11–20 of 71 posts
My favorite hack back in MFC days was a combo box which I stored the pointer address in the text (to the right after a lot of spaces so was hidden). When a user chooses an item, parse the pointer and de-reference it back to an object.
Very old Macs used this trick to squeeze their ROM routines down a bit, operating with 24 bit addressing and using the top bits for flags and whatnot. Of course they ran into trouble when machines with 16MB of memory started appearing. If you do this you might be making more work for yourself in the future when you buy a new machine with 256EB of main memory.
The amount of data that can be used for the tag is architecture-dependant, and the routine discards any tag bits that don't fit into the tagged pointer without telling the caller.
To me, this seems ridiculous - why not just use a struct with a tag and a pointer, and not run the risk of your tag being destroyed without you knowing because the architecture can't fit that many bits?
But the Go folks are smart, and must be doing this for a reason. Can anyone explain the thinking here?
[0] https://github.com/golang/go/blob/master/src/runtime/tagptr_...
I looked at the Go implementation of this in "tagged pointers" [0] The amount of data that can be used for the tag is architecture-dependant, and the routine discards any tag bits that don't fit into the tagged pointer without telling the caller. To me, this seems ridiculous - why not just use a struct with a tag and a pointer, and not run the risk of your tag being destroyed without you knowing because the architect…
Very old Macs used this trick to squeeze their ROM routines down a bit, operating with 24 bit addressing and using the top bits for flags and whatnot. Of course they ran into trouble when machines with 16MB of memory started appearing. If you do this you might be making more work for yourself in the future when you buy a new machine with 256EB of main memory.
Then it later added explicit automatic masking, which also avoids the problem. As long as your program can make do with smaller amounts of memory, there are no downsides.
Very old Macs used this trick to squeeze their ROM routines down a bit, operating with 24 bit addressing and using the top bits for flags and whatnot. Of course they ran into trouble when machines with 16MB of memory started appearing. If you do this you might be making more work for yourself in the future when you buy a new machine with 256EB of main memory.
I looked at the Go implementation of this in "tagged pointers" [0] The amount of data that can be used for the tag is architecture-dependant, and the routine discards any tag bits that don't fit into the tagged pointer without telling the caller. To me, this seems ridiculous - why not just use a struct with a tag and a pointer, and not run the risk of your tag being destroyed without you knowing because the architect…
I looked at the Go implementation of this in "tagged pointers" [0] The amount of data that can be used for the tag is architecture-dependant, and the routine discards any tag bits that don't fit into the tagged pointer without telling the caller. To me, this seems ridiculous - why not just use a struct with a tag and a pointer, and not run the risk of your tag being destroyed without you knowing because the architect…
My favorite hack back in MFC days was a combo box which I stored the pointer address in the text (to the right after a lot of spaces so was hidden). When a user chooses an item, parse the pointer and de-reference it back to an object.