wrong weak alias optimization with llvm-gcc-4.2

[llvmbot]

Bugzilla Link	1678
Resolution	FIXED
Resolved on	Sep 03, 2008 02:26
Version	unspecified
OS	Linux
Depends On	llvm/llvm-bugzilla-archive#2742
Reporter	LLVM Bugzilla Contributor
CC	@nlewycky

Extended Description

This bug only effects llvm-gcc-4.2. Here is the testcase:

extern void B (void);
static __typeof(B) A attribute ((weakref("B")));
int active (void)
{
static void *const p = extension (void *) &A;
return p != 0;
}

This is optimized to:

@​A = alias weak void ()* @​B
define i32 @​active() {
entry:
ret i32 1
}

which is wrong (see #2018 ). The cause is quite interesting though:
llvm-gcc emits the function @​active and runs per function passes on
it. At that point A has not yet been declared an alias for B! It
is simply an ordinary global. Thus the optimizers correctly decide
that &A is not null. The alias is only output later.

This shows a danger with per-function passes: if a function refers
to globals then those globals had better be in their final state
before any passes are run.

[nlewycky]

Would this be solved if llvm-gcc created A with ghost linkage? That seems to be the closest fit for what llvm-gcc is doing here.

[llvmbot]

What is ghost linkage?

[nlewycky]

Ghost linkage is used when only parts of a file have been streamed out of a .bc -- ghost linkage indicates that something exists but the full definition isn't available. I think that makes sense in this case.

[llvmbot]

I still don't get it. Got an example?
PS: linkage types don't seem to be documented in the LangRef.

[nlewycky]

Linkage types are documented, http://llvm.org/docs/LangRef.html#linkage but you'll note that ghost is missing (how mysterious).

The comment in include/llvm/GlobalValue.h documents it as "Stand-in functions for streaming fns from BC files".

Recall that you don't need to read in an entire bitcode file to use part of it; just JIT the functions you need without paying for the I/O of reading every function out of the bitcode. The functions which haven't been read yet are marked 'ghost' linkage.

I'm suggesting that this has the exact semantics that we want; it means that we know this global exists, but its full definition isn't visible in the IR yet. llvm-gcc could emit the global as ghost linkage then come back later and fill it in properly.

Sorry, but I don't have a concrete example.

[llvmbot]

The #linkage section doesn't seem to exist; instead there is a
section on visibility styles that's not mentioned in the contents!

Anyway, I'm closing this bug because it has been fixed! The
output is now:

@​A = alias weak void ()* @​B ; <void ()*> [#uses=1]

define i32 @​active() nounwind {
entry:
ret i32 zext (i1 icmp ne (void ()* @​A, void ()* null) to i32)
}

declare extern_weak void @​B()

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#2742