[llvm-bugs] [Bug 39427] New: gcc ABI incompatibility when passing llvm::Optional

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39427

            Bug ID: 39427
           Summary: gcc ABI incompatibility when passing llvm::Optional
           Product: clang
           Version: 7.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: tneumann at users.sourceforge.net
                CC: llvm-bugs at lists.llvm.org

Created attachment 21034
  --> https://bugs.llvm.org/attachment.cgi?id=21034&action=edit
ABI demonstration

Calling llvm::ConstantExpr::getGetElementPtr crashes when LLVM 7 is compiled
with gcc 8.2 and the caller is compiled with clang 7.

The reason is an ABI incompatibility: clang wants to pass the
llvm::Optional<unsigned> parameter by value (i.e., as a single i64), while gcc
expects the value to be passed by reference.

I have attached a simple test program that demonstrates the problem explicitly,
regardless how the underlying LLVM was compiled. (It still uses the LLVM 7
headers, but the problem is triggered purely in the example code here).

To reproduce, compile and run the attached program like this:

clang++-7 -c -opart1.o foo.cpp `llvm-config-7 --cxxflags`
g++ -c -opart2.cpp -DPART2 foo.cpp `llvm-config-7 --cxxflags`
clang++-7 -ofoo part1.o part2.o `llvm-config-7 --cxxflags`
./foo

This leads to a crash. Using either g++ or clang++ for both files is fine, but
mixing them leads to a crash. By inspecting the assembler code we can see that
clang++ passes by value, and g++ passes by reference.

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