ThinLTO on Aarch64 fails to handle aliases to overridden weak functions

[samitolvanen]

Bugzilla Link	42542
Resolution	FIXED
Resolved on	Aug 23, 2019 08:24
Version	trunk
OS	Linux
CC	@nickdesaulniers,@pcc,@samitolvanen

Extended Description

C code that defines an alias to a weak function, which is then referred to in assembly code that also overrides the same weak function, fails to compile with -flto=thin on Aarch64:

$ cat alias.c
attribute((weak)) int __a(int n)
{
return n;
}
int a(int n) attribute((alias("__a")));

int main()
{
return __a(0);
}

$ cat alias.s
.arch armv8-a

__a:
b a
.globl __a

$ clang -flto=thin -fvisibility=hidden -fuse-ld=lld --target=aarch64-linux-gnu- alias.s alias.c
Alias must point to a definition
i32 (i32)* @​a
LLVM ERROR: Broken module found, compilation aborted!
clang-9: error: linker command failed with exit code 1 (use -v to see invocation)

Using -flto instead of -flto=thin works.

This bug prevents us from compiling the Linux kernel with ThinLTO, as explained here:
ClangBuiltLinux/linux#509

[llvmbot]

Reproduced and working on a fix.

[llvmbot]

Fix sent for review in D66264. It took some time as I found multiple issues.

[samitolvanen]

Fix sent for review in D66264.

Thanks! I can confirm that this fixes the alias issues with ThinLTO when compiling the kernel.

[llvmbot]

Fix just went in as r369766.