[PATCH] D16843: [Sema] Fix bug in TypeLocBuilder::pushImpl

[Akira Hatanaka via cfe-commits]

ahatanak updated this revision to Diff 48241.
ahatanak added a comment.

Address some of Manman's review comments.

- Add comments that explain what the code is doing (I should add some high-level comments too later).
- Simplify the if-else statement.

I haven't implemented the other changes you've suggested, because I wasn't sure whether imitating the code in "LocalAlignment == 4" would improve readability of the code. For example, I can declare variable "unsigned Padding = NumBytesAtAlign4  %8", but the variable alone doesn't tell you whether there is padding or not (if we haven't inserted any 8-byte elements, we won't have any paddings, but NumBytesAtAlign4 % 8 can still be non-zero).

Maybe I didn't understand what you were suggesting. Could you elaborate a bit more on which part of this code is perhaps hard to understand and how we can improve it?


http://reviews.llvm.org/D16843

Files:
  lib/Sema/TypeLocBuilder.cpp
  test/SemaObjCXX/typeloc-data-alignment.mm

Index: test/SemaObjCXX/typeloc-data-alignment.mm
===================================================================
--- /dev/null
+++ test/SemaObjCXX/typeloc-data-alignment.mm
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+// Make sure this doesn't crash.
+
+ at protocol P
+ at end
+template <class T>
+id<P> foo(T) {
+  int i;
+  foo(i);
+}
Index: lib/Sema/TypeLocBuilder.cpp
===================================================================
--- lib/Sema/TypeLocBuilder.cpp
+++ lib/Sema/TypeLocBuilder.cpp
@@ -115,11 +115,32 @@
       NumBytesAtAlign4 += LocalSize;
     }
   } else if (LocalAlignment == 8) {
-    if (!NumBytesAtAlign8 && NumBytesAtAlign4 % 8 != 0) {
-      // No existing padding and misaligned members; add in 4 bytes padding
-      memmove(&Buffer[Index - 4], &Buffer[Index], NumBytesAtAlign4);
-      Index -= 4;
+    // If the new index is not aligned on an 8-byte boundary, adjust the
+    // current index.
+    bool AdjustIndex = (Index - LocalSize) % 8;
+
+    if (AdjustIndex) {
+      // We know there is a 4-byte padding if an 8-byte aligned element has been
+      // inserted and the total size of the 4-byte aligned elements that have
+      // been inserted since the last 8-byte aligned element was inserted is not
+      // a multiple of 4.
+      bool CurrentlyHasPadding = NumBytesAtAlign8 && NumBytesAtAlign4 % 8;
+
+      if (CurrentlyHasPadding) {
+        // There is a 4-byte padding that starts at Index+NumBytesAtAlign4, so
+        // we can remove it and shift the 4-byte elements backwards to make sure
+        // the new element is 8-byte aligned.
+        memmove(&Buffer[Index + 4], &Buffer[Index], NumBytesAtAlign4);
+        Index += 4;
+      } else {
+        // There is no 4-byte padding we can remove, so we have to insert a
+        // 4-byte padding and shift the 4-byte elements forward to make sure
+        // the new element is 8-byte aligned.
+        memmove(&Buffer[Index - 4], &Buffer[Index], NumBytesAtAlign4);
+        Index -= 4;
+      }
     }
+
     // Forget about any padding.
     NumBytesAtAlign4 = 0;
     NumBytesAtAlign8 += LocalSize;


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