[PATCH] D81678: Introduce noundef attribute at call sites for stricter poison analysis

[Richard Smith - zygoloid via Phabricator via llvm-commits]

rsmith added inline comments.


================
Comment at: clang/lib/CodeGen/CGCall.cpp:2110
+      DetermineNoUndef(RetTy, getTypes(), DL, RetAI)) {
+    RetAttrs.addAttribute(llvm::Attribute::NoUndef);
+  }
----------------
rsmith wrote:
> This only applies in C++. In C, it's valid for a function to omit its `return` statement if the result value is unused, and in that case the returned value will legitimately be `undef`.
> 
> Even in C++, we're conservative about enforcing the constraint that functions return a proper value and control flow doesn't just fall off the bottom of the function. See the full set of checks here: https://github.com/llvm/llvm-project/blob/master/clang/lib/CodeGen/CodeGenFunction.cpp#L1322
It would seem sensible to suppress this for `extern "C"` functions too, on the basis that the definition of such functions is probably in C code, and thus they might return `undef`.


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  https://reviews.llvm.org/D81678/new/

https://reviews.llvm.org/D81678