CodeGen: ConstStructBuilder must verify packed constraints after padding

This reverts commit r220169 which reverted r220153.  However, it also
contains additional changes:
- We may need to add padding *after* we've packed the struct.  This
  occurs when the aligned next field offset is greater than the new
  field's offset.  When this occurs, we make the struct packed.
  *However*, once packed the next field offset might be less than the
  new feild's offset.  It is in this case that we might further pad the
  struct.
- We would pad structs which were perfectly sized!  This behavior is
  immensely old.  This behavior came from blindly subtracting
  NextFieldOffsetInChars from RecordSize.  This doesn't take into
  account the fact that the struct might have a greater overall
  alignment than the last field.

llvm-svn: 220175

1 files changed, 30 insertions, 14 deletions

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 421e7deccc3..088a5d08b9b 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -104,16 +104,7 @@ AppendBytes(CharUnits FieldOffsetInChars, llvm::Constant *InitCst) {
 
   // Round up the field offset to the alignment of the field type.
   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;
-  }
+      NextFieldOffsetInChars.RoundUpToAlignment(FieldAlignment);
 
   if (AlignedNextFieldOffsetInChars < FieldOffsetInChars) {
     // We need to append padding.
@@ -122,6 +113,24 @@ 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();
+
+    // After we pack the struct, we may need to insert padding.
+    if (NextFieldOffsetInChars < FieldOffsetInChars) {
+      // We need to append padding.
+      AppendPadding(FieldOffsetInChars - NextFieldOffsetInChars);
+
+      assert(NextFieldOffsetInChars == FieldOffsetInChars &&
+             "Did not add enough padding!");
+    }
     AlignedNextFieldOffsetInChars = NextFieldOffsetInChars;
   }
 
@@ -486,10 +495,14 @@ llvm::Constant *ConstStructBuilder::Finalize(QualType Ty) {
     // No tail padding is necessary.
   } else {
     // Append tail padding if necessary.
-    AppendTailPadding(LayoutSizeInChars);
-
     CharUnits LLVMSizeInChars =
-      NextFieldOffsetInChars.RoundUpToAlignment(LLVMStructAlignment);
+        NextFieldOffsetInChars.RoundUpToAlignment(LLVMStructAlignment);
+
+    if (LLVMSizeInChars != LayoutSizeInChars)
+      AppendTailPadding(LayoutSizeInChars);
+
+    LLVMSizeInChars =
+        NextFieldOffsetInChars.RoundUpToAlignment(LLVMStructAlignment);
 
     // Check if we need to convert the struct to a packed struct.
     if (NextFieldOffsetInChars <= LayoutSizeInChars &&
@@ -501,7 +514,10 @@ llvm::Constant *ConstStructBuilder::Finalize(QualType Ty) {
              "Converting to packed did not help!");
     }
 
-    assert(LayoutSizeInChars == NextFieldOffsetInChars &&
+    LLVMSizeInChars =
+        NextFieldOffsetInChars.RoundUpToAlignment(LLVMStructAlignment);
+
+    assert(LayoutSizeInChars == LLVMSizeInChars &&
            "Tail padding mismatch!");
   }