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Low level code has to take extreme care of avoiding as many allocs as possible, wonder when the compiler will be smart enough to do these optimizations automati

Low level code has to take extreme care of avoiding as many allocs as possible, wonder when the compiler will be smart enough to do these optimizations automatically...

deleted 5y ago
[deleted]
VHRanger 5y ago
The compiler can't really reason very well about the data layout - it can reason about the instructions
- jimmyed 5y ago
  It can also reason about control flow. It does constant folding etc, is it at least theoretically possible to do alloc optimization?
  - chakkepolja 5y ago
    Yep but that would probably break predictability. In low level code I wouldn't rely on some malloc being optimized away, and rather write code explicitly without allocations. In High level languages, Escape analysis exists.
nextaccountic 5y ago
C and C++ compilers can sometimes lower a small heap allocation into a stack allocation, at least in simpler cases. Here's some links about it * https://www.youtube.com/watch?v=lTgERgPNTF8 https://www.youtube.com/watch?v=lTgERgPNTF8 a 5 minutes talk about llvm doing this * https://speice.io/2019/02/compiler-optimizations.html https://speice.io/2019/02/compiler-optimizations.html discusses this happening in Rust (the Rust compiler uses llvm) * http://www.cs.cmu.edu/afs/cs.cmu.edu/user/jatina/www/CS_15_745_Final_Report.pdf http://www.cs.cmu.edu/afs/cs.cmu.edu/user/jatina/www/CS_15_7... a 12 page paper, also on llvm (ps: gcc can do it too, it was just easier to find llvm sources) * https://en.wikipedia.org/wiki/Escape_analysis#Optimizations https://en.wikipedia.org/wiki/Escape_analysis#Optimizations (the optimization that enables this is called escape analysis: it tells whether the heap allocation "escapes" the current function, or if it's local enough that stack allocation is a good fit. inlining increases the likelihood that heap allocation won't escape) * https://stackoverflow.com/questions/47075871/can-the-compiler-optimize-from-heap-to-stack-allocation https://stackoverflow.com/questions/47075871/can-the-compile... asking if compilers are actually allowed to do that (they sometimes are, but not always)
- Thorrez 5y ago
  Go doesn't even have a language-level distinction between heap and stack, everything is on the heap from the language standpoint. However, values that don't escape and that aren't too big are put on the stack by the compiler.

Low level code has to take extreme care of avoiding as many allocs as possible, wonder when the compiler will be smart enough to do these optimizations automatically...

deleted 5y ago

[deleted]

VHRanger 5y ago

The compiler can't really reason very well about the data layout - it can reason about the instructions

jimmyed 5y ago
It can also reason about control flow. It does constant folding etc, is it at least theoretically possible to do alloc optimization?
- chakkepolja 5y ago
  Yep but that would probably break predictability. In low level code I wouldn't rely on some malloc being optimized away, and rather write code explicitly without allocations. In High level languages, Escape analysis exists.

nextaccountic 5y ago

C and C++ compilers can sometimes lower a small heap allocation into a stack allocation, at least in simpler cases. Here's some links about it * https://www.youtube.com/watch?v=lTgERgPNTF8 https://www.youtube.com/watch?v=lTgERgPNTF8 a 5 minutes talk about llvm doing this * https://speice.io/2019/02/compiler-optimizations.html https://speice.io/2019/02/compiler-optimizations.html discusses this happening in Rust (the Rust compiler uses llvm) * http://www.cs.cmu.edu/afs/cs.cmu.edu/user/jatina/www/CS_15_745_Final_Report.pdf http://www.cs.cmu.edu/afs/cs.cmu.edu/user/jatina/www/CS_15_7... a 12 page paper, also on llvm (ps: gcc can do it too, it was just easier to find llvm sources) * https://en.wikipedia.org/wiki/Escape_analysis#Optimizations https://en.wikipedia.org/wiki/Escape_analysis#Optimizations (the optimization that enables this is called escape analysis: it tells whether the heap allocation "escapes" the current function, or if it's local enough that stack allocation is a good fit. inlining increases the likelihood that heap allocation won't escape) * https://stackoverflow.com/questions/47075871/can-the-compiler-optimize-from-heap-to-stack-allocation https://stackoverflow.com/questions/47075871/can-the-compile... asking if compilers are actually allowed to do that (they sometimes are, but not always)

Thorrez 5y ago
Go doesn't even have a language-level distinction between heap and stack, everything is on the heap from the language standpoint. However, values that don't escape and that aren't too big are put on the stack by the compiler.