Disable MandatoryCopyPropagation.

Mandatory copy propagation was primarily a stop-gap until lexcial
lifetimes were implemented. It supposedly made variables lifetimes
more consistent between -O and -Onone builds. Now that lexical
lifetimes are enabled, it is no longer needed for that purpose (and
will never satisfactorily meet that goal anyway).

Mandatory copy propagation may be enabled again later as a -Onone "
optimization. But that requires a more careful audit of the effect on
debug information.

For now, it should be disabled.

Author: atrick

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -912,10 +912,7 @@ SILPassPipelinePlan::getOnonePassPipeline(const SILOptions &Options) {
   P.startPipeline("non-Diagnostic Enabling Mandatory Optimizations");
   P.addForEachLoopUnroll();
   P.addMandatoryCombine();
-  if (P.getOptions().CopyPropagation == CopyPropagationOption::On) {
-    // 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

@@ -570,6 +570,8 @@ void CopyPropagation::run() {
   }
 }
 
+// MandatoryCopyPropagation is not currently enabled in the -Onone pipeline
+// because it may negatively affect the debugging experience.
 SILTransform *swift::createMandatoryCopyPropagation() {
   return new CopyPropagation(/*pruneDebug*/ true, /*canonicalizeAll*/ true,
                              /*canonicalizeBorrows*/ false,
@@ -581,4 +583,3 @@ SILTransform *swift::createCopyPropagation() {
                              /*canonicalizeBorrows*/ EnableRewriteBorrows,
                              /*poisonRefs*/ false);
 }
-