If I cargo build --release a Rust binary on an AMD CPU and then run it on an Intel (or vice versa), could that be a problem (compatibility issues and/or considerable performance sacrifice)? I know we can use a target-cpu=<cpu> flag and that should result in a potentially more optimized machine code for the target platform. My questions are:
Practically speaking, if we build for one platform but run on the other, should we expect a significant runtime performance penalty?
If we build on AMD with target-cpu=intel (or vice versa), could the compilation itself be:
Note: Linux will be the OS for both compilation and running.
In general, if you just do a cargo build --release without further configuration, then you will get a binary that runs on any machine of the relevant architecture. That is, it will run on either an Intel or AMD CPU that's x86-64. It will also be optimized for a general CPU of that architecture, regardless of what type of CPU you build it on. The specific settings are going to depend on whatever rustc and LLVM are configured for, but unless you've done a custom build, that's usually the case.
Usually that is sufficient for most people's needs and building for a target CPU is unnecessary. However, if you specify a particular CPU, then it will be optimized for that CPU and may contain instructions that don't run on other CPUs. For example, the architectural definition for x86-64 doesn't contain things like AVX, which is a later addition, so if you compile for a CPU providing those instructions then rustc may use them, which may cause it to perform worse or not at all elsewhere.
It's impossible to say more about your particular situation without knowing more about your code and performance needs. My recommendation is just to ues cargo build --release and not to optimize for a specific CPU unless you have measured the code and determined that there is a particular section which is slow and which would benefit from that. Most people benefit greatly from the additional portability of the code and don't need CPU-specific optimizations.
Everything I've said here is also true of other sets of architectures. If you compile for aarch64-unknown-linux-gnu or riscv64gc-unknown-linux-gnu, it will build for a generic CPU of that type which works on all systems of that type unless you specify different options. The exception tends to be on systems like macOS, where it is specifically known that all CPUs that run on that OS have some specific set of features, and thus a compilation for e.g. x86-64 CPUs on macOS might optimize for Intel CPUs with given features since macOS only runs on hardware that uses those CPUs.