https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63278
Bug ID: 63278
Summary: Fails to compute loop bound from constant string
Product: gcc
Version: 4.9.1
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: rguenth at gcc dot gnu.org
For
inline constexpr size_t hashString(const char* str, size_t hash = 5381)
{
return (*str) ? hashString(str + 1, hash + (hash << 5) + *str) : hash;
}
inline size_t hashString2(const char* str) __attribute__ ((pure));
size_t hashString2(const char* str)
{
size_t hash = 5381;
while (*str) {
hash += (hash << 5) + *str;
str++;
}
return hash;
}
size_t foo()
{
return hashString("foo") + hashString2("foo");
}
that is, the loop created by tail-recursion and the manual loop we fail
to compute the number of iterations and thus fail to unroll them.
Both look like
<bb 2>:
<bb 3>:
# str_28 = PHI <"foo"(2), str_10(4)>
# _29 = PHI <102(2), _4(4)>
# hash_30 = PHI <5381(2), hash_9(4)>
_6 = hash_30 << 5;
_8 = (long unsigned int) _29;
_23 = _6 + _8;
hash_9 = _23 + hash_30;
str_10 = str_28 + 1;
_4 = *str_10;
if (_4 != 0)
goto <bb 4>;
else
goto <bb 8>;
<bb 4>:
goto <bb 3>;
Similar cases may occur when iterating over a constant initializer
guarded by a NULL pointer.
SCEV cannot handle the above (obviously), but eventually hacking up
some special-case code may be worth the trouble.
