mrs $ cat /tmp/t2.cc
typedef long GLint;
void aglChoosePixelFormat(const GLint *);
void find(const int *alistp) {
const int *blist;
int list[32];
if (alistp)
blist = alistp;
else {
list[3] = 42; /* this store disappears with -fstrict-aliasing */
blist = list;
}
aglChoosePixelFormat((GLint*)blist);
}
mrs $ ./xgcc -B./ -S -O1 /tmp/t2.cc -fno-strict-aliasing && grep 42 t2.s
li r0,42
mrs $ ./xgcc -B./ -S -O1 /tmp/t2.cc -fstrict-aliasing && grep 42 t2.s
mrs $
The store cannot be removed, as the converted pointer can be used inside the
aglChoosePixelFormat function to access the value. Since the optimizer can't
see that it isn't used, the optimizer must assume it can as the function isn't
marked pure. If it had been, then the optimization would be ok.
t2.cc.035t.dce1 is the first pass that doesn't have the store, t2.cc.034t.fre
has the store.
This worked in 3.3, but not 4.0.1. This doesn't work in gcc version 4.2.0
20060815 (experimental) on powerpc-apple-darwin9.0.0d1, nor on
i686-apple-darwin8.
--
Summary: strict-aliasing bug
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: mrs at apple dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28778
