support __attribute__((used)

[llvmbot]

Bugzilla Link	3566
Resolution	FIXED
Resolved on	Feb 13, 2009 16:09
Version	unspecified
OS	All
Reporter	LLVM Bugzilla Contributor

Extended Description

clang should support attribute((used)

ddunbar@ozzy:tmp$ cat t.c
static int a attribute((used)) = 52;
ddunbar@ozzy:tmp$ xcc -c t.c
ddunbar@ozzy:tmp$ nm t.o
nm: no name list

[asl]

*** Bug llvm/llvm-bugzilla-archive#3572 has been marked as a duplicate of this bug. ***

[llvmbot]

this is currently blocking FreeBSD kernel linking....

[llvmbot]

Basic implementation.
This is just a quick basic implementation of the attribute

[llvmbot]

the patch makes it work without optimizations, when I switch on -O1 the symbols get removed... anyway, it's an improvement

[llvmbot]

anders posted another patch:

Index: lib/CodeGen/CodeGenModule.cpp

--- lib/CodeGen/CodeGenModule.cpp (revision 64449)
+++ lib/CodeGen/CodeGenModule.cpp (working copy)
@@ -648,7 +649,9 @@
}

// Set the llvm linkage type as appropriate.

• if (D->getStorageClass() == VarDecl::Static)

• if (D->getAttr() != 0)
• GV->setLinkage(llvm::Function::ExternalLinkage);
• else if (D->getStorageClass() == VarDecl::Static)
  GV->setLinkage(llvm::Function::InternalLinkage);
  else if (D->getAttr())
  GV->setLinkage(llvm::Function::DLLImportLinkage);

which with the first one lets me use -O2 compilation, but I am getting some symbols defined twice... I dont know if it's a bug in the patch or if it's
an unrelated issue

[llvmbot]

I just analyzed it... it's another problem... the two patches from anders combined work ok for me, if it's accepted/commited this can be closed

[lattner]

The trick is that attribute(used) should cause codegen to add a pointer to the global to the magic llvm.used array. Try plopping an example into llvm.org/demo to see what I mean.

[llvmbot]

I was not aware of llvm.used. I'll try and make a new patch and post back here if I'm successful or not.

[llvmbot]

New patch that uses a "magic" llvm.used
I think i found a okay solution. There is still some miner stuff to wokr on here, but I'd like to get some feedback on the way i make the array.

Thanks.

[llvmbot]

Sema part here: http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090209/012293.html

The codegen part needs a bit more work than that I believe, will look at now.

[llvmbot]

Fixed here: http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090209/012310.html

[asl]

mentioned in issue llvm/llvm-bugzilla-archive#3572