[Mem2Reg] Don't canonicalize undef.

They aren't the kind of values whose lifetimes are eligible for
canonicalization.

rdar://113121424

Author: nate-chandler

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -2195,6 +2195,8 @@ void MemoryToRegisters::canonicalizeValueLifetimes(
       /*pruneDebug=*/true, /*maximizeLifetime=*/!f.shouldOptimize(), &f,
       accessBlockAnalysis, domInfo, calleeAnalysis, deleter);
   for (auto value : owned) {
+    if (isa<SILUndef>(value))
+      continue;
     auto root = CanonicalizeOSSALifetime::getCanonicalCopiedDef(value);
     if (auto *copy = dyn_cast<CopyValueInst>(root)) {
       if (SILValue borrowDef = CanonicalizeBorrowScope::getCanonicalBorrowedDef(
@@ -2207,6 +2209,8 @@ void MemoryToRegisters::canonicalizeValueLifetimes(
   }
   CanonicalizeBorrowScope borrowCanonicalizer(&f, deleter);
   for (auto value : guaranteed) {
+    if (isa<SILUndef>(value))
+      continue;
     auto borrowee = CanonicalizeBorrowScope::getCanonicalBorrowedDef(value);
     if (!borrowee)
       continue;

test/SILOptimizer/mem2reg_ossa.sil

@@ -730,3 +730,15 @@ exit:
   %retval = tuple ()
   return %retval : $()
 }
+
+// CHECK-LABEL: sil [ossa] @dont_canonicalize_undef : {{.*}} {
+// CHECK:         [[RETVAL:%[^,]+]] = tuple
+// CHECK:         return [[RETVAL]]
+// CHECK-LABEL: } // end sil function 'dont_canonicalize_undef'
+sil [ossa] @dont_canonicalize_undef : $@convention(thin) () -> () {
+  %addr = alloc_stack $()
+  store undef to [trivial] %addr : $*()
+  dealloc_stack %addr : $*()
+  %retval = tuple ()
+  return %retval : $()
+}