There always was an "Intel advantage" to compilers for decades (admittedly Intel invested in compilers more than AMD, but they also were sneaky about trying to nerf AMD in compilers), but with AMD being such a clear leader for so many years, I would hope at least GCC has started supporting AMD flavors of compilation better.
Anyone know if this has happened with GCC and AMD silicon? Or at least is there a better body of knowledge of what GCC flags help AMD more?
But in general I think it's a bit of a red herring for the thrust of my original post; first off you always have to target the benchmark to test a hypothesis, you don't run them in isolation for zero reason. My hypothesis when I ran my own for instance was "General execution of bog standard scalar code is only up by about 50-60%" and using the exact same binary instructions was the baseline criteria for that; it was not "Does targeting a specific microarchitecture scheduling model yield specific gains." If you want to test the second one, you need to run another benchmark.
There are too many factors for any particular machine for any such post to be comprehensive, as I'm sure you're aware. I'm just speaking in loose generalities.
Note that -march will use instructions that might be unavailable on other CPUs of the target. -mtune (which is implied by -march) is the flag that sets the cost tables used by instruction selection, cache line sizes, etc.
The microarchitectures have improved significantly though, which does matter. For instance, Haswell-era AVX2 implementations were significantly poorer than the modern ones in, say, Tiger Lake or Zen 3. The newer ones have completely different power usage and per-core performance characteristics for AVX code; even if you could run AVX2 on older processors, it might not have actually been a good idea if the cumulative slowdowns they cause impacts the whole system (because the chips had to downclock the whole system so they wouldn't brownout). So it's not just a matter of instruction sets, but their individual performance characteristics.
And also, it is not just CPUs that have improved. If anything, the biggest improvements have been in storage devices across the stack, which now have significantly better performance and parallelism, and the bandwidth has improved too (many more PCIe lanes). I can read gigabytes a second from a single NVMe drive; millions of IOPS a second, which is vastly better than you could 7 years ago on a consumer-level budget. Modern machines do not just crunch scalar code in isolation, and neither did older ones; we could just arbitrage CPU cycles more often than we can now in an era where a lot of the performance "cliffs" have been dealt with. Isolating things to just look at how fast the CPU can retire instructions is a good metric for CPU designers, but it's a very incomplete view when viewing the system as a whole as an application developer.