cmd/gc: optimize assignment of empty string literals

[dvyukov]

Currently gc generates the following code:

    var s1 string
  400c19:   48 c7 44 24 08 00 00    movq   $0x0,0x8(%rsp)
  400c22:   48 c7 44 24 10 00 00    movq   $0x0,0x10(%rsp)

    s2 := ""
  400c2b:   48 8d 1c 25 20 63 42    lea    0x426320,%rbx
  400c33:   48 8b 2b                mov    (%rbx),%rbp
  400c36:   48 89 6c 24 18          mov    %rbp,0x18(%rsp)
  400c3b:   48 8b 6b 08             mov    0x8(%rbx),%rbp
  400c3f:   48 89 6c 24 20          mov    %rbp,0x20(%rsp)

    s3 = ""
  400c44:   48 8d 1c 25 20 63 42    lea    0x426320,%rbx
  400c4c:   48 8b 2b                mov    (%rbx),%rbp
  400c4f:   48 89 2c 25 f0 34 46    mov    %rbp,0x4634f0
  400c57:   48 8b 6b 08             mov    0x8(%rbx),%rbp
  400c5b:   48 89 2c 25 f8 34 46    mov    %rbp,0x4634f8

Ideally it is:

    var s1 string
  400c19:   48 c7 44 24 08 00 00    movq   $0x0,0x8(%rsp)
  400c22:   48 c7 44 24 10 00 00    movq   $0x0,0x10(%rsp)

    s2 := ""
  400c19:   48 c7 44 24 08 00 00    movq   $0x0,0x8(%rsp)
  400c22:   48 c7 44 24 10 00 00    movq   $0x0,0x10(%rsp)

    s3 = ""
  400c19:   48 c7 44 24 08 00 00    movq   $0x0,0x8(%rsp)
  400c22:   48 c7 44 24 10 00 00    movq   $0x0,0x10(%rsp)

For := "", compiler can just remove the initializer.
For = "", compiler can recognize "" and store zeros.

[rsc]

Comment 1:

Status changed to Duplicate.

Merged into issue #1914.