cmd/compile: miscompilation in pointer operations [1.18 backport]

[gopherbot]

@bcmills requested issue #52953 to be considered for backport to the next 1.18 minor release.

@gopherbot, please backport to Go 1.17 and 1.18. This bug causes miscompilation of otherwise-correct Go programs.

[randall77]

I'm not sure this needs backporting. The bug was only found by fuzzing the compiler, not in a real program. The code to trigger it is unusual (reinitializing a variable using a pointer to that very same variable), and easily worked around.

That said, the fix is pretty safe, and if you do hit the bug it causes incorrect behavior.

[bcmills]

The bug was only found by fuzzing the compiler, not in a real program. The code to trigger it is unusual (reinitializing a variable using a pointer to that very same variable), and easily worked around.

That may be true, but the behavior is also pretty subtle — I wouldn't be at all surprised if there are a few real-world programs that do trigger it but don't notice the incorrect behavior, and of course we don't have any way to identify them if they're still on an older Go release.