[PATCH] D85473: [Clang] Add option to allow marking pass-by-value args as noalias.

[Florian Hahn via Phabricator via cfe-commits]

fhahn added inline comments.


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Comment at: clang/include/clang/Driver/Options.td:4287-4290
+def fpass_by_value_noalias: Flag<["-"], "fpass-by-value-noalias">,
+  HelpText<"Allows assuming no references to passed by value escape before "
+           "transferring execution to the called function. Note that this "
+           "does not hold for C++">;
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rjmccall wrote:
> rsmith wrote:
> > This should be in `Group<f_Group>`.
> The "Note" clause seems to muddy more than it clarifies.  Maybe "has no effect on non-trivially-copyable classes in C++"?  Or add proper documentation somewhere instead of trying to jam this into the help text.
I've added the clarification, thanks!


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Comment at: clang/lib/CodeGen/CGCall.cpp:2198
+        // reference to the underlying object. Mark it accordingly.
+        Attrs.addAttribute(llvm::Attribute::NoAlias);
+
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rjmccall wrote:
> This definitely can't be added unconditionally to all types; you need to rule out non-trivial C++ class types, as well as types with ObjC weak references.
Updated to rule out non trivially-copyable types.

I am not sure how to best handle types with weak references. I did not manage to find any helpers that traverse a whole type to check if it contains weak references. 

I added `clang/test/CodeGenObjC/pass-by-value-noalias.m` and IIUC `noalias` should not be added for that case.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D85473/new/

https://reviews.llvm.org/D85473