<p dir="ltr">I'm curious why we need to support forward declared types. It seems really brittle. Have you looked into this much? If not, I can.</p>
<div class="gmail_quote">On Dec 22, 2014 7:56 PM, "Alexey Samsonov" <<a href="mailto:vonosmas@gmail.com">vonosmas@gmail.com</a>> wrote:<br type="attribution"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi chandlerc, rsmith,<br>
<br>
Use large alignment when calculating empty/tombstone keys<br>
for T*. We can't use alignof(T), as T may be forward-declared at<br>
this point, and PointerLikeTypeTraits<T*>::NumLowBitsAvailable may<br>
be too small.<br>
<br>
<a href="http://reviews.llvm.org/D6768" target="_blank">http://reviews.llvm.org/D6768</a><br>
<br>
Files:<br>
  include/llvm/ADT/DenseMapInfo.h<br>
<br>
Index: include/llvm/ADT/DenseMapInfo.h<br>
===================================================================<br>
--- include/llvm/ADT/DenseMapInfo.h<br>
+++ include/llvm/ADT/DenseMapInfo.h<br>
@@ -30,14 +30,21 @@<br>
 // Provide DenseMapInfo for all pointers.<br>
 template<typename T><br>
 struct DenseMapInfo<T*> {<br>
+  // Objects of types T may require large alignment, which we might not know (if<br>
+  // T is forward-declared). Generate empty/tombstone keys to account for that.<br>
+  enum { MaxAlignmentLog = 5 };<br>
   static inline T* getEmptyKey() {<br>
     uintptr_t Val = static_cast<uintptr_t>(-1);<br>
-    Val <<= PointerLikeTypeTraits<T*>::NumLowBitsAvailable;<br>
+    Val <<= PointerLikeTypeTraits<T *>::NumLowBitsAvailable < MaxAlignmentLog<br>
+                ? MaxAlignmentLog<br>
+                : PointerLikeTypeTraits<T *>::NumLowBitsAvailable;<br>
     return reinterpret_cast<T*>(Val);<br>
   }<br>
   static inline T* getTombstoneKey() {<br>
     uintptr_t Val = static_cast<uintptr_t>(-2);<br>
-    Val <<= PointerLikeTypeTraits<T*>::NumLowBitsAvailable;<br>
+    Val <<= PointerLikeTypeTraits<T *>::NumLowBitsAvailable < MaxAlignmentLog<br>
+                ? MaxAlignmentLog<br>
+                : PointerLikeTypeTraits<T *>::NumLowBitsAvailable;<br>
     return reinterpret_cast<T*>(Val);<br>
   }<br>
   static unsigned getHashValue(const T *PtrVal) {<br>
<br>
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<br></blockquote></div>