[PATCH] D87149: [InstCombine] erase instructions leading up to unreachable

[Chandler Carruth via Phabricator via llvm-commits]

chandlerc added a comment.

Regarding access to volatile memory...

If volatile memory access is known to not trap, then the guidance on LLVM's own blog is wrong:
https://blog.llvm.org/posts/2011-05-13-what-every-c-programmer-should-know/

And in fact, Clang's error message is wrong as it specifically suggests re-writing a dereference of a null pointer to use volatile in order to ensure a trap.

If we insist on following the LangRef rules, we would need to change the lowering of volatile memory accesses in Clang, and that doesn't seem especially useful. I think the LangRef is simply wrong on this point, and we need to admit that access to volatile memory, much like a call to an unknown external function, may trap, unwind, never return, etc. There is a significant amount of real C and C++ code written that relies on these rules, no doubt because LLVM and Clang advertised them. =]


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