Markus Schoepflin wrote:
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.
aCC supports #pragma unalign [1|2|4|8|16] which is not as convenient, in my opinion, as __unaligned type specifier in cxx compiler. With aCC, you'd have to do something like: #pragma unalign 1 typedef bitfield_t ua_bitfield_t; // bitfield_t with 1 byte alignment ua_bitfield_t& data() const { ... } Thanks, Boris ----- Original Message ----- From: "Markus Schöpflin" <markus.schoepflin@comsoft.de> To: <boost@lists.boost.org> Sent: Wednesday, November 14, 2007 7:46 AM Subject: Re: [boost] [GIL] 64-bit alignment issue
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
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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
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