Re: [boost] [GIL] 64-bit alignment issue

From: Markus Schöpflin (markus.schoepflin_at_[hidden])
Date: 2007-11-14 07:46:12

Lubomir Bourdev wrote:

[...]

> I believe this is indeed the problem. GIL needs to read/write a type
> bigger than a byte that starts at a char* address, so it is possibly not
> aligned.
> 
> One approach is to special-case platforms which cannot dereference at a
> non-byte boundary (I couldn't find a config flag that detects this,
> anyone?) and provide a special solution for such platforms. The solution
> I imagine will involve static recursion that reads/writes each byte and
> shifts it in place.

[...]

For Tru64/CXX there is an easy workaround available, please refer to the 
attached patch for this. Of course this isn't a general solution, although 
I would think that acc supports something similar, which then could be 
wrapped into a macro hiding the platform dependency.

But you should be aware that unaligned accesses usually hurt performance a 
lot, even on platforms where the hardware is able to deal with it.

HTH,
Markus

Index: channel.hpp
===================================================================
--- channel.hpp	(revision 41078)
+++ channel.hpp	(working copy)
@@ -292,8 +292,8 @@
     data_ptr_t operator &() const {return _data_ptr;}
 protected:
     static const integer_t max_val    = (1<<NumBits) - 1;
-    const bitfield_t& const_data() const { return *static_cast<const bitfield_t*>(_data_ptr); }
-          bitfield_t& data()       const { return *static_cast<      bitfield_t*>(_data_ptr); }
+    __unaligned const bitfield_t& const_data() const { return *static_cast<const bitfield_t*>(_data_ptr); }
+    __unaligned       bitfield_t& data()       const { return *static_cast<      bitfield_t*>(_data_ptr); }
 private:
     void set(integer_t value) const {     // can this be done faster??
         const integer_t num_values = max_val+1;
@@ -383,7 +383,7 @@
     integer_t get()                               const { return integer_t((this->const_data()&channel_mask) >> FirstBit); }
     void set_unsafe(integer_t value)              const { this->data() = (this->const_data() & ~channel_mask) | (value<<FirstBit); }
 private:
-    void set_from_reference(const BitField& other_bits) const { this->data() = (this->const_data() & ~channel_mask) | (other_bits & channel_mask); }
+    void set_from_reference(__unaligned const BitField& other_bits) const { this->data() = (this->const_data() & ~channel_mask) | (other_bits & channel_mask); }
 };
 
 } }  // namespace boost::gil