"A decent compiler should know this sort of thing!"

A decent 8051 compiler like Keil C51 does have the means to know by virtue of its SFR extensions for on-chip registers. For off-chip device registers the problem still exists. Strictly conforming compilers (i.e., without extensions) have the means for expressing "read-only" (i.e., the 'const' qualifier), but lack a means for qualifying something as "write-only".

Probably the best one could hope for, is that qualifying a register declaration with "volatile" would suggest to the compiler that after a writing a value to the register, the register contents might change and thus should not be read as the source to propagate a value across a series of right-to-left associative assignment operators in a single statement.

In the end, it is up to the coder to either recognize that their compiler generates the desired code productions (bad practice), or to code things in an unambiguous manner that avoids this type of issue altogether.