Assertion failure when emitting constructors and destructors as aliases

[MaggieYingYi]

Compile the following code:

class test {
  test();
};
test::test() {}

using

$ rm clang.db
$ clang -O0 -c -target x86_64-pc-linux-gnu-repo -o test.o test.cpp
$ clang -O0 -c -target x86_64-pc-linux-gnu-repo -o test.o test.cpp

The test.cpp is compiled with clang twice.
The first time success and second time failed with following assert error:

Alias must point to a definition
void (%class.test*)* @_ZN4testC1Ev
fatal error: error in backend: Broken module found, compilation aborted!

[MaggieYingYi]

Triage:

The issue is caused by the constructor alias and program repository pruning.
The following IR code is generated by the first compilation:

@_ZN4testC1Ev = alias void (%class.test*), void (%class.test*)* @_ZN4testC2Ev

define void @_ZN4testC2Ev(%class.test* %this) unnamed_addr #0 align 2 !repo_ticket !2 {
entry:
  …
}

By default, the constructor alias is enabled by clang option of ‘-mconstructor-aliases’. It emitted the constructor _ZN4testC1Ev as an alias of _ZN4testC2Ev. When compiling the code again, the constructor _ZN4testC2Ev already exist in the database clang.db. The program repository pruning pass converts the constructor _ZN4testC2Ev from definition to declaration as shown below:

@_ZN4testC1Ev = alias void (%class.test*), void (%class.test*)* @_ZN4testC2Ev

declare !repo_ticket !2 void @_ZN4testC2Ev(%class.test*) unnamed_addr #0 align 2

The constructor _ZN4testC1Ev is an alias of a declared constructor _ZN4testC2Ev, which caused the assertion failure: Alias must point to a definition.

[MaggieYingYi]

Possible Solution:

Short term solution: disable constructor alias for repo target.
Long term solution: support the function alias, which point to a declaration in the module and a definition in the database.

Short term solution:

Change the code in llvm\tools\clang\lib\Driver\ToolChains\Clang.cpp (line 3407).

From:

  if (!RawTriple.isOSDarwin() && !RawTriple.isNVPTX())
    CmdArgs.push_back("-mconstructor-aliases");

To:

  if (!RawTriple.isOSDarwin() && !RawTriple.isNVPTX() &&
      !RawTriple.isOSBinFormatRepo())
    CmdArgs.push_back("-mconstructor-aliases");

[MaggieYingYi]

A note has been added into the readme file (git commit: c871a1c401f82e833).

[paulhuggett]

Closing. The "short term" solution has been with us for a while now. We should revisit implementing support for function aliases later.