What do you mean by "these UB optimizations"? C is a low-level language; it's basically impossible for a compiler to reason about the code unless it makes certain assumptions.
It needs to assume the code is not self-modifying to do pretty much any code-generation more intelligent than a macro assembler. It needs to assume the code isn't messing with the stack frames/return addresses in order to inline functions. It needs to assume the code isn't using an out-of-bounds pointer to access a neighboring local variable, so that it can move local variables into registers.
"gcc -O0" is a good approximation for the performance you get if the compiler isn't allowed to optimize based on UB.
Yes, that means C without optimizing without UB is slower than Java. Optimizations need some form of reasoning about what's happening. For Java it's optimizing based on guarantees provided by the language (there's no raw pointers that could mess with the things listed above). But C doesn't provide any hard guarantees, so instead it needs to blindly assume that the code will behave sanely.
Also note that for many of the more manageable sources of UB, most compilers provide a choice (-fwrapv, -fno-strict-aliasing, ...). Yet few projects use these options, even when they use other gcc/clang-specific features. Doesn't that indicate that C programmers indeed want to sacrifice safety for optimizations?
Yes, that means C without optimizing without UB is slower than Java. Optimizations need some form of reasoning about what's happening. For Java it's optimizing based on guarantees provided by the language (there's no raw pointers that could mess with the things listed above). But C doesn't provide any hard guarantees, so instead it needs to blindly assume that the code will behave sanely.
Also note that for many of the more manageable sources of UB, most compilers provide a choice (-fwrapv, -fno-strict-aliasing, ...). Yet few projects use these options, even when they use other gcc/clang-specific features. Doesn't that indicate that C programmers indeed want to sacrifice safety for optimizations?