[all-commits] [llvm/llvm-project] 166c8c: [msan][CodeGen] Set noundef for C return value

[Vitaly Buka via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 166c8cccde010ea01fdc20945a2a25fa1971cb73
      https://github.com/llvm/llvm-project/commit/166c8cccde010ea01fdc20945a2a25fa1971cb73
  Author: Vitaly Buka <vitalybuka at google.com>
  Date:   2022-12-05 (Mon, 05 Dec 2022)

  Changed paths:
    M clang/lib/CodeGen/CGCall.cpp
    M clang/test/CodeGen/cleanup-destslot-simple.c
    M clang/test/CodeGen/msan-param-retval.c

  Log Message:
  -----------
  [msan][CodeGen] Set noundef for C return value

Msan needs noundef consistency between interface and implementation. If
we call C++ from C we can have noundef on C++ side, and no noundef on
caller C side, noundef implementation will not set TLS for return value,
no noundef caller will expect it. Then we have false reports in msan.

The workaround could be set TLS to zero even for noundef return values.
However if we do that always it will increase binary size by about 10%.
If we do that selectively we need to handle "address is taken"
functions, any non local functions, and probably all function which have
musttail callers. Which is still a lot.

The existing implementation of HasStrictReturn refers to C standard as
the reason not enforcing noundef. I believe it applies only to the case
when return statement is omitted. Testing on Google codebase I never see
such cases, however I've see tens of cases where C code returns actual
uninitialized variables, but we ignore that it because of "omitted
return" case.

So this patch will:
1. fix false-positives with TLS missmatch.
2. detect bugs returning uninitialized variables for C as well.
3. report "omitted return" cases stricter than C, which is already a
   warning and very likely a bug in a code anyway.

Reviewed By: kda

Differential Revision: https://reviews.llvm.org/D139296