swift234

When binding address-only let declarations to let arguments, use the 
buffer being passed in, instead of creating a temporary one and copying
into it.

For:

func generic_identity<T>(let a : T) -> T {
  return a
}

We now get:

sil @_TF2t216generic_identityU__FT1aQ__Q_ : $@thin <$T_0_0> (@out $T_0_0, @in $T_0_0) -> () {
bb0(%0 : $*T, %1 : $*T):
  copy_addr [take] %1 to [initialization] %0 : $*T // id: %2
  %3 = tuple ()                                   // user: %4
  return %3 : $()                                 // id: %4
}

We used to produce:

sil @_TF2t216generic_identityU__FT1aQ__Q_ : $@thin <$T_0_0> (@out $T_0_0, @in $T_0_0) -> () {
bb0(%0 : $*T, %1 : $*T):
  %2 = alloc_box $T  // let a                     // users: %5, %4, %3
  copy_addr [take] %1 to [initialization] %2#1 : $*T // id: %3
  copy_addr %2#1 to [initialization] %0 : $*T     // id: %4
  strong_release %2#0 : $Builtin.ObjectPointer    // id: %5
  %6 = tuple ()                                   // user: %7
  return %6 : $()                                 // id: %7
}

and then optimize it into:

sil @_TF2t216generic_identityU__FT1aQ__Q_ : $@thin <$T_0_0> (@out $T_0_0, @in $T_0_0) -> () {
bb0(%0 : $*T, %1 : $*T):
  %2 = alloc_stack $T  // let a                   // users: %5, %3, %4
  copy_addr [take] %1 to [initialization] %2#1 : $*T // id: %3
  copy_addr [take] %2#1 to [initialization] %0 : $*T // id: %4
  dealloc_stack %2#0 : $*@local_storage T         // id: %5
  %6 = tuple ()                                   // user: %7
  return %6 : $()                                 // id: %7
}



Swift SVN r11941