Changes between Version 6 and Version 7 of Commentary/CmmExceptions

Jan 16, 2007 6:01:45 AM (7 years ago)

update for #1042 fix


  • Commentary/CmmExceptions

    v6 v7  
    1515 * FPU exceptions may be unavoidable, especially if several FPU operations are serially performed at the machine level so the higher level software has no opportunity to check the results in between operations.  
    17 There is at least one problem in GHC that requires FPU exception handling.  See bug ticket #1042.  The bug occurs in 'show'ing the number (conversion from base_2 to base_10).  Note that the FPU exception does not occur on PowerPC machines. 
     17There has been at least one problem in GHC that would benefit from exception handling--in some cases, for `Integral`s.  See bug ticket #1042.  The bug occurs in `show`ing the number, in [GhcFile(libraries/base/GHC/Show.lhs) GHC.Show], `showSignedInt`, before conversion from base_2 to base_10, where a negative `Int` (always `Int32`) is negated in order to process it as a positive value when converting it to a string, base_10, causing an overflow error on some architectures.  Note that the exception example in #1042 does not occur on PowerPC machines, which dutifully print the two's complement of {{{(-2147483648::Int) `div` (-1::Int)}}}: `0`.  (`-2147483648` is the minimum bound for signed Ints, so negating it should properly become, bitwise, a positive `2147483647` (all but bit 31 set); once negated again when divided by `-1` this would be `0`; `-0` is converted to `0`.)  On some architectures such as Intel 64 and IA-32, negating the minimum bound does not wrap around to `0` but overflows, which is reported as a floating point "overflow" (`#O`) exception.  The workaround was to avoid negating `minBound` `Int`s.   
    1919There was a long message thread on the Haskell-prime mailing list, "realToFrac Issues," beginning with [ John Meacham's message] and ending with [ Simon Marlow's message].  The following code for converting a Float to a Double will ''fail'' to produce a floating point exception or NaN on x86 machines (recall that 0.0/0.0 is NaN ''and'' a definite FPU exception):