Bug 12134 - long double precision lost
Summary: long double precision lost
Status: NEW
Alias: None
Product: gdb
Classification: Unclassified
Component: gdb (show other bugs)
Version: unknown
: P2 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
Depends on:
Reported: 2010-10-18 21:07 UTC by Jakub Jelinek
Modified: 2010-10-18 21:07 UTC (History)
0 users

See Also:
Target: x86_64-linux
Last reconfirmed:


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Description Jakub Jelinek 2010-10-18 21:07:43 UTC
double d = __DBL_DENORM_MIN__;
long double l1 = (long double) __DBL_DENORM_MIN__;
long double l2 = __LDBL_MIN__;
long double l3 = __LDBL_MIN__ * 0x1p200L;

main (void)
  return 0;

gcc -g -o tt tt.c
gdb ./tt
b main
(gdb) p d
$1 = 4.9406564584124654e-324
(gdb) p l1
$2 = 4.9406564584124654417656879286822137e-324
(gdb) p l2
$3 = 0
(gdb) p l3
$4 = 0
(gdb) x/2gx &l2
0x600830 <l2>: 0x8000000000000000 0x0000000000000001
(gdb) x/2gx &l3
0x600840 <l3>: 0x8000000000000000 0x00000000000000c9

From the above it seems that the long double value read from the vars is somewhere in gdb internally cast to double, then probably cast back to long double and printed (or goes through text representation or whatever), because 
l2 and l3 definitely are not zero.

Also, it would be nice if p/x {d,l1,l2,l3} printed the double/long double vars
using ISO C99 hexadecimal notation (%a/%La for printf) and if hex float literals could be used in gdb expressions.