In attached code:
in find(), generated code computes offset using multiplication/division.
in set(), generated code computes &data[10] and compares &data[i] with that (to
verify find() fails to be optimised because of overflow rules)
tested gcc: 4.3.4, 4.4.1, 4.5.0-alpha20090827
command line:
gcc main.c -o main.o -c -O3
-f(no-)strict-overflow or -m32 doesn't have any effect on that behaviour
---------------------------------
static struct {
int a, b, c;
char pad[200]; /* padding so offset computation has to use expensive
div/mul */
} data[1000]; /* 1000 so loop isn't unrolled */
int find(int val)
{
int i;
for (i = 0; i < 1000; i++) {
if (data[i].a == val) return &data[i] - data; /* does *212,
/212 */
}
return -1;
}
void set(int val)
{
int i;
for (i = 0; i < 1000; i++) {
data[i].b = &data[i] - data < 10 || data[i].a == val; /*
compares with &data[10] */
}
}
---------------------------------
Relevant generated ASM:
(eax is "i", sar+imul is converted division by 212)
---------------------------------
cdqe
imul rax, rax, 212
sar rax, 2
imul eax, eax, -1944890851
ret
---------------------------------
--
Summary: "&data[i] - data" isn't converted to "i"
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: zsojka at seznam dot cz
GCC host triplet: x86_64-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41244
