5 ms·

Additionally, he keeps talking about dereferencing the pointer, which I don't think is right. The pointer never gets deferenced in the code shown. I'm not an x

Additionally, he keeps talking about dereferencing the pointer, which I don't think is right. The pointer never gets deferenced in the code shown. I'm not an x86 guru, but I think that "movq ptr(%rip), %rsi" is different because ptr needs to be moved from relative to the instruction pointer (because it is on the stack, as a non-const variable).

kccqzy 9y ago
You are right. This is RIP-relative addressing.
bonzini 9y ago
It's a global variable, it's not on the stack. It's in the data section. $arr is copying the address of "arr", so it must use an immediate move (possibly 64-bit). It could also use RIP-relative with the "lea" (load effective address) instruction. $ptr(%rip) is accessing memory, so it uses RIP-relative addressing with the "mov" instruction.
- magnetic 9y ago
  > It's a global variable, it's not on the stack. It's in the data section. And because it's a const, it's in the .rodata section... (read-only) Playing too liberally with the data often leads to a SEGV (as it should).
novia 9y ago
This is a little off topic, but does anyone have any good resources to help me wrap my head around pointers in C? Right now they are very confusing to me.
- Zelizz 9y ago
  Imagine all memory is a big array. A pointer is just an index into that array. A pointer dereference is like accessing something at an array index. A pointer to a pointer is array index to a location where you'll find another array index.
  - novia 9y ago
    I understand pointers conceptually. It's the syntax (in C specifically) that is giving me trouble. I am fine with pointers in ASM.
    - iainmerrick 9y ago
      Hmm, it's been a long time since I read it, but this sounds like something that good old K&R probably does well. It's famously short and well-written. It's old-fashioned but I assume (maybe others can correct me) the latest edition is up to date enough that it won't teach you any outright bad habits.
      - novia 9y ago
        Thank you very much! I'll give it a look.
AbacusAvenger 9y ago
The code is more interesting if you -do- actually dereference the pointer, too. I've changed the prototype of "bogus" to take a single char as the first argument, and dereference the two pointers inside the do_arr/do_ptr functions: https://goo.gl/jZYx0f https://goo.gl/jZYx0f So the "do_arr" version "knows" what value is at * arr because "arr" is immutable, so the compiler can just choose to load a constant. The "do_ptr" version has to load from memory instead. But what if we tell the compiler that the value of "ptr" won't change (i.e. "char * const" instead of "const char * ")? The code becomes the same: https://goo.gl/iNaUHe https://goo.gl/iNaUHe So basically "const char []" and "char * const" are more logically equivalent here.

kccqzy 9y ago

You are right. This is RIP-relative addressing.

bonzini 9y ago

It's a global variable, it's not on the stack. It's in the data section. $arr is copying the address of "arr", so it must use an immediate move (possibly 64-bit). It could also use RIP-relative with the "lea" (load effective address) instruction. $ptr(%rip) is accessing memory, so it uses RIP-relative addressing with the "mov" instruction.

magnetic 9y ago
> It's a global variable, it's not on the stack. It's in the data section. And because it's a const, it's in the .rodata section... (read-only) Playing too liberally with the data often leads to a SEGV (as it should).

novia 9y ago

This is a little off topic, but does anyone have any good resources to help me wrap my head around pointers in C? Right now they are very confusing to me.

Zelizz 9y ago
Imagine all memory is a big array. A pointer is just an index into that array. A pointer dereference is like accessing something at an array index. A pointer to a pointer is array index to a location where you'll find another array index.
- novia 9y ago
  I understand pointers conceptually. It's the syntax (in C specifically) that is giving me trouble. I am fine with pointers in ASM.
  - iainmerrick 9y ago
    Hmm, it's been a long time since I read it, but this sounds like something that good old K&R probably does well. It's famously short and well-written. It's old-fashioned but I assume (maybe others can correct me) the latest edition is up to date enough that it won't teach you any outright bad habits.
    - novia 9y ago
      Thank you very much! I'll give it a look.

AbacusAvenger 9y ago

The code is more interesting if you -do- actually dereference the pointer, too. I've changed the prototype of "bogus" to take a single char as the first argument, and dereference the two pointers inside the do_arr/do_ptr functions: https://goo.gl/jZYx0f https://goo.gl/jZYx0f So the "do_arr" version "knows" what value is at * arr because "arr" is immutable, so the compiler can just choose to load a constant. The "do_ptr" version has to load from memory instead. But what if we tell the compiler that the value of "ptr" won't change (i.e. "char * const" instead of "const char * ")? The code becomes the same: https://goo.gl/iNaUHe https://goo.gl/iNaUHe So basically "const char []" and "char * const" are more logically equivalent here.