gcc ABI incompatibility when passing llvm::Optional

[llvmbot]

Bugzilla Link	39427
Resolution	FIXED
Resolved on	Mar 07, 2019 00:13
Version	7.0
OS	Linux
Blocks	#38454
Attachments	ABI demonstration
Reporter	LLVM Bugzilla Contributor
CC	@aleden,@d0k,@dwblaikie,@zmodem,@cuviper,@slacka,@zygoloid,@rnk,@serge-sans-paille,@sylvestre,@TNorthover,@tstellar
Fixed by commit(s)	r346985

Extended Description

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 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.

[rnk]

Yeah, this is bad. It's been discussed on llvm-dev and the cause is known, but the ship has sailed:
http://lists.llvm.org/pipermail/llvm-dev/2018-September/126564.html
http://llvm.org/viewvc/llvm-project?view=revision&revision=342637

Maybe we can fix it for 7.0.1.

Surprisingly, I don't think we had a bug for it in the tracker until now, just mailing list discussion.

[tstellar]

Is it correct that the consequences of backporting r342637 are:

• 7.0.0->7.0.1 ABI break for libLLVM-7.so that was compiled by gcc.
• gcc < 7.3 won't be able to compile LLVM 7.0.1.

[zmodem]

Is it correct that the consequences of backporting r342637 are:

• 7.0.0->7.0.1 ABI break for libLLVM-7.so that was compiled by gcc.
• gcc < 7.3 won't be able to compile LLVM 7.0.1.

No, r342637 just reverted trunk back to the state we were in when branching for 7.0.0 IIRC.

This isn't fixed on trunk yet.

My proposal is to go back to what we had before r322838.

[cuviper]

I believe they're hitting this in the Debian bug I just linked, where clang-built libLLVM.so is called by rustc's src/rustllvm built with GCC.

[sylvestre]

Tom, do you know how you are going to fix that?
Thanks

[tstellar]

Tom, do you know how you are going to fix that?
Thanks

I think Han's suggestion for fixing it is fine for trunk.

For our Fedora builds I know we are going to fix it by keeping the ABI the same on gcc builds and changing the ABI on clang builds to match gcc (i.e removing the #if block that only is active for clang).

For the upstream 7.0.1 release, I don't see a way to fix this without breaking the ABI for at least one of the compilers. Since clang is an llvm project, my suggestion for the release_70 branch would be to fix it the opposite of how we will fix it in Fedora, which means to keep the clang ABI the same and change the gcc ABI to match it.

[zmodem]

It's confusing to keep track of the changes here.

I believe this is the change that introduced the Clang-GCC ABI difference, by adding the #ifdefs:

---

commit 933d1a7
Author: Benjamin Kramer benny.kra@googlemail.com
Date: Thu Jan 18 16:23:40 2018 +0000

[ADT] Just give up on GCC, I can't fix this.

While the memmove workaround fixed it for GCC 6.3. GCC 4.8 and GCC 7.1
are still broken. I have no clue what's going on, just blacklist GCC for
now.

Needless to say this code is ubsan, asan and msan-clean.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322862 91177308-0d34-0410-b5e6-96231b3b80d8

---

IIUC that was the end of a chain of commits trying to make Optional trivially copyable, see #35326 .

The reason for #ifdefs is that several versions of GCC miscompiles #ifdefed out code otherwise.

My suggestion is that for trunk, we back out the whole thing. It's caused more problems than it's solving I think. I believe that means backing out to the state before r322838.

For our Fedora builds I know we are going to fix it by keeping the ABI the same on gcc builds and changing the ABI on clang builds to match gcc (i.e removing the #if block that only is active for clang).

I think removing the clang-only #if block is the correct fix for the branch.

For the upstream 7.0.1 release, I don't see a way to fix this without breaking the ABI for at least one of the compilers. Since clang is an llvm project, my suggestion for the release_70 branch would be to fix it the opposite of how we will fix it in Fedora, which means to keep the clang ABI the same and change the gcc ABI to match it.

If we do this, many versions of GCC will misscompile the code. That's why the #if's were added. So I think removing the clang-only #if block is a better fix, even if it disturbs the ABI more (i.e. changes it for clang-built binaries).

Ben, do you have any input on this?

[d0k]

We had 3 more attempts at fixing this that broke with random versions of GCC. Let's just revert it, I don't think there are any dependencies on it in trunk at the moment.

It will make clang-tidy emit many more warnings and you can't put llvm::Optional into a union anymore, but that's better than an ABI break.

[sylvestre]

In Debian & Ubuntu, I am taking the risk to break the ABI.
AFAIK, "only" rustc was impacted

I uploaded a new version of the package with this patch:
https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/blob/7/debian/patches/pr39427-misscompile.diff
It fixed the rust crash.

However, the test case from this bug still fail (there is a small typo in comment #​0)
Should be
g++ -c -opart2.o -DPART2 foo.cpp llvm-config-7 --cxxflags
(instead of g++ -c -opart2.cpp -DPART2 foo.cpp llvm-config-7 --cxxflags)

#​0 0x00000000004005c0 in bar(llvm::Type*, llvm::Constant*, llvm::ArrayRefllvm::Value*, bool, llvm::Optional, llvm::Type*) ()
#​1 0x00000000004005ad in main ()

[tstellar]

Patch to fix this in trunk: https://reviews.llvm.org/D54540

[tstellar]

This has been merged to trunk as r346985. For the stable release, what do people think about holding this out of 7.0.1 and then doing another release after 7.0.1 that includes only this patch (either 7.0.2 or 7.1.0), so that users can get all the fixes without the ABI change if they want.

[llvmbot]

Ping. What happens now?

[rnk]

The way I see it, users care about GCC/clang library ABI compatibility, so we need to put out a release with this bug fixed somewhere, even though it technically breaks our ABI stability promise. I seem to recall Tom Stellard suggesting that we put out two simultaneous releases: 7.0.1 and 7.0.2, which are the same, except 7.0.2 takes the patch to revert the ifdefs from llvm::Optional. Personally, this seems like extra complexity, especially because most Linux distros are carrying the patch to revert the llvm::Optional ifdefs anyway. Pulling the revert into 7.0.1 simplifies their lives. Ultimately, Tom is the 7.0.1 release manager, so it is his decision as to whether it is worth the extra complexity and effort to make two releases with two sets of pre-built packages.

[llvmbot]

Reid, 7.0.1 is out already. Hence, I'm pinging for some kind of progress here ;-).

There were also suggestions to make it 7.1.0 but I don't know if the progress on removing minor version number didn't already made that impracticable. One thing to note is that we need to change SOVERSION for this release.

[tstellar]

I am planning to put this fix into a 7.1.0 release. I will update this issue when the branch is ready for testing/review.