On 1/2/2012 5:24 PM, Rolf Eike Beer wrote:
It looks like gcc simply optimizes out any reference to the tested symbol.
Given the fact that we don't really ever use that symbol anywhere it is even
correct to do so. Just that this is not what we want.
Yuck. Clearly a less-optimizable approach is needed.
I pushed a topic test-symbol-exists to stage that verifies that this is indeed
a problem in Check{,CXX}SymbolExists, it iterates through all build
configurations (were this problem does _not_ happen) and later explicitely
adds a -O3 test if it is gcc/g++ which then _does_ fail.
So, there are 3 points that I need advise:
-what should happen with this topic? Should I merge it to next? Then we will
get a ton of failures on at least everything that is using GNU compilers.
Certainly we should add tests for these modules. They are critical to
many projects. However, let's not add a test when we know it will break
immediately. Please also rebase the topic so it does not depend on your
FindDoxygen topic. Add a commit prior to the test to fix the behavior
as discussed below.
-which other compilers can be tested this way, I mean using -O3? From what I
know Clang does understand this and even -O4, what is about the Intel one?
Let's start with the GNU case and add more later. I suspect that if we can
convince GCC -O3 to keep the symbol (see below) then no other compiler will
be able to optimize it out.
-how to fix this thing at the end? I was thinking about just doing
#include<stdio.h>
printf("%s is at %p\n", ${SYMBOL},&${SYMBOL}));
Is anyone aware of some place where this wouldn't work? Or should I just put
this in and try out?
We don't want the test to depend on any symbols other than the one tested,
even runtime library symbols. Some cross-compilers may not provide everything
by default. We just need to make the return value depend on the symbol. I
worked this out in "Modules/CMakeCCompilerId.c.in". Note that the return
value should also depend on a runtime argument (argc) so the optimizer cannot
make any assumptions.
I think this will work:
int main(int argc, char* argv[])
{
(void)argv;
return ((int*)(&SYMBOL))[argc];
}
-Brad
--
Powered by www.kitware.com
Visit other Kitware open-source projects at
http://www.kitware.com/opensource/opensource.html
Please keep messages on-topic and check the CMake FAQ at:
http://www.cmake.org/Wiki/CMake_FAQ
Follow this link to subscribe/unsubscribe:
http://public.kitware.com/cgi-bin/mailman/listinfo/cmake-developers
