[llvm-bugs] [Bug 38666] Replace pass by const reference with pass by value

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38666

David Blaikie <dblaikie at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dblaikie at gmail.com
         Resolution|---                         |WORKSFORME
             Status|NEW                         |RESOLVED

--- Comment #1 from David Blaikie <dblaikie at gmail.com> ---
LLVM already performs this optimization (if you're curious about which
optimizations LLVM performs, asking on llvm-dev might be more suitable than
filing bugs) where possible:

  $ cat arg_promo.cpp
    void f1(int);
    static __attribute__((noinline)) void f2(const int &a) { f1(a); }
    void f3(int a) { f2(a); }
  $ clang++-tot arg_promo.cpp -emit-llvm -S && cat arg_promo.ll | grep
define.*f2
    define internal void @_ZL2f2RKi(i32* dereferenceable(4) %a) #0 {
  $ clang++-tot arg_promo.cpp -emit-llvm -S -O3 && cat arg_promo.ll | grep
define.*f2
    define internal fastcc void @_ZL2f2RKi(i32 %a.val) unnamed_addr #1 {

It cannot be performed on externally visible functions because it changes the
way the calling code works - and is contingent on the implementation of the
function, so only when both the implementation and all callers are visible/can
be changed, can the optimization be performed. (otherwise the function might be
modified, but a caller could still end up trying to pass a reference, etc)

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