4 ms·

Sure, but back then the standard was not relevant. You had your x86 computer, your compiler (MSVC, DJGPP, CodeWarrior, Borland...) and maybe a "how to write C/C

Sure, but back then the standard was not relevant. You had your x86 computer, your compiler (MSVC, DJGPP, CodeWarrior, Borland...) and maybe a "how to write C/C++" book. You did have weird things like segmented addressing, but for most purposes, pointers were just memory addresses, and you could use casts to reinterpret data as you wished. In fact, using bit-cast-magic and doing pointer arithmetics were the preferred way of doing things. It's strange that we put so much effort into learning all this arcane knowledge and now we're supposed to unlearn it...

wruza 8y ago
I can’t tell for sure, since I was too young back then and didn’t write something complex enough, but I suspect that compilers were not evil genies back then. It was probably later gcc who first used UB as an excuse to evil optimizations. I also don’t understand a reason behind dragging all that legacy into current standards. Does a real application of it make at least 0.1% of all usage? You cannot even buy a chip that implements segments, tagged pointers, etc etc. at least they could make a special mode where you can span across symbol whatever it means or see a pointer as a cpu sees it. All this can be solved with simple ptr_untag(p), ptr_span(expr) and similar constructs, but instead we have resort to outsmarting the specific compiler logic or introducing ourself with complicated type systems. That went insane. Personally I just want my bytes in linear address space and a way to tell which bytes point to which and which do not. I liked asm and then C, they were two of my first three languages, but what C became today is just a horrible mess.
- ChrisSD 8y ago
  But memory isn't a linear address space. If you're forced to treat memory as strictly linear than you're denied many optimizations.
  - twtw 8y ago
    > But memory isn't a linear address space Would you mind expanding on this point a bit?
    - wruza 8y ago
      I think it was mostly aliasing-related, and that’s why I mentioned “tell which bytes point at which”. Managing C today is like managing that girlfriend who doesn’t say anything straight and avoids direct conversations. I’m not against optimizations, I’m against compiler pretending to be smarter than average me and spoiling our relationship when I only have basic demands. If not that, I’m also curious.
      - Too 8y ago
        Sorry but it's the other way around. You are the one pretending to be smarter than the compiler by imagining objects are laid out sequentially in an infinite char[] when they they are in fact just abstract objects laid out in whichever way the compiler thinks is best. Just keep it this way and it will be much easier to manage C, it doesn't need micro management and neither does girlfriends.
        wruza 8y ago
        I'm saying that I don't like this model, not that I want to see everything as char[] in it. You insist that I should just embrace the model, since char[] doesn't work in it. I know, ok? If that tauthology was an argument, it completely missed the point. Below the compiler, I'm pretty familiar how things are laid out, since there is ABI, packing/alignment rules, documented hardware and 15 years of dealing with them. Please don't mock up magic and wisdom out of few conventions.
    - ChrisSD 8y ago
      Modern memory is layered. You have registers, you have three layers of CPU cache, etc, etc. At higher levels of abstraction we pretend like it's all flat but we do need to allow for optimizations to occur.
      - wruza 8y ago
        So what that does with linear addressing and optimization? How the presence of registers and caches depend on [non-]linearity?
- jcranmer 8y ago
  > You cannot even buy a chip that implements segments Not only can you buy a chip that implements segments, the computer you wrote that statement on probably has such a chip.
  - wruza 8y ago
    It is arm-based, so I believe it doesn’t. If you’re about x86s, then segment registers are there, but were not actually used (except for fs-gs utility cases) for almost a couple of decades, afaik. Segmented memory model is simply slow, cumbersome and unnecessary in the presence of decent pmmu. Edit: though strictly speaking I was obviously wrong on that, clarifications are welcome.
    - slededit 8y ago
      While it may be “cumbersome” from the programmers perspective - it’s certainly a lot faster than an MMU. There’s no universe where traversing a page table (actually a tree) in memory is faster than an offset and a bounds check.
      - wruza 8y ago
        What are you talking about? On x86 page tables were cached in TLB since their introduction. No mmu at all (80286) means that you're subject to fragmentation, and swapping segments is as expensive as a syscall since you have to lookup the descriptor through GDTR. x86 segments are scarce, expensive and cumbersome resources invented to cover bus width mismatch. Today it is not ever an issue. Even TSS is shared one per cpu, so much useless and slow its hardware part is. We could imagine more segment registers and bigger descriptor tables, but that would be just poor man's manual TLB (manual always failed in cpus). If you concerned with constant checks, you may order yourself a cpu with BOUND instruction support and put it everywhere with the same result. Oh, it is already in x86-64, nevermind. >actually a tree It is 2 level "tree", afair. Directory and table. Please make your homework and research why segmented model was kicked off software arena by virtually everyone involved.
        8y ago
pjmlp 8y ago
When I was writing portable UNIX software in the late 90's, early 2000's, the standard was quite relevant.

wruza 8y ago

I can’t tell for sure, since I was too young back then and didn’t write something complex enough, but I suspect that compilers were not evil genies back then. It was probably later gcc who first used UB as an excuse to evil optimizations. I also don’t understand a reason behind dragging all that legacy into current standards. Does a real application of it make at least 0.1% of all usage? You cannot even buy a chip that implements segments, tagged pointers, etc etc. at least they could make a special mode where you can span across symbol whatever it means or see a pointer as a cpu sees it. All this can be solved with simple ptr_untag(p), ptr_span(expr) and similar constructs, but instead we have resort to outsmarting the specific compiler logic or introducing ourself with complicated type systems. That went insane. Personally I just want my bytes in linear address space and a way to tell which bytes point to which and which do not. I liked asm and then C, they were two of my first three languages, but what C became today is just a horrible mess.

ChrisSD 8y ago
But memory isn't a linear address space. If you're forced to treat memory as strictly linear than you're denied many optimizations.
- twtw 8y ago
  > But memory isn't a linear address space Would you mind expanding on this point a bit?
  - wruza 8y ago
    I think it was mostly aliasing-related, and that’s why I mentioned “tell which bytes point at which”. Managing C today is like managing that girlfriend who doesn’t say anything straight and avoids direct conversations. I’m not against optimizations, I’m against compiler pretending to be smarter than average me and spoiling our relationship when I only have basic demands. If not that, I’m also curious.
    - Too 8y ago
      Sorry but it's the other way around. You are the one pretending to be smarter than the compiler by imagining objects are laid out sequentially in an infinite char[] when they they are in fact just abstract objects laid out in whichever way the compiler thinks is best. Just keep it this way and it will be much easier to manage C, it doesn't need micro management and neither does girlfriends.
      - wruza 8y ago
        I'm saying that I don't like this model, not that I want to see everything as char[] in it. You insist that I should just embrace the model, since char[] doesn't work in it. I know, ok? If that tauthology was an argument, it completely missed the point. Below the compiler, I'm pretty familiar how things are laid out, since there is ABI, packing/alignment rules, documented hardware and 15 years of dealing with them. Please don't mock up magic and wisdom out of few conventions.
  - ChrisSD 8y ago
    Modern memory is layered. You have registers, you have three layers of CPU cache, etc, etc. At higher levels of abstraction we pretend like it's all flat but we do need to allow for optimizations to occur.
    - wruza 8y ago
      So what that does with linear addressing and optimization? How the presence of registers and caches depend on [non-]linearity?
jcranmer 8y ago
> You cannot even buy a chip that implements segments Not only can you buy a chip that implements segments, the computer you wrote that statement on probably has such a chip.
- wruza 8y ago
  It is arm-based, so I believe it doesn’t. If you’re about x86s, then segment registers are there, but were not actually used (except for fs-gs utility cases) for almost a couple of decades, afaik. Segmented memory model is simply slow, cumbersome and unnecessary in the presence of decent pmmu. Edit: though strictly speaking I was obviously wrong on that, clarifications are welcome.
  - slededit 8y ago
    While it may be “cumbersome” from the programmers perspective - it’s certainly a lot faster than an MMU. There’s no universe where traversing a page table (actually a tree) in memory is faster than an offset and a bounds check.
    - wruza 8y ago
      What are you talking about? On x86 page tables were cached in TLB since their introduction. No mmu at all (80286) means that you're subject to fragmentation, and swapping segments is as expensive as a syscall since you have to lookup the descriptor through GDTR. x86 segments are scarce, expensive and cumbersome resources invented to cover bus width mismatch. Today it is not ever an issue. Even TSS is shared one per cpu, so much useless and slow its hardware part is. We could imagine more segment registers and bigger descriptor tables, but that would be just poor man's manual TLB (manual always failed in cpus). If you concerned with constant checks, you may order yourself a cpu with BOUND instruction support and put it everywhere with the same result. Oh, it is already in x86-64, nevermind. >actually a tree It is 2 level "tree", afair. Directory and table. Please make your homework and research why segmented model was kicked off software arena by virtually everyone involved.
      - 8y ago

pjmlp 8y ago

When I was writing portable UNIX software in the late 90's, early 2000's, the standard was quite relevant.