That Time The Windows Kernel Fought Gamma Rays Corrupting Its Processor Cache (microsoft.com)

Long-time Microsoft programmer Raymond Chen recently shared a memory about an unusual single-line instruction that was once added into the Windows kernel code -- accompanied by an "incredulous" comment from the Microsoft programmer who added it:

;
; Invalidate the processor cache so that any stray gamma
; rays (I'm serious) that may have flipped cache bits
; while in S1 will be ignored.
;
; Honestly. The processor manufacturer asked for this.
; I'm serious.
invd

"Less than three weeks later, the INVD instruction was commented out," writes Chen. "But the comment block remains.

"In case we decide to resume trying to deal with gamma rays corrupting the the processor cache, I guess."

Comments

That's a great comment

[NotSoHeavyD3]

Since it explains the reasoning why that code is there.(Since another developer could come by and wonder why that code is there.) I've seen way too many people put in a comment like ;invalidate cache and call it a day.

Commented out code

[DigressivePoser]

The comment block was descriptive and necessary, but it should also include processor errata info to trace back to published documentation. Perhaps this was something newly discovered and the processor and software engineers were in close communications.

"Less than three weeks later, the INVD instruction was commented out," writes Chen. "But the comment block remains.

I don't like seeing commented out code. If it's commented out then it has no business being in the source code file - even if there's an explanation in the comment block. The code's removal along with its comment block should be documented in whatever revision control system is in use. Maybe I'm bias because I worked in safety critical environments where commented out code is a no-no.