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637 :: Undefined behaviour in signed integer overflows (as in C)

• def (orginal) [2014-11-23T02:35:50+01:00] view original

  
  import strutils
  
  proc GetInt(): int = stdin.readLine.parseInt
  
  var
    Chunk = GetInt()
    ChunkSize = 1024
    Offset = Chunk * ChunkSize
  
  if Chunk < 0 or Offset < 0:
    quit 0
  
  echo Offset
  quit 1
  Run

  Run with

  
  $ nim -d:release c minbug.nim && echo 1000000000000000000 | ./minbug
  -9017668127734890496
  Run

  Depending on the underlying C compiler and optimization level Offset can be printed in the end, even when it's negative, because of signed integer overflows. The behaviour of a program with signed integer overflow is undefined in C, so gcc 4.8 with -O2 optimizes the Offset < 0 away, while gcc 4.7 does not.

  This behaviour has long been known, see for example: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30475 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49820 https://gcc.gnu.org/ml/gcc-help/2011-07/msg00219.html

  I don't know whether this behaviour is wanted in Nim.

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• Araq (orginal) [2014-11-23T02:47:16+01:00] view original

  Yes, this is what we want (for now). Well we don't want "completely undefined" behaviour really, but the optimizer is allowed to assume no overflow happens for signed arithmetic. At runtime it should raise the OverflowError exception or wrap around. Personally I'm a fan of saturated arithmetic instead of wrap around but that's not easy to do with today's hardware.

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