[GHC] #9564: Floating point subnormals overrounded on output

[GHC]

#9564: Floating point subnormals overrounded on output
-------------------------------------+-------------------------------------
       Reporter:  jrp                |                   Owner:  simonmar
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Runtime System     |                 Version:  7.8.3
       Keywords:                     |        Operating System:  MacOS X
   Architecture:  x86_64 (amd64)     |         Type of failure:  Incorrect
     Difficulty:  Unknown            |  result at runtime
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 The following test suggests that GHC is overrounding Float subnormals,
 when printing them out
 {{{
 Prelude> 2**(-149)
 1.401298464324817e-45
 Prelude> 2**(-149) :: Float
 1.0e-45
 Prelude> isDenormalized it
 True
 Prelude>
 Prelude> 2**(-149) * 2**(100) * 2**(49) :: Float
 1.0
 }}}


 By comparison


 {{{
 #include <stdio.h>
 #include <stdlib.h>

 int main (void)
 {
  float clang_dec, clang_hex, libc_dec, libc_hex;

  clang_dec = 1.401298464324817e-45;
  clang_hex = 0x0.000002P-126;
  printf("clang (from decimal)   = %a\n",clang_dec);
  printf("clang (from decimal)   = %g\n",clang_dec);
  printf("clang (from hex)       = %a\n",clang_hex);
  printf("clang (from hex)       = %g\n",clang_hex);

  libc_dec = strtod("1.401298464324817e-45",NULL);
  libc_hex = strtod("0x0.000002P-126",NULL);
  printf("libc (from decimal) = %a\n",libc_dec);
  printf("libc (from decimal) = %g\n",libc_dec);
  printf("libc (from hex)     = %a\n",libc_hex);
  printf("libc (from hex)     = %g\n",libc_hex);
 }

 }}}

 produces


 {{{
 clang (from decimal)   = 0x1p-149
 clang (from decimal)   = 1.4013e-45
 clang (from hex)       = 0x1p-149
 clang (from hex)       = 1.4013e-45
 libc (from decimal) = 0x1p-149
 libc (from decimal) = 1.4013e-45
 libc (from hex)     = 0x1p-149
 libc (from hex)     = 1.4013e-45

 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9564>
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