llvm dev - Apr 2008 - [LLVMdev] CodeGen fails for CallInst with label

It seems like LLVM happily creates function calls that pass in labels
but doesn't know how to emit them.

For the source c code:
__asm__ __volatile__ ( ".byte 0x0f, 0x36, 0x00" : : "a"
(&&label) : "memory" );

llvm-g++ -emit-llvm generates:
tail call void asm sideeffect ".byte 0x0f, 0x36, 0x00",
"{ax},~{dirflag},~{fpsr},~{flags},~{memory}"( label %label ) nounwind

However, trying to assemble x86 binaries fails:
cc1plus:
/mounts/zion/disks/0/localhome/tbrethou/2.2/testing/llvm-2.2/include/llvm/CodeGen/ValueTypes.h:212:
unsigned int llvm::MVT::getSizeInBits(llvm::MVT::ValueType): Assertion
`isExtendedVT(VT) && "ValueType has no known size!"'
failed.
At global scope: internal compiler error: Aborted

This also happens if I craft my own function to take a label and llvm-as:
declare void @foo(label)
call void @bar( label %label )

llvm-as[0x826fa54]
llvm-as[0x826fd1a]
[0x8cd420]
llvm-as(llvm::PATypeHolder::operator llvm::Type*() const+0x11)[0x816e93b]
llvm-as(llvm::Value::getType() const+0x14)[0x816e952]
llvm-as(llvmAsmparse()+0xccc4)[0x818a14e]
llvm-as[0x818c15c]
llvm-as(llvm::RunVMAsmParser(std::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&,
_IO_FILE*)+0x4a)[0x818c5aa]
llvm-as(llvm::ParseAssemblyFile(std::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&,
llvm::ParseError*)+0xf3)[0x8178d65]
llvm-as(main+0x84)[0x815aeb6]
/lib/libc.so.6(__libc_start_main+0xdc)[0x6e8dec]
llvm-as(__gxx_personality_v0+0x2a9)[0x815aa81]

llvm-as:  llvm version 2.0 DEBUG build with assertions.
llvm-g++: gcc version 4.2.1 (Based on Apple Inc. build 5546) (LLVM build 2.2)

Ed

On Apr 19, 2008, at 1:30 AM, Edward Lee wrote:
> It seems like LLVM happily creates function calls that pass in labels
> but doesn't know how to emit them.
Yep, this isn't supported.  We can't quite enforce it as invalid at  
this moment, but don't expect it to work.  The only think you can do  
with labels is branch/switch/invoke to them and use them as the block  
operand to phi nodes.

Note that using the address of a label in GCC for anything other than  
an indirect jump is also undefined and doesn't work in general.

-Chris

On Sat, Apr 19, 2008 at 1:27 PM, Chris Lattner <sabre at nondot.org>
wrote:
> On Apr 19, 2008, at 1:30 AM, Edward Lee wrote:
>  > It seems like LLVM happily creates function calls that pass in labels
>  > but doesn't know how to emit them.
>  Yep, this isn't supported.  We can't quite enforce it as invalid
at
>  this moment, but don't expect it to work.
That's a little bit disappointing for me. Is there some way in LLVM to
get the address of a label and put it in a register for use within
that function?

I'm using a special instruction that effectively branches to the value
in that register.

pseudo-LLVM:
%tmp = label %label
br label %tmp

Except it doesn't behave like a (un)conditional branch of course;
otherwise, I would just use the standard LLVM BranchInst.

Ed