[PATCH] D32001: [APInt] Reorder fields to avoid a hole in the middle of the class

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

APInt is currently implemented with an unsigned BitWidth field first and then a uint_64/pointer union. Due to the 64-bit size of the union there is a hole after the bitwidth.

Putting the union first allows the class to be packed. Making it 12 bytes instead of 16 bytes. An APSInt goes from 20 bytes to 16 bytes.

This shows a 4k reduction on the size of the opt binary on my local x86-64 build. So this enables some other improvement to the code as well.


https://reviews.llvm.org/D32001

Files:
  include/llvm/ADT/APInt.h


Index: include/llvm/ADT/APInt.h
===================================================================
--- include/llvm/ADT/APInt.h
+++ include/llvm/ADT/APInt.h
@@ -79,22 +79,22 @@
   };
 
 private:
-  unsigned BitWidth; ///< The number of bits in this APInt.
-
   /// This union is used to store the integer value. When the
   /// integer bit-width <= 64, it uses VAL, otherwise it uses pVal.
   union {
     uint64_t VAL;   ///< Used to store the <= 64 bits integer value.
     uint64_t *pVal; ///< Used to store the >64 bits integer value.
   };
 
+  unsigned BitWidth; ///< The number of bits in this APInt.
+
   friend struct DenseMapAPIntKeyInfo;
 
   /// \brief Fast internal constructor
   ///
   /// This constructor is used only internally for speed of construction of
   /// temporaries. It is unsafe for general use so it is not public.
-  APInt(uint64_t *val, unsigned bits) : BitWidth(bits), pVal(val) {}
+  APInt(uint64_t *val, unsigned bits) : pVal(val), BitWidth(bits) {}
 
   /// \brief Determine if this APInt just has one word to store value.
   ///
@@ -290,7 +290,7 @@
   }
 
   /// \brief Move Constructor.
-  APInt(APInt &&that) : BitWidth(that.BitWidth), VAL(that.VAL) {
+  APInt(APInt &&that) : VAL(that.VAL), BitWidth(that.BitWidth) {
     that.BitWidth = 0;
   }
 
@@ -305,7 +305,7 @@
   ///
   /// This is useful for object deserialization (pair this with the static
   ///  method Read).
-  explicit APInt() : BitWidth(1), VAL(0) {}
+  explicit APInt() : VAL(0), BitWidth(1) {}
 
   /// \brief Returns whether this instance allocated memory.
   bool needsCleanup() const { return !isSingleWord(); }


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D32001.95077.patch
Type: text/x-patch
Size: 1625 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170413/345de858/attachment.bin>