Re: Maybe a bug about built-in string to cstring converter

[Ward]

I don't think adding a nil check will become the performance bottleneck. Here 
is my test code:
    
    
    import times
    
    proc MulDiv*(P1: int32, P2: int32, P3: int32): int32 {.stdcall, 
discardable, dynlib: "kernel32", importc.}
    
    proc test(s: cstring) =
      # do someting, to make sure the loop will not be eliminated by the 
compiler
      MulDiv(cast[int32](s), cast[int32](s), cast[int32](s))
    
    var
      s1: string
      s2 = "abc"
      count = 100000000
      start: float
    
    start = cpuTime()
    for i in 1..count:
      test(cast[cstring](0))
      test(cast[cstring](-1))
    echo cpuTime() - start, " (loop only)"
    
    start = cpuTime()
    for i in 1..count:
      test(s1)
      test(s2)
    echo cpuTime() - start, " (default convert)"
    
    start = cpuTime()
    for i in 1..count:
      test(cast[cstring](cast[uint](s1) + (if s1.isNil: 0 else: 8)))
      test(cast[cstring](cast[uint](s2) + (if s2.isNil: 0 else: 8)))
    echo cpuTime() - start, " (safe convert)"
    
    start = cpuTime()
    for i in 1..count:
      test(if s1.isNil: nil else: s1)
      test(if s2.isNil: nil else: s2)
    echo cpuTime() - start, " (worst code)"
    

Output 
    
    
    3.33 (loop only)
    3.504 (default convert)
    3.476999999999999 (safe convert)
    24.2 (worst code)
    

nim -d:release --opt:speed 
    
    
    0.855 (loop only)
    1.107 (default convert)
    1.138 (safe convert)
    3.684 (worst code)
    

After optimization, there is no conditional jump in the asm output for safe 
convert: 
    
    
    mov edx, DWORD PTR _s1_121012_755421110
    mov DWORD PTR _i_121068_755421110, ebx
    cmp edx, 1
    sbb eax, eax
    not eax
    and eax, 8
    add eax, edx
    

Most people will use the WORST CODE to check nil, right? So in my opinion, add 
the check by compiler is clever.