With the tip of the 4.2 branch, the following program returns 1. Mainline
returns 0. Is this defined behavior? I could not find anything on the
subject.
struct S {
unsigned b4:4;
unsigned b6:6;
} s;
int main(void){
s.b6 = 31;
s.b4 = s.b6;
s.b6 = s.b4;
return s.b6 == 15 ? 0 : 1;
}
before FRE (-fdump-tree-ccp):
;; Function main (main)
main ()
{
short unsigned int D.1882;
short unsigned int D.1881;
int D.1880;
<unnamed type> D.1879;
<unnamed type> D.1878;
<unnamed type> D.1877;
<unnamed type> D.1876;
<bb 2>:
s.b6 = 31;
D.1876_3 = s.b6;
D.1877_4 = (<unnamed type>) D.1876_3;
s.b4 = D.1877_4;
D.1878_7 = s.b4;
D.1879_8 = (<unnamed type>) D.1878_7;
s.b6 = D.1879_8;
D.1881_10 = BIT_FIELD_REF <s, 16, 0>;
D.1882_11 = D.1881_10 & 1008;
D.1880_12 = D.1882_11 != 240;
return D.1880_12;
}
after FRE (-fdump-tree-fre):
;; Function main (main)
main ()
{
short unsigned int D.1882;
short unsigned int D.1881;
int D.1880;
<unnamed type> D.1879;
<unnamed type> D.1878;
<unnamed type> D.1877;
<unnamed type> D.1876;
<bb 2>:
s.b6 = 31;
D.1876_3 = 31;
D.1877_4 = (<unnamed type>) D.1876_3;
s.b4 = D.1877_4;
D.1878_7 = D.1877_4;
D.1879_8 = 31;
s.b6 = D.1879_8;
D.1881_10 = BIT_FIELD_REF <s, 16, 0>;
D.1882_11 = D.1881_10 & 1008;
D.1880_12 = D.1882_11 != 240;
return D.1880_12;
}
D.1879_8 was replaced by 31, ignoring the fact that the value should have been
truncated to 15 when assigned to s.b4.
--
Summary: FRE ignores bit-field truncation
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: TabonyEE at austin dot rr dot com
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31136
