addSemanticARCOpts();
 
   // Devirtualizes differentiability witnesses into functions that reference them.
@@ -785,7 +793,9 @@ SILPassPipelinePlan::getPerformancePassPipeline(const SILOptions &Options) {
   // Run one last copy propagation/semantic arc opts run before serialization/us
   // lowering ownership.
   if (P.getOptions().EnableOSSAModules) {
-    P.addCopyPropagation();
+    if (!P.getOptions().DisableCopyPropagation) {
+      P.addCopyPropagation();
+    }
     P.addSemanticARCOpts();
   }
 
@@ -835,8 +845,10 @@ SILPassPipelinePlan::getOnonePassPipeline(const SILOptions &Options) {
   P.startPipeline("non-Diagnostic Enabling Mandatory Optimizations");
   P.addForEachLoopUnroll();
   P.addMandatoryCombine();
-  // MandatoryCopyPropagation should only be run at -Onone, not -O.
+  if (P.getOptions().EnableCopyPropagation) {
-  P.addMandatoryCopyPropagation();
+    // MandatoryCopyPropagation should only be run at -Onone, not -O.
+    P.addMandatoryCopyPropagation();
+  }
   // TODO: MandatoryARCOpts should be subsumed by CopyPropagation. There should
   // be no need to run another analysis of copies at -Onone.
   P.addMandatoryARCOpts();

lib/SILOptimizer/Transforms/CopyPropagation.cpp

@@ -136,11 +136,9 @@ void CopyPropagation::run() {
 }
 
 SILTransform *swift::createMandatoryCopyPropagation() {
-  return new CopyPropagation(/*pruneDebug*/ true, /*unownedRemnant*/ true,
+  return new CopyPropagation(/*pruneDebug*/ true, /*canonicalizeAll*/ true);
-                             /*canonicalizeAll*/ true);
 }
 
 SILTransform *swift::createCopyPropagation() {
-  return new CopyPropagation(/*pruneDebug*/ true, /*unownedRemnant*/ false,
+  return new CopyPropagation(/*pruneDebug*/ true, /*canonicalizeAll*/ false);
-                             /*canonicalizeAll*/ false);
 }