Problem with fld on i686

[John Hedges <john@drystone.co.uk>]

I've come across what seems to be a debugger problem with fld. The build is 
i686 linux pc. The failing code comes from static_cast<double>(float) which 
looks ok.:


(gdb) l
138           __ostream_type& 
139           operator<<(float __f)
140           { return this->operator<<(static_cast<double>(__f)); }

(gdb) disassemble
0x80aa350 <_ZNSolsEf>:  push   %ebp
0x80aa351 <_ZNSolsEf+1>:        mov    %esp,%ebp
0x80aa353 <_ZNSolsEf+3>:        sub    $0x8,%esp
0x80aa356 <_ZNSolsEf+6>:        sub    $0x4,%esp
0x80aa359 <_ZNSolsEf+9>:        flds   0xc(%ebp)
0x80aa35c <_ZNSolsEf+12>:       lea    0xfffffff8(%esp,1),%esp
0x80aa360 <_ZNSolsEf+16>:       fstpl  (%esp,1)
0x80aa363 <_ZNSolsEf+19>:       pushl  0x8(%ebp)
0x80aa366 <_ZNSolsEf+22>:       call   0x80abf00 <_ZNSolsEd>
0x80aa36b <_ZNSolsEf+27>:       add    $0x10,%esp
0x80aa36e <_ZNSolsEf+30>:       mov    %ebp,%esp
0x80aa370 <_ZNSolsEf+32>:       pop    %ebp
0x80aa371 <_ZNSolsEf+33>:       ret    



The contents of ebp+0x0c (0.8) and registers before the flds instruction:

(gdb) x $ebp+12
0xbf1ff790:     0x3f4ccccd

(gdb) info all
eax            0xbf1ff84c       -1088423860
ecx            0x0      0
edx            0xbf1ff764       -1088424092
ebx            0x400ea7f0       1074702320
esp            0xbf1ff778       0xbf1ff778
ebp            0xbf1ff784       0xbf1ff784
esi            0x0      0
edi            0x810b0b8        135311544
eip            0x80aa359        0x80aa359
eflags         0x386    902
cs             0x23     35
ss             0x2b     43
ds             0x2b     43
es             0x2b     43
fs             0x0      0
gs             0x0      0
st0            -0e+4922 (raw 0xffff0000000000000000)
st1            -0e+4922 (raw 0xffff0000000000000000)
st2            -0e+4922 (raw 0xffff0000000000000000)
st3            0        (raw 0x00000000000000000000)
st4            1        (raw 0x3fff8000000000000000)
st5            1        (raw 0x3fff8000000000000000)
st6            3        (raw 0x4000c000000000000000)
st7            1.0909090909090909091204782410677865     (raw 
0x3fff8ba2e8ba2e8ba2e9)
fctrl          0x37f    895
fstat          0x0      0
ftag           0x0      0
fiseg          0x23     35
fioff          0x401fa178       1075814776
foseg          0x2b     43
fooff          0x4020b648       1075885640
fop            0x0      0


Registers after flds:

(gdb) si
0x080aa35c      140           { return 
this->operator<<(static_cast<double>(__f)); }

(gdb) info all
eax            0xbf1ff84c       -1088423860
ecx            0x0      0
edx            0xbf1ff764       -1088424092
ebx            0x400ea7f0       1074702320
esp            0xbf1ff778       0xbf1ff778
ebp            0xbf1ff784       0xbf1ff784
esi            0x0      0
edi            0x810b0b8        135311544
eip            0x80aa35c        0x80aa35c
eflags         0x386    902
cs             0x23     35
ss             0x2b     43
ds             0x2b     43
es             0x2b     43
fs             0x0      0
gs             0x0      0
st0            -nan(0xc000000000000000) (raw 0xffffc000000000000000)
st1            -0e+4922 (raw 0xffff0000000000000000)
st2            -0e+4922 (raw 0xffff0000000000000000)
st3            -0e+4922 (raw 0xffff0000000000000000)
st4            0        (raw 0x00000000000000000000)
st5            1        (raw 0x3fff8000000000000000)
st6            1        (raw 0x3fff8000000000000000)
st7            3        (raw 0x4000c000000000000000)
fctrl          0x37f    895
fstat          0x3a41   14913
ftag           0x0      0
fiseg          0x23     35
fioff          0x80aa359        134914905
foseg          0x2b     43
fooff          0xbf1ff790       -1088424048
fop            0x0      0


The st0 register gets -nan and so this is the result of the static_cast. From 
what I can make of fpu docs, the stack overflow bit is set in fstat. The 
problem does not occur when run outside the debugger, nor in a minimal test 
program.

John