https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87303
Bug ID: 87303
Summary: DFmode FP constants are not correctly truncated when
promoted to XFmode
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ubizjak at gmail dot com
Target Milestone: ---
I was looking at the following testsuite failure with
-fexcess-precision=standard on 32bit x86 target:
FAIL: gcc.c-torture/execute/ieee/acc1.c execution
--cut here--
double func (const double *array)
{
double d = *array;
if (d == 0.0)
return d;
else
return d + func (array + 1);
}
int main ()
{
double values[] = { 0.1e-100, 1.0, -1.0, 0.0 };
if (func (values) != 0.1e-100)
abort ();
exit (0);
}
--cut here--
gcc -O2 -fexcess-precision=standard -m32 creates following optimized tree dump:
--cut here--
<bb 2> [local count: 1073741824]:
values[0] = 1.00000000000000005171617276904849891057306773641595375548e-101;
values[1] = 1.0e+0;
values[2] = -1.0e+0;
values[3] = 0.0;
_9 = func (&MEM[(void *)&values + 8B]);
_10 = (long double) _9;
_11 = _10 + 1.00000000000000005171617276904849891057306773641595375548e-101;
_12 = (double) _11;
_1 = (long double) _12;
if (_1 != 9.9999999999999999997834478666160678572417847571029307194e-102)
goto <bb 3>; [0.00%]
else
goto <bb 4>; [99.96%]
<bb 3> [count: 0]:
abort ();
<bb 4> [local count: 1073312328]:
exit (0);
--cut here--
Please note that the last compare is with XFmode constant that is not correctly
truncated to DFmode. The comparison in extended to XFmode (due to
-fexcess-precision=standard option), but the constant is not truncated to
DFmode value, even if it is alter loaded via XFmode load instruction.
(gdb) disass
Dump of assembler code for function main:
0x08048350 <+0>: lea 0x4(%esp),%ecx
...
0x08048384 <+52>: call 0x80484e0 <func>
0x08048389 <+57>: faddl 0x80485c0
0x0804838f <+63>: add $0x10,%esp
0x08048392 <+66>: fstpl -0x30(%ebp)
0x08048395 <+69>: fldt 0x80485d0
0x0804839b <+75>: fldl -0x30(%ebp)
=> 0x0804839e <+78>: fucompp
0x080483a0 <+80>: fnstsw %ax
0x080483a2 <+82>: sahf
0x080483a3 <+83>: jp 0x80483b1 <main+97>
0x080483a5 <+85>: jne 0x80483b1 <main+97>
0x080483a7 <+87>: sub $0xc,%esp
0x080483aa <+90>: push $0x0
0x080483ac <+92>: call 0x8048320 <exit@plt>
0x080483b1 <+97>: call 0x8048340 <abort@plt>
End of assembler dump.
(gdb) i r flo
st0 1.00000000000000005172e-101 (raw 0x3eafb32df8e9f3546800)
st1 9.99999999999999999978e-102 (raw 0x3eafb32df8e9f3546564)
st2 0 (raw 0x00000000000000000000)
st3 0 (raw 0x00000000000000000000)
st4 0 (raw 0x00000000000000000000)
st5 0 (raw 0x00000000000000000000)
st6 0 (raw 0x00000000000000000000)
st7 0 (raw 0x00000000000000000000)
