This is not a proposal, but rather a starting point for a discussion.
I've been running a fair amount of ostensibly portable code through clang, and I'm getting a bunch of warnings that, while correct, are (I believe) unhelpful.
Here are two examples:
1) libjpeg tests for integer size mismatch/overflows with:
if ((long) jd_samplesperrow != samplesperrow)
and clang helpfully responds with (when compiling on x86_64):
jdmaster.c:285:31: warning: Both operands to '!=' always have the same value
2) crypto++ has a template that looks like this:
template <class T>
std::string IntToString(T a, unsigned int base = 10)
{
if (a == 0)
return "0";
bool negate = false;
if (a < 0)
{
negate = true;
… and so on.
and when instantiated like this
IntToString<unsigned long>
clang pipes up with:
./misc.h:414:8: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
if (a < 0)
I can find other examples if people want to see them.
Both of these warnings are correct (in the current environment), and yet, I don't think they're that helpful (in these cases).
(For the crypto++ case, I edited the makefile to add -Wno-tautological-compare)
Anyone have any ideas about what can be done to recognize these kinds of cases and not warn on them (as opposed to other
places where the warnings are more useful)?
Thanks in advance!
-- Marshall
This is not a proposal, but rather a starting point for a discussion.
I've been running a fair amount of ostensibly portable code through clang,
and I'm getting a bunch of warnings that, while correct, are (I believe)
unhelpful.
Here are two examples:
1) libjpeg tests for integer size mismatch/overflows with:
if ((long) jd_samplesperrow != samplesperrow)
and clang helpfully responds with (when compiling on x86_64):
jdmaster.c:285:31: warning: Both operands to '!=' always have the same value
This message comes from the static analyzer. In the general case, the static
analyzer may have proven interesting things about these values in making this
conclusion, so I would imagine this is often a useful warning. What is it
about this specific case which makes it unhelpful?
2) crypto++ has a template that looks like this:
template <class T> std::string IntToString(T a, unsigned int base = 10)
{
if (a == 0) return "0"; bool negate = false; if (a < 0) {
negate = true;
… and so on.
and when instantiated like this IntToString<unsigned long>
clang pipes up with: ./misc.h:414:8: warning: comparison of unsigned
expression < 0 is always false [-Wtautological-compare] if (a < 0)
This is known, and already filed as PR8682. My view is that we should only
issue this warning (and possibly others like it, on a case-by-case basis) if
it is true for all possible instantiations. One conservative approach to this
is to only warn if we detect the problem while parsing the template (this
would incidentally also fix the issue that, for the non-dependent problem
cases, we currently see this warning once for the template and once in each
instantiation).
I can find other examples if people want to see them.
The more specific you can be about problems, the better.
Richard