Hi,
I have some code that computes a result from values in the type punning struct below. The struct is initialised with a 64 bit integer and the computations use the values of l1 and l2, i.e. it relies on type punning. The computation accesses all members via the union.
struct pun {
union {
struct {
uint32_t l1;
uint32_t l2;
};
uint64_t v;
};
pun (uint64_t v) : v(v) { }
};
When I compile my code thus:
g++ -std=c++0x -g -fnon-call-exceptions -fasynchronous-unwind-tables -ffast-math -msse4 -MMD -O3 -fno-threadsafe-statics -fno-omit-frame-pointer --param inline-unit-growth=70 -Wstrict-aliasing -Wformat -Werror=format -Wno-multichar -Werror=sequence-point -Werror=overflow -Werror=nonnull -Werror=return-type -Werror=unused-function -Werror=array-bounds
there are instances when the compiler chooses to pre-compute the value at compile time, which is nice. Unfortunately the resulting value doesn’t match the result of executing the same computation at run-time. It appears that the value at compile time is the value that would be derived if l2 (above) was zero rather than the non-zero value it takes in this instance.
I’ve read http://gcc.gnu.org/onlinedocs/gcc-4.4.1/gcc/Optimize-Options.html#Type_002dpunning which I interpret as meaning the above is ok.
Is the above unsafe on gcc 4.8? Have I tripped up over a known issue?
Andy
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