CodeGen: ConstStructBuilder must verify packed constraints after padding

Before, ConstStructBuilder::AppendBytes would check packed constraints
prior to padding being added before the field's offset.  However, adding
this padding might force our struct to be packed.  Because we wouldn't
check *after* adding padding, ConstStructBuilder would be in an
inconsistent state leading to a crash.

This fixes PR21300.

llvm-svn: 220153

1 files changed, 10 insertions, 9 deletions

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 421e7deccc3..dd87dd4657e 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -106,15 +106,6 @@ AppendBytes(CharUnits FieldOffsetInChars, llvm::Constant *InitCst) {
   CharUnits AlignedNextFieldOffsetInChars =
     NextFieldOffsetInChars.RoundUpToAlignment(FieldAlignment);
 
-  if (AlignedNextFieldOffsetInChars > FieldOffsetInChars) {
-    assert(!Packed && "Alignment is wrong even with a packed struct!");
-
-    // Convert the struct to a packed struct.
-    ConvertStructToPacked();
-
-    AlignedNextFieldOffsetInChars = NextFieldOffsetInChars;
-  }
-
   if (AlignedNextFieldOffsetInChars < FieldOffsetInChars) {
     // We need to append padding.
     AppendPadding(FieldOffsetInChars - NextFieldOffsetInChars);
@@ -122,6 +113,16 @@ AppendBytes(CharUnits FieldOffsetInChars, llvm::Constant *InitCst) {
     assert(NextFieldOffsetInChars == FieldOffsetInChars &&
            "Did not add enough padding!");
 
+    AlignedNextFieldOffsetInChars =
+      NextFieldOffsetInChars.RoundUpToAlignment(FieldAlignment);
+  }
+
+  if (AlignedNextFieldOffsetInChars > FieldOffsetInChars) {
+    assert(!Packed && "Alignment is wrong even with a packed struct!");
+
+    // Convert the struct to a packed struct.
+    ConvertStructToPacked();
+
     AlignedNextFieldOffsetInChars = NextFieldOffsetInChars;
   }