问题
When compiling/linking C/C++ libraries or programs that are meant to work on all implementations of an ISA (e.g. x86-64), what optimization flags are safe from the correctness and run-time performance perspectives? I want optimizations that yield correct results and won't be detrimental performance-wise for a particular CPU. E.g I would like to avoid optimization flags that yield run-time performance improvements on an 8th-gen Intel Core i7, but result in performance degradation on an AMD Ryzen.
Are PGO, LTO, and -O3 safe? Is it solely dependent on -march and -mtune (or the absence thereof)?
回答1:
They're all supposed to be "safe", assuming that your code is well defined.
If you don't want to specialize for a particular CPU family then just leave -march and -mtune alone; the default suits a generic x86_64.
PGO is always a good idea, it's mostly used for avoiding branches.
LTO and -O3 can have different effects on different code-bases. For example, if your code benefits from vectorization then -O3 is a big win over -O2, but the extra inlining and unrolling can lead to larger code sizes, and that can be a disadvantage on systems with more limited caches.
In the end, the only advice that ever really means anything here is: measure it and see what's good for your code.
来源：https://stackoverflow.com/questions/52435573/what-gcc-optimization-flags-and-techniques-are-safe-across-cpus