https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62074
Bug ID: 62074
Summary: "right shift count >= width of type" warning on dead
branch in template code
Product: gcc
Version: 4.10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: brooks at gcc dot gnu.org
Consider the following code:
------------------cut here---------------------
typedef unsigned int uint32;
template <uint32 offset>
int foo(uint32 x) {
if (offset <= 1) {
return x << offset;
} else {
// this branch is dead for offset = 0, but it still generates a warning
return (x << (offset - 1)) +
(x >> (32 - offset));
}
}
int main(int argc, char** argv) {
return foo<0>(argc);
}
------------------cut here---------------------
As noted in the comment, the "else" branch is dead code for the offset=0
instantiation, but (at r213772 on trunk) we still generate warnings for the
shifts that are out of range in the offset=0 case:
$ /home/bmoses/gcc-archive/213772/bin/g++ b16825807.cc -o b16825807
b16825807.cc: In instantiation of ‘int foo(uint32) [with unsigned int offset =
0u; uint32 = unsigned int]’:
b16825807.cc:15:21: required from here
b16825807.cc:9:15: warning: left shift count >= width of type
return (x << (offset - 1)) +
^
b16825807.cc:10:15: warning: right shift count >= width of type
(x >> (32 - offset));
^
(This is also present in the google/gcc-4_9 branch, and thus I suspect present
on upstream 4.9.1 although I have not yet tested it.)
Google bug: 16825807
