That is not at all how compilers & cpu engineers work together.
Compilers are not reverse-engineering the CPU, and different CPUs do not behave that radically different to make this non-portable.
The biggest issue by far are the languages themselves. Java, for example, is inherently anti-CPU-friendly by design. You can't get flattened objects, almost everything is done via reference instead of value, etc... This was done for other tradeoffs, but the majority of languages in use were not built for raw CPU throughput.
Compilers are not reverse-engineering the CPU, and different CPUs do not behave that radically different to make this non-portable.
The biggest issue by far are the languages themselves. Java, for example, is inherently anti-CPU-friendly by design. You can't get flattened objects, almost everything is done via reference instead of value, etc... This was done for other tradeoffs, but the majority of languages in use were not built for raw CPU throughput.