[Animats]

What a mess. To support a stupid HTML feature, the browser's parser has to be set up like a superscalar CPU, retirement unit and all. Hopefully the discard operation doesn't happen very often.

i86 CPUs have to do something like this if you store into code just ahead of execution. That was an optimization technique marginally useful in the 1980s. Today it's a performance hit. The CPU is happily looking ahead and decoding instructions, when one of the superscalar pipelines has a store into the instruction stream. The retirement unit catches the conflict between an instruction fetch on one stream and a store into the same location in another. The CPU stalls as instructions up to the changed instructions are committed. Then the CPU is flushed and cleared as for a page fault or context switch, and starts from the newly stored instruction.

Only x86 machines do this, for backwards compatibility with the DOS era. The same thing seems to have happened in the browser area.

(Prospective fetching should be disabled on devices where you pay for data traffic. Is it?)