[sil-combine] When folding (unchecked_trivial_bit_cast (unchecked_ref_cast)), make sure to move the unchecked_trivial_bit_cast to before the unchecked_ref_cast.

The reason that this needs to be done is that if the unchecked_ref_cast is an
owned value, it will end the lifetime of its operand meaning that we can't just
use it for a later unchecked_trivial_bit_cast.

Author: gottesmm

lib/SILOptimizer/SILCombiner/SILCombinerCastVisitors.cpp

@@ -747,28 +747,36 @@ SILCombiner::visitRawPointerToRefInst(RawPointerToRefInst *rawToRef) {
   return nullptr;
 }
 
-SILInstruction *
-SILCombiner::
-visitUncheckedTrivialBitCastInst(UncheckedTrivialBitCastInst *UTBCI) {
+SILInstruction *SILCombiner::visitUncheckedTrivialBitCastInst(
+    UncheckedTrivialBitCastInst *utbci) {
   // (unchecked_trivial_bit_cast Y->Z
   //                                 (unchecked_trivial_bit_cast X->Y x))
   //   ->
   // (unchecked_trivial_bit_cast X->Z x)
-  SILValue Op = UTBCI->getOperand();
-  if (auto *OtherUTBCI = dyn_cast<UncheckedTrivialBitCastInst>(Op)) {
-    return Builder.createUncheckedTrivialBitCast(UTBCI->getLoc(),
-                                                 OtherUTBCI->getOperand(),
-                                                 UTBCI->getType());
+  SILValue operand = utbci->getOperand();
+  if (auto *otherUTBCI = dyn_cast<UncheckedTrivialBitCastInst>(operand)) {
+    return Builder.createUncheckedTrivialBitCast(
+        utbci->getLoc(), otherUTBCI->getOperand(), utbci->getType());
   }
 
-  // (unchecked_trivial_bit_cast Y->Z
-  //                                 (unchecked_ref_cast X->Y x))
+  // %y = unchecked_ref_cast %x X->Y
+  // ...
+  // %z = unchecked_trivial_bit_cast %y Y->Z
+  //
   //   ->
-  // (unchecked_trivial_bit_cast X->Z x)
-  if (auto *URBCI = dyn_cast<UncheckedRefCastInst>(Op)) {
-    return Builder.createUncheckedTrivialBitCast(UTBCI->getLoc(),
-                                                 URBCI->getOperand(),
-                                                 UTBCI->getType());
+  //
+  // %z = unchecked_trivial_bit_cast %x X->Z
+  // %y = unchecked_ref_cast %x X->Y
+  // ...
+  if (auto *urbci = dyn_cast<UncheckedRefCastInst>(operand)) {
+    // We just move the unchecked_trivial_bit_cast to before the
+    // unchecked_ref_cast and then make its operand the unchecked_ref_cast
+    // operand. Then we return the cast so we reprocess given that we changed
+    // its operands.
+    utbci->moveBefore(urbci);
+    utbci->setDebugLocation(urbci->getDebugLocation());
+    utbci->setOperand(urbci->getOperand());
+    return utbci;
   }
 
   return nullptr;

test/SILOptimizer/sil_combine_ossa.sil

@@ -4769,3 +4769,34 @@ bb2(%6c : @guaranteed $__ContiguousArrayStorageBase):
   %14 = struct $MyInt (%13 : $Builtin.Int32)
   return %14 : $MyInt
 }
+
+// Make sure that we hoist the unchecked_trivial_bit_cast up the def-use chain
+// to before copy1 and make its operand copy1.
+//
+// CHECK-LABEL: sil [ossa] @unchecked_trivial_bit_cast_hoist_up_def_use_chain : $@convention(thin) (@guaranteed AnyObject) -> UInt {
+// CHECK: bb0([[ARG:%.*]] :
+// CHECK:   [[ARG_COPY:%.*]] = copy_value [[ARG]]
+// CHECK:   [[RESULT:%.*]] = unchecked_trivial_bit_cast [[ARG_COPY]]
+// CHECK:   destroy_value [[ARG_COPY]]
+// CHECK:   cond_br undef, [[BB_UNREACHABLE:bb[0-9]*]], [[BB_RESULT:bb[0-9]*]]
+//
+// CHECK: [[BB_UNREACHABLE]]:
+// CHECK-NEXT:   unreachable
+//
+// CHECK: [[BB_RESULT]]:
+// CHECK-NEXT: return [[RESULT]]
+// CHECK: } // end sil function 'unchecked_trivial_bit_cast_hoist_up_def_use_chain'
+sil [ossa] @unchecked_trivial_bit_cast_hoist_up_def_use_chain : $@convention(thin) (@guaranteed AnyObject) -> UInt {
+bb0(%0 : @guaranteed $AnyObject):
+  %copy1 = copy_value %0 : $AnyObject
+  %cast = unchecked_ref_cast %copy1 : $AnyObject to $Builtin.NativeObject
+  cond_br undef, bb3, bb4 
+
+bb3:
+  unreachable
+
+bb4:
+  %2 = unchecked_trivial_bit_cast %cast : $Builtin.NativeObject to $UInt
+  destroy_value %cast : $Builtin.NativeObject
+  return %2 : $UInt
+}