When instrumenting allocas, we use a SmallSet (i.e. SmallPtrSet). When there are fewer elements than the SmallSet size, it behaves like a vector, offering stable iteration order. Once we have too many allocas to instrument, the iteration order becomes unstable. This manifests as non-deterministic builds because of the global constant we create while instrumenting the alloca.
The test added is a simple IR file, but was discovered while building libcxx/src/filesystem/operations.cpp from libc++. A reduced C++ example from that:
// clang++ -fsanitize=memory -fsanitize-memory-track-origins \
// -fno-discard-value-names -S -emit-llvm \
// -c op.cpp -o op.ll
struct Foo {
~Foo();
};
bool func1(Foo);
void func2(Foo);
void func3(int) {
int f_st, t_st;
Foo f, t;
func1(f) || func1(f) || func1(t) || func1(f) && func1(t);
func2(f);
}