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

Thu Sep 3 22:36:25 PDT 2020

rjmccall added a comment.

Is it acceptable to leave this as a -cc1 option while we're pursuing this with the language committee?  Do we have any intent to pursue this with the language committee?

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Comment at: clang/include/clang/Basic/LangOptions.def:372
+LANGOPT(PassByValueIsNoAlias, 1, 0, "assumption that by-value parameters do "
+                                    "not alias any other values")

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This should be a code-gen option, I think.  I can't really imagine how this would affect language processing.

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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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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.

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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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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.

Repository:
  rG LLVM Github Monorepo

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

https://reviews.llvm.org/D85473