Most floating point tests fail when GDB is built with GCC trunk for mips-linux.
I assume it shows up on all targets. The miscompiled function is
doublest.c:put_field. Here is a reduced test case:
int put_field (unsigned int start, unsigned int len)
{
int cur_bitshift = ((start + len) % 8) - 8;
if (cur_bitshift > -8)
not_removed ();
}
When compiled with -O2, the resulting assembly file does not include a call to
not_removed. But in fact it should be called in seven out of eight cases.
((start + len) % 8) is an unsigned int in [0, 7]. "((start + len) % 8) - 8" is
an unsigned int in [UINT_MAX - 8, UINT_MAX]. cur_bitshift is an int, which can
not represent UINT_MAX; this is an implementation defined conversion, and the
GCC manual says that the value is reduced modulo (UINT_MAX+1) until it fits in
the range of int. So the defined result of the conversion is in [-8, -1].
But VRP says:
Value ranges after VRP:
start_1(D): VARYING
len_2(D): VARYING
D.1559_3: [0, +INF] EQUIVALENCES: { } (0 elements)
D.1560_4: [0, 7] EQUIVALENCES: { } (0 elements)
D.1561_5: [4294967288, +INF] EQUIVALENCES: { } (0 elements)
cur_bitshift_6: [-INF(OVF), -2147483648] EQUIVALENCES: { } (0 elements)
Folding predicate cur_bitshift_6 >= -7 to 0
Removing basic block 3
Merging blocks 2 and 4
put_field (start, len)
{
int cur_bitshift;
unsigned int D.1561;
unsigned int D.1560;
unsigned int D.1559;
<bb 2>:
D.1559_3 = start_1(D) + len_2(D);
D.1560_4 = D.1559_3 & 7;
D.1561_5 = D.1560_4 + 4294967288;
cur_bitshift_6 = (int) D.1561_5;
return;
}
--
Summary: VRP eliminates a useful test due to overflow
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: drow at gcc dot gnu dot org
GCC target triplet: mips-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31605
