llvm dev - Mar 2013 - [LLVMdev] Hidden-visibility aliases to default-visibility globals

Hi,

I am trying to compile a dynamic loader using LLVM.  Part of the IR
for this loader looks like this:

@_rtld_local = hidden alias %struct.rtld_global* @_rtld_global
@_rtld_global = unnamed_addr global %struct.rtld_global { ... }

The purpose of _rtld_local is to allow _rtld_global to be referenced
without using the GOT, as this global is accessed before the dynamic
loader initialises the GOT.  However, LLVM sees through the alias
and resolves references to _rtld_local to _rtld_global, resulting in
segfaults when the dynamic loader is used.

I'm trying to figure out the best way to fix this.  The
GlobalValue::mayBeOverridden() function controls whether LLVM resolves
aliases, and hacking this function to return true if a global is hidden
gets around this particular problem, but converts the function into a
misnomer (a hidden alias may _not_ be overridden, e.g. by LD_PRELOAD,
although I'm pretty sure mayBeOverridden is not intended to care about
LD_PRELOAD) and results in a lot of deoptimisation.  I'm thinking
of introducing a new function GlobalAlias::mayBeResolved() which
returns true if mayBeOverridden() returns false and the visibility
of the alias is the same as that of the aliasee, and converting over
the relevant clients of mayBeOverridden to use this new function.

Thanks,
-- 
Peter

On Wed, Mar 20, 2013 at 02:22:47PM -0700, Peter Collingbourne
wrote:
> I am trying to compile a dynamic loader using LLVM.  Part of the IR
> for this loader looks like this:
> 
> @_rtld_local = hidden alias %struct.rtld_global* @_rtld_global
> @_rtld_global = unnamed_addr global %struct.rtld_global { ... }
Have you tried using protected visibility?

Joerg

On Wed, Mar 20, 2013 at 02:22:47PM -0700, Peter Collingbourne
wrote:
> Hi,
> 
> I am trying to compile a dynamic loader using LLVM.  Part of the IR
> for this loader looks like this:
> 
> @_rtld_local = hidden alias %struct.rtld_global* @_rtld_global
> @_rtld_global = unnamed_addr global %struct.rtld_global { ... }
> 
> The purpose of _rtld_local is to allow _rtld_global to be referenced
> without using the GOT, as this global is accessed before the dynamic
> loader initialises the GOT.  However, LLVM sees through the alias
> and resolves references to _rtld_local to _rtld_global, resulting in
> segfaults when the dynamic loader is used.
> 
> I'm trying to figure out the best way to fix this.  The
> GlobalValue::mayBeOverridden() function controls whether LLVM resolves
> aliases, and hacking this function to return true if a global is hidden
> gets around this particular problem, but converts the function into a
> misnomer (a hidden alias may _not_ be overridden, e.g. by LD_PRELOAD,
> although I'm pretty sure mayBeOverridden is not intended to care about
> LD_PRELOAD) and results in a lot of deoptimisation.  I'm thinking
> of introducing a new function GlobalAlias::mayBeResolved() which
> returns true if mayBeOverridden() returns false and the visibility
> of the alias is the same as that of the aliasee, and converting over
> the relevant clients of mayBeOverridden to use this new function.
This is http://llvm-reviews.chandlerc.com/D606

Thanks,
-- 
Peter

On Thu, Mar 21, 2013 at 02:12:27PM +0900, Joerg Sonnenberger
wrote:
> On Wed, Mar 20, 2013 at 02:22:47PM -0700, Peter Collingbourne wrote:
> > I am trying to compile a dynamic loader using LLVM.  Part of the IR
> > for this loader looks like this:
> > 
> > @_rtld_local = hidden alias %struct.rtld_global* @_rtld_global
> > @_rtld_global = unnamed_addr global %struct.rtld_global { ... }
> 
> Have you tried using protected visibility?
Oops, missed this message.  I think there was some reason why I thought
protected visibility would not work, but I just tried it and it turns
out that it does.  I'll see what upstream thinks and if they're ok
with it I'll drop the patch.

Thanks,
-- 
Peter

I think this is a bug in global opt. We should probably not consider
this a correctness issue in general, but

* It is fine for something low level like a dynamic linker to depend
on some implementation details.
* This is not profitable. Global opt should figure out that accesses
via a hidden alias are faster than via a non-hidden symbol and avoid
the transformation.

On 20 March 2013 17:22, Peter Collingbourne <peter at pcc.me.uk>
wrote:
> Hi,
>
> I am trying to compile a dynamic loader using LLVM.  Part of the IR
> for this loader looks like this:
>
> @_rtld_local = hidden alias %struct.rtld_global* @_rtld_global
> @_rtld_global = unnamed_addr global %struct.rtld_global { ... }
>
> The purpose of _rtld_local is to allow _rtld_global to be referenced
> without using the GOT, as this global is accessed before the dynamic
> loader initialises the GOT.  However, LLVM sees through the alias
> and resolves references to _rtld_local to _rtld_global, resulting in
> segfaults when the dynamic loader is used.
>
> I'm trying to figure out the best way to fix this.  The
> GlobalValue::mayBeOverridden() function controls whether LLVM resolves
> aliases, and hacking this function to return true if a global is hidden
> gets around this particular problem, but converts the function into a
> misnomer (a hidden alias may _not_ be overridden, e.g. by LD_PRELOAD,
> although I'm pretty sure mayBeOverridden is not intended to care about
> LD_PRELOAD) and results in a lot of deoptimisation.  I'm thinking
> of introducing a new function GlobalAlias::mayBeResolved() which
> returns true if mayBeOverridden() returns false and the visibility
> of the alias is the same as that of the aliasee, and converting over
> the relevant clients of mayBeOverridden to use this new function.
>
> Thanks,
> --
> Peter
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev