3 ms·

I wish for this in x86_64, I know the difference but we need to move on from 32 bit.

by ungamed 10y ago

gens 10y ago
While amd64 is better in that it gives more registers for compilers to use, we rarely need to "move on". In fact many programs will work better (more efficient, faster) with 32bit pointers (as many programs would with 16bit integers instead of 32bit ones, where possible ofc). Realistically nobody codes for performance anymore so x86 or amd64 is not much difference (for math stuff, sse(1-4)/avx(1/2) don't care about amd64 vs x86). Anyway, learning amd64 when knowing x86 is easy as they are mostly the same.
- ant6n 10y ago
  I've been wondering whether it would be possible to write programs that have most of their memory in 32 bits, so most pointers would be 32 bits. Also, such program would have int as a 32 bit value unless specifically declared to be larger - we could write programs that use less memory, but still use more registers, and use 64bit pointers and values as necessary
  - mox1 10y ago
    I guess this is theoretically possible (...maybe..), but I would definitely require cooperation from the underlying operating system and some of the hardware. Additionally, processors have "modes". You tell the processor to go into 64-bit or 32-bit mode. I don't think quickly switching back and forth between those in real-time is a very good idea.
    - ant6n 10y ago
      64-bit mode is kind of a strict superset of 32-bit mode. So the program can run in 64bit mode. But for example, if a program stores a pointer in memory, it's stored as a 32bit number. Once it's loaded, it gets 0-extended to a 64bit integer. The main thing is having a differen ABI (within the program), and using the linker/memmapping to ensure that the stack, code, and a heap is in the lower 4G. Another heap can be in the 64bit space, using 64bit pointers. edit: one could even use trickery related to the alignment of pointers (i.e. 32bit alignment) to shift values on load or access, using the fact that the lower bits of the address. This could allow 36bit addresses that cover 16GB of memory.
  - messe 10y ago
    Look up the x32 ABI for Linux. It's exactly that.
  - psi-squared 10y ago
    One of the less-well-known features of Linux is that you can do this! Theres's a thing called the "x32 ABI" (use the option -mx32 with gcc or clang; you'll need all your libraries compiled with it too) where: * As far as the processor itself is concerned, the code runs in 64-bit mode, so you get the extra (and wider) registers from that. * But pointers are still 32 bits, so you get the memory savings of 32-bit mode. In principle, as long as you're using <4GiB of memory, it should be at least as fast as the best of 32-bit or 64-bit mode for any particular program. But I haven't heard of it being used much. https://en.wikipedia.org/wiki/X32_ABI https://en.wikipedia.org/wiki/X32_ABI
    - tomcam 10y ago
      Like GP, I thought I had cleverly thought of this idea up myself. So glad that smarter people than I have did this up right!
      - ant6n 10y ago
        The only thing missing, I guess, is 64bit 'himem' pointers.
    - ant6n 10y ago
      does something like this exist for aarch64?
  - GFK_of_xmaspast 10y ago
    So basically going back to segmented memory with near (32bit) and far (64bit) pointers?
- mtgx 10y ago
  Well, for one the ASLR security feature is much weaker on 32-bit platforms.
tedunangst 10y ago
Sometimes the instruction ends in a q for 64 bits.
- mox1 10y ago
  It is a lot more than that. Calling conventions, register names and usage, etc. etc. I have a "cheatsheet" sitting on my desk that I wrote up last time I need to do some x86-64 reversing. Do no sit down with a x86-64 binary and use this manual as a guide, you will not get very far.
  - trishume 10y ago
    Would you be willing to post this cheat sheet, or link to it if you got it from the internet?
- pcwalton 10y ago
  The extra registers affect the way programs are written. You get a more RISC-like feel: this shows up most notably in the calling conventions, which pass all arguments in registers most of the time. You see more RISC-like three register arithmetic because the compiler has more opportunities to use LEA. Etc.
  - astocko 10y ago
    Piggy backing on this comment. Agner Fog has some nice tables at http://www.agner.org/optimize/calling_conventions.pdf http://www.agner.org/optimize/calling_conventions.pdf describing the register usage and calling coventions. Chapter 6 & 7

I wish for this in x86_64, I know the difference but we need to move on from 32 bit.

by ungamed 10y ago

I wish for this in x86_64, I know the difference but we need to move on from 32 bit.

gens 10y ago

While amd64 is better in that it gives more registers for compilers to use, we rarely need to "move on". In fact many programs will work better (more efficient, faster) with 32bit pointers (as many programs would with 16bit integers instead of 32bit ones, where possible ofc). Realistically nobody codes for performance anymore so x86 or amd64 is not much difference (for math stuff, sse(1-4)/avx(1/2) don't care about amd64 vs x86). Anyway, learning amd64 when knowing x86 is easy as they are mostly the same.

ant6n 10y ago
I've been wondering whether it would be possible to write programs that have most of their memory in 32 bits, so most pointers would be 32 bits. Also, such program would have int as a 32 bit value unless specifically declared to be larger - we could write programs that use less memory, but still use more registers, and use 64bit pointers and values as necessary
- mox1 10y ago
  I guess this is theoretically possible (...maybe..), but I would definitely require cooperation from the underlying operating system and some of the hardware. Additionally, processors have "modes". You tell the processor to go into 64-bit or 32-bit mode. I don't think quickly switching back and forth between those in real-time is a very good idea.
  - ant6n 10y ago
    64-bit mode is kind of a strict superset of 32-bit mode. So the program can run in 64bit mode. But for example, if a program stores a pointer in memory, it's stored as a 32bit number. Once it's loaded, it gets 0-extended to a 64bit integer. The main thing is having a differen ABI (within the program), and using the linker/memmapping to ensure that the stack, code, and a heap is in the lower 4G. Another heap can be in the 64bit space, using 64bit pointers. edit: one could even use trickery related to the alignment of pointers (i.e. 32bit alignment) to shift values on load or access, using the fact that the lower bits of the address. This could allow 36bit addresses that cover 16GB of memory.
- messe 10y ago
  Look up the x32 ABI for Linux. It's exactly that.
- psi-squared 10y ago
  One of the less-well-known features of Linux is that you can do this! Theres's a thing called the "x32 ABI" (use the option -mx32 with gcc or clang; you'll need all your libraries compiled with it too) where: * As far as the processor itself is concerned, the code runs in 64-bit mode, so you get the extra (and wider) registers from that. * But pointers are still 32 bits, so you get the memory savings of 32-bit mode. In principle, as long as you're using <4GiB of memory, it should be at least as fast as the best of 32-bit or 64-bit mode for any particular program. But I haven't heard of it being used much. https://en.wikipedia.org/wiki/X32_ABI https://en.wikipedia.org/wiki/X32_ABI
  - tomcam 10y ago
    Like GP, I thought I had cleverly thought of this idea up myself. So glad that smarter people than I have did this up right!
    - ant6n 10y ago
      The only thing missing, I guess, is 64bit 'himem' pointers.
  - ant6n 10y ago
    does something like this exist for aarch64?
- GFK_of_xmaspast 10y ago
  So basically going back to segmented memory with near (32bit) and far (64bit) pointers?
mtgx 10y ago
Well, for one the ASLR security feature is much weaker on 32-bit platforms.

tedunangst 10y ago

Sometimes the instruction ends in a q for 64 bits.

mox1 10y ago
It is a lot more than that. Calling conventions, register names and usage, etc. etc. I have a "cheatsheet" sitting on my desk that I wrote up last time I need to do some x86-64 reversing. Do no sit down with a x86-64 binary and use this manual as a guide, you will not get very far.
- trishume 10y ago
  Would you be willing to post this cheat sheet, or link to it if you got it from the internet?
pcwalton 10y ago
The extra registers affect the way programs are written. You get a more RISC-like feel: this shows up most notably in the calling conventions, which pass all arguments in registers most of the time. You see more RISC-like three register arithmetic because the compiler has more opportunities to use LEA. Etc.
- astocko 10y ago
  Piggy backing on this comment. Agner Fog has some nice tables at http://www.agner.org/optimize/calling_conventions.pdf http://www.agner.org/optimize/calling_conventions.pdf describing the register usage and calling coventions. Chapter 6 & 7