Verifier allows invalid aliases

[llvmbot]

Bugzilla Link	2144
Resolution	FIXED
Resolved on	Mar 22, 2008 03:38
Version	trunk
OS	Windows XP
Attachments	fix and test case
Reporter	LLVM Bugzilla Contributor
CC	@asl

Extended Description

The verifier check for aliases is not strong enough, in particular it does not check that the aliasee is a function or variable. Patch and test case attached.

I'm assuming that the (assembly language) documentation is correct and that aliases to aliases are illegal. However, there is an issue in that the documentation says that bitcasts of global values are legal. I think should be bitcasts of global variables or functions, but this is not reflected in my patch.

[asl]

In general, aliases to aliases are legal. GCC even contains the code to resolve aliasing chains. I recently added something similar to LLVM also. The only tricky case is asm emission, where we need to emit the 'last' aliasee always. This is currently not implemented.

Alias to something bitcasted is perfectly legal. Pretty typical situation - alias to some external declaration, which was overridden later during linking.

[llvmbot]

Okay. Would be nice to update the assembly documentation to describe this. I assume bitcast of alias is fine. So the actual criterion is that the alias chain should end in a function or variable? And cycles are probably not allowed...

Should I expect bytecode linking with chained aliases to work? It doesn't seem to but I haven't investigated because I thought the problem was the chained alias.

This should invalidate llvm/llvm-bugzilla-archive#2145 .

[asl]

I added first support for chained aliases only yesterday during adding of TLS bits (resolveAliasedGlobal()). So, most probably, stuff can be broken in several places. However, I don't see any 'bad' things during linking. Maybe I'm wrong.

I don't have access to repository right now, but I'll upgrade docs and check different stuff as soon as possible.

[asl]

Documentation and Verifier was updated in:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20080317/060065.html

Testcase here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20080317/060066.html

Please reopen, if something 'bad' with chained aliases will be found.