4 ms·

Just looking from afar, I don't have time to analyze every other claim, but this one: > Both C++ and Rust have generated identical assembly listings; both have

by acqq 6y ago

Just looking from afar, I don't have time to analyze every other claim, but this one: > Both C++ and Rust have generated identical assembly listings; both have added push rbx for the sake of stack alignment. Q.E.D. seems to be completely wrong: a decent compiler is able to align the stack without "touching" it. For the variables inside of the function to be pushed to the aligned stack position, only different offsets have to be calculated. For the stack itself to get to be aligned, only the register has to be updated, surely nothing has to be pushed. So something else must have been happening there, and I don't have time to analyze what, but I'm sure push is surely not necessary for alignment alone.

pitaj 6y ago
It may just be for alignment. A push may be just (having a specific case in the CPU) as fast as updating the SP.
ridiculous_fish 6y ago
You are correct, it has nothing to do with alignment. rbx is a callee-saved register, so the callee saves it.
- deleted 6y ago
  [deleted]
- thayne 6y ago
  Do you mean thar rax is caller-saved?
  - ridiculous_fish 6y ago
    rax is indeed caller-saved, but I was referring to rbx. In this code: int bar(int a, int b) { int z = a * b; foo(); return z; } https://godbolt.org/z/a8Y35r https://godbolt.org/z/a8Y35r Where to put `z`? It has to be someplace where `foo` won't clobber it, like a callee-saved register - in this case ebx. But now `bar` is responsible for saving the value of ebx for its caller. That explains the push and pop. Alignment is unrelated; this is just about calling conventions.
Someone 6y ago
Counterintuitively, on some x86 architectures, push ¿is/can be? faster than decreasing the stack pointer’s value because the CPU uses dedicated hardware to speed up subroutine calls (https://stackoverflow.com/a/36633556 https://stackoverflow.com/a/36633556)

by acqq 6y ago

pitaj 6y ago

It may just be for alignment. A push may be just (having a specific case in the CPU) as fast as updating the SP.

ridiculous_fish 6y ago

You are correct, it has nothing to do with alignment. rbx is a callee-saved register, so the callee saves it.

deleted 6y ago
[deleted]
thayne 6y ago
Do you mean thar rax is caller-saved?
- ridiculous_fish 6y ago
  rax is indeed caller-saved, but I was referring to rbx. In this code: int bar(int a, int b) { int z = a * b; foo(); return z; } https://godbolt.org/z/a8Y35r https://godbolt.org/z/a8Y35r Where to put `z`? It has to be someplace where `foo` won't clobber it, like a callee-saved register - in this case ebx. But now `bar` is responsible for saving the value of ebx for its caller. That explains the push and pop. Alignment is unrelated; this is just about calling conventions.

Someone 6y ago

Counterintuitively, on some x86 architectures, push ¿is/can be? faster than decreasing the stack pointer’s value because the CPU uses dedicated hardware to speed up subroutine calls (https://stackoverflow.com/a/36633556 https://stackoverflow.com/a/36633556)