Trivial fix for an LICM assertion in projectLoadValue

This seems to compile correctly in release builds. But it does go
through an llvm_unreachable path, so really isn't safe to leave unfixed.

When the accessPath has an offset, propagate it through the recursive
calls. This may have happened when the offset was moved outside of the
sequence of path indices. The code rebuilds a path from the indices
without adding back the offset.

LICM asserts during projectLoadValue when it needs to rematerialize a
loaded value within the loop using projections and the loop-invariant
address is an index_addr.

Basically:

  %a = index_addr %4 : $*Wrapper, %1 : $Builtin.Word
  store %_ to %a : $*Wrapper
  br loop:

loop:
  %f = struct_element_addr %a
  %v = load %f : $Value
  %s = struct $Wrapper (%v : $Value)
  store %s to %a : $*Wrapper

Where the store inside the loop is deleted. And where the load is
hoisted out of the loop, but now loads $Wrapper instead of $Value.

Fixes rdar://92191909 (LICM assertion: isSubObjectProjection(), MemAccessUtils.h, line 1069)

Author: atrick

lib/SILOptimizer/LoopTransforms/LICM.cpp

@@ -1173,7 +1173,8 @@ static SILValue projectLoadValue(SILValue addr, AccessPath accessPath,
     assert(ProjectionIndex(SEI).Index == elementIdx);
     SILValue val = projectLoadValue(
         SEI->getOperand(),
-        AccessPath(accessPath.getStorage(), pathNode.getParent(), 0),
+        AccessPath(accessPath.getStorage(), pathNode.getParent(),
+                   accessPath.getOffset()),
         rootVal, rootAccessPath, beforeInst);
     SILBuilder B(beforeInst);
     return B.createStructExtract(beforeInst->getLoc(), val, SEI->getField(),
@@ -1183,7 +1184,8 @@ static SILValue projectLoadValue(SILValue addr, AccessPath accessPath,
     assert(ProjectionIndex(TEI).Index == elementIdx);
     SILValue val = projectLoadValue(
         TEI->getOperand(),
-        AccessPath(accessPath.getStorage(), pathNode.getParent(), 0),
+        AccessPath(accessPath.getStorage(), pathNode.getParent(),
+                   accessPath.getOffset()),
         rootVal, rootAccessPath, beforeInst);
     SILBuilder B(beforeInst);
     return B.createTupleExtract(beforeInst->getLoc(), val, TEI->getFieldIndex(),

test/SILOptimizer/licm.sil

@@ -8,6 +8,13 @@ sil_stage canonical
 import Builtin
 import Swift
 
+class Storage {
+  init()
+}
+
+// globalArray
+sil_global @globalArray : $Storage
+
 // CHECK-LABEL: @memset
 
 // CHECK: bb0
@@ -1397,3 +1404,52 @@ bb6:
   return %15 : $()
 }
 
+struct UInt64 {
+  @_hasStorage var _value: Builtin.Int64 { get set }
+  init(_value: Builtin.Int64)
+}
+
+public struct UInt64Wrapper {
+  @_hasStorage public var rawValue: UInt64 { get set }
+  private init(_ rawValue: UInt64)
+  public init()
+}
+
+// rdar://92191909 (LICM assertion: isSubObjectProjection(), MemAccessUtils.h, line 1069)
+//
+// projectLoadValue needs to rematerialize a loaded value within the
+// loop using projections and the loop-invariant address is an
+// index_addr.
+//
+// The store inside the loop is deleted, and the load is hoisted such
+// that it now loads the UInt64Wrapper instead of Builtin.Int64
+sil @testTailProjection : $@convention(thin) () -> () {
+bb0:
+  %0 = integer_literal $Builtin.Int64, 0
+  %1 = integer_literal $Builtin.Word, 1
+  %2 = integer_literal $Builtin.Word, 2
+  %3 = alloc_ref [tail_elems $UInt64Wrapper * %2 : $Builtin.Word] $Storage
+  %4 = ref_tail_addr %3 : $Storage, $UInt64Wrapper
+  %5 = index_addr %4 : $*UInt64Wrapper, %1 : $Builtin.Word
+  %6 = struct $UInt64 (%0 : $Builtin.Int64)
+  %7 = struct $UInt64Wrapper (%6 : $UInt64)
+  store %7 to %5 : $*UInt64Wrapper
+  %9 = load %5 : $*UInt64Wrapper
+  br bb1(%0 : $Builtin.Int64, %9 : $UInt64Wrapper)
+
+bb1(%11 : $Builtin.Int64, %12 : $UInt64Wrapper):
+  cond_br undef, bb3, bb2
+
+bb2:
+  %14 = struct_element_addr %5 : $*UInt64Wrapper, #UInt64Wrapper.rawValue
+  %15 = struct_element_addr %14 : $*UInt64, #UInt64._value
+  %16 = load %15 : $*Builtin.Int64
+  %17 = struct $UInt64 (%16 : $Builtin.Int64)
+  %18 = struct $UInt64Wrapper (%17 : $UInt64)
+  store %18 to %5 : $*UInt64Wrapper
+  br bb1(%16 : $Builtin.Int64, %18 : $UInt64Wrapper)
+
+bb3:
+  %21 = tuple ()
+  return %21 : $()
+}