Can Rust optimise away the bit-wise copy during move of an object someday? -


consider snippet

struct foo {     dummy: [u8; 65536], }  fn bar(foo: foo) {     println!("{:p}", &foo) }  fn main() {     let o = foo { dummy: [42u8; 65536] };     println!("{:p}", &o);     bar(o); }

a typical result of program is

0x7fffc1239890 0x7fffc1229890

where addresses different.

apparently, large array dummy has been copied, expected in compiler's move implementation. unfortunately, can have non-trivial performance impact, dummy large array. impact can force people choose passing argument reference instead, when function "consumes" argument conceptually.

since foo not derive copy, object o moved. since rust forbids access of moved object, preventing bar "reuse" original object o, forcing compiler generate potentially expensive bit-wise copy? there fundamental difficulty, or see compiler someday optimise away bit-wise copy?

given in rust (unlike c or c++) address of value not considered matter, there nothing in terms of language prevents elision of copy.

however, today rustc not optimize anything: optimizations delegated llvm, , seems have hit limitation of llvm optimizer here (it's unclear whether limitation due llvm being close c's semantics or omission).

so, there 2 avenues of improving code generation this:

  • teaching llvm perform optimization (if possible)
  • teaching rustc perform optimization (optimization passes coming rustc has mir)

but might want avoid such large objects being allocated on stack, can box example.


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