[Issue 24836] New: struct return by hidden struct arg does not need copy to become an rvalue

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24836

          Issue ID: 24836
           Summary: struct return by hidden struct arg does not need copy
                    to become an rvalue
           Product: D
           Version: D2
          Hardware: All
                OS: Windows
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: bugzilla at digitalmars.com

Related: 9666

Given:

  import core.stdc.stdio;

  struct S {
    this(ref S) { printf("this(ref S)\n"); }
    void opAssign(ref S) { printf("opAssign(ref S)\n"); }
    void opAssign(S) { printf("opAssign(S)\n"); }
    void opMove(ref S) { printf("opMove(ref S)\n"); }
  }

  void main() {
    S s;
    S t;

    S test() { S* p = &s; return *p; }
    t = test();
  }

The code generated for main() is:

_Dmain:
    push      RBP
    mov       RBP,RSP
    sub       RSP,010h
    xor       EAX,EAX
    mov       -8[RBP],AL  // S s;
    mov       -7[RBP],AL  // S t;
    lea       RSI,-6[RBP] // address of struct return value
    mov       RDI,RBP     // static link
    call      test
    mov       RSI,RAX     // address of struct return value as source
    lea       RDI,-5[RBP] // address of temporary
    movsb                 // copy return value to temporary
    lea       RSI,-5[RBP] // address of temporary
    lea       RDI,-7[RBP] // address of t
    call      opAssign    // opAssign(S)
    xor       EAX,EAX
    leave
    ret

The copy is not necessary, and is a bug.

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