func foo(e interface{}) error {
    switch x := e.(type) {
    case error:
        return x
    }
    return nil
}

We call efacethash only to throw the result away.  We should only call this function
(and equivalently ifacethash) only if we need the result (for binary search over types?).

0000000000400c00 <main.foo>:
  400c00:       64 48 8b 0c 25 f0 ff    mov    %fs:0xfffffffffffffff0,%rcx
  400c07:       ff ff 
  400c09:       48 3b 21                cmp    (%rcx),%rsp
  400c0c:       77 07                   ja     400c15 <main.foo+0x15>
  400c0e:       e8 6d bd 01 00          callq  41c980 <runtime.morestack32>
  400c13:       eb eb                   jmp    400c00 <main.foo>
  400c15:       48 83 ec 40             sub    $0x40,%rsp
  400c19:       48 8b 4c 24 48          mov    0x48(%rsp),%rcx
  400c1e:       48 8b 44 24 50          mov    0x50(%rsp),%rax
  400c23:       48 89 4c 24 30          mov    %rcx,0x30(%rsp)
  400c28:       48 89 0c 24             mov    %rcx,(%rsp)
  400c2c:       48 89 44 24 38          mov    %rax,0x38(%rsp)
  400c31:       48 89 44 24 08          mov    %rax,0x8(%rsp)
  400c36:       e8 45 3c 00 00          callq  404880 <runtime.efacethash>
  400c3b:       8b 5c 24 10             mov    0x10(%rsp),%ebx
  400c3f:       48 c7 04 24 80 27 42    movq   $0x422780,(%rsp)
  400c46:       00 
  400c47:       48 8b 5c 24 30          mov    0x30(%rsp),%rbx
  400c4c:       48 89 5c 24 08          mov    %rbx,0x8(%rsp)
  400c51:       48 8b 5c 24 38          mov    0x38(%rsp),%rbx
  400c56:       48 89 5c 24 10          mov    %rbx,0x10(%rsp)
  400c5b:       e8 80 37 00 00          callq  4043e0 <runtime.assertE2I2>
  400c60:       48 8b 54 24 18          mov    0x18(%rsp),%rdx
  400c65:       48 8b 4c 24 20          mov    0x20(%rsp),%rcx
  400c6a:       48 0f b6 5c 24 28       movzbq 0x28(%rsp),%rbx
  400c70:       80 fb 00                cmp    $0x0,%bl
  400c73:       74 0f                   je     400c84 <main.foo+0x84>
  400c75:       48 89 54 24 58          mov    %rdx,0x58(%rsp)
  400c7a:       48 89 4c 24 60          mov    %rcx,0x60(%rsp)
  400c7f:       48 83 c4 40             add    $0x40,%rsp
  400c83:       c3                      retq   
  400c84:       48 c7 44 24 58 00 00    movq   $0x0,0x58(%rsp)
  400c8b:       00 00 
  400c8d:       48 c7 44 24 60 00 00    movq   $0x0,0x60(%rsp)
  400c94:       00 00 
  400c96:       48 83 c4 40             add    $0x40,%rsp
  400c9a:       c3                      retq