It would be easy to have an "auto" clobber list option. The inline assembler would note which registers were touched (mostly trivial, a few instructions have implicit destinations) and would add them all to the clobber list. In about 99% of cases this would be sufficient. I am not aware of any code that conditionally uses a register _and_ conditionally preserves it.
So, 1% of assembly code would use the manual clobber list, but the other 99% would be guaranteed (barring bugs in the compiler) to not have this bug. It seems like the right tradeoff.
Or, instead of an "auto" clobber list the compiler could have a warning if a register is used without being in the clobber list. The programmer could silence that warning in the rare cases where they need to optimize register preservation, and the bugs would be greatly reduced.
If your numbers are roughly correct then I agree it's worth trying to do this. My guess was that the assembly we actually use tends to be aggressively hand-optimised to solve very nice problems in the most optimal way and therefore would be more rather than less likely to trip up analysis, but I haven't experimented.
My experience is that most non-trivial assembly language is doing processing of large chunks of data (high-precision math, encrypting blocks of data, FFTs, etc.) and therefore the startup cost of the function is not significant, so a tiny bit of inefficiency in that area is not something people care about.
Or, put another way, writing tiny little assembly language functions is probably not worth it because the mere fact that you are using assembly language instead of (say) C/C++ means that you have missed many opportunities (code reordering, inlining, etc.) so assembly language functions _should_ be doing enough work to justify their calling cost.
But, I'm not working on an assembler or even using one so I don't think I'll even file a feature request.
So, 1% of assembly code would use the manual clobber list, but the other 99% would be guaranteed (barring bugs in the compiler) to not have this bug. It seems like the right tradeoff.
Or, instead of an "auto" clobber list the compiler could have a warning if a register is used without being in the clobber list. The programmer could silence that warning in the rare cases where they need to optimize register preservation, and the bugs would be greatly reduced.