llvm dev - Apr 2011 - [LLVMdev] gcroot + `section not found for addresss ...' ???

Hi,

If I type

define i32 @main() gc "shadow-stack" {
entry:
  %0 = alloca i8*
  %1 = call i8* @malloc(i64 1)
  store i8* %1, i8** %0
  call void @llvm.gcroot(i8** %0, i8* null)
  ret i32 0
}

declare i8* @malloc(i64)

declare void @llvm.gcroot(i8**, i8*) nounwind

in test.ll

and then do
> llc test.ll
> gcc test.s
I get the error

ld: in /var/folders/Ea/EakEx6b+GBSo6cUn2AnFZk+++TI/-Tmp-//cc5d3RCx.o, section
not found for address 0x64200E00000004CD

If I remove line containing malloc and the line after that, no such error
happens. If I remove
the gcroot line then no such error happens.

Any hints as to what this error means would be great!

Cheers,
N

The linker is going off in the weeds trying to parse the dwarf unwind info.  
The CIE has:


Leh_frame_common_begin0:
	.long	0                       ## CIE Identifier Tag
	.byte	1                       ## DW_CIE_VERSION
	.asciz	 "zLR"                  ## CIE Augmentation
	.byte	1                       ## CIE Code Alignment Factor
	.byte	120                     ## CIE Data Alignment Factor
	.byte	16                      ## CIE Return Address Column
	.byte	2                       ## Augmentation Size
	.byte	16                      ## LSDA Encoding = pcrel
	.byte	16                      ## FDE Encoding = pcrel


But the FDE has:

	.byte	 0                       ## Augmentation size

The linker is expecting that since the CIE said there was an LSDA in each FDE
that is pcrel encoded, that there would in fact be an LSDA in each FDE.  But in
this case, the compiler is trying to omit the LSDA by marking that the FDE has a
zero sized augmentation data.

I'll have to research if this is a valid dwarf unwind optimization.  Even if
it is, it is tripping up the existing linker.

-Nick


On Apr 10, 2011, at 6:06 AM, Nicolas Ojeda Bar wrote:
> Hi,
> 
> If I type
> 
> define i32 @main() gc "shadow-stack" {
> entry:
>  %0 = alloca i8*
>  %1 = call i8* @malloc(i64 1)
>  store i8* %1, i8** %0
>  call void @llvm.gcroot(i8** %0, i8* null)
>  ret i32 0
> }
> 
> declare i8* @malloc(i64)
> 
> declare void @llvm.gcroot(i8**, i8*) nounwind
> 
> in test.ll
> 
> and then do
> 
>> llc test.ll
>> gcc test.s
> 
> I get the error
> 
> ld: in /var/folders/Ea/EakEx6b+GBSo6cUn2AnFZk+++TI/-Tmp-//cc5d3RCx.o,
section not found for address 0x64200E00000004CD
> 
> If I remove line containing malloc and the line after that, no such error
happens. If I remove
> the gcroot line then no such error happens.
> 
> Any hints as to what this error means would be great!
> 
> Cheers,
> N
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

This is an interesting problem. The GC code is being converted into
'invokes' instead of calls:

define i32 @main() gc "shadow-stack" {
entry:
  %gc_frame = alloca %gc_stackentry.main
  %gc_currhead = load %gc_stackentry** @llvm_gc_root_chain
  %gc_frame.map = getelementptr %gc_stackentry.main* %gc_frame, i32 0, i32 0,
i32 1
  store %gc_map* getelementptr inbounds (%gc_map.0* @__gc_main, i32 0, i32 0),
%gc_map** %gc_frame.map
  %0 = getelementptr %gc_stackentry.main* %gc_frame, i32 0, i32 1
  store i8* null, i8** %0
  %gc_frame.next = getelementptr %gc_stackentry.main* %gc_frame, i32 0, i32 0,
i32 0
  %gc_newhead = getelementptr %gc_stackentry.main* %gc_frame, i32 0, i32 0
  store %gc_stackentry* %gc_currhead, %gc_stackentry** %gc_frame.next
  store %gc_stackentry* %gc_newhead, %gc_stackentry** @llvm_gc_root_chain
  %1 = invoke i8* @malloc(i64 1)
          to label %entry.cont unwind label %gc_cleanup

entry.cont:                                       ; preds = %entry
  store i8* %1, i8** %0
  %gc_frame.next1 = getelementptr %gc_stackentry.main* %gc_frame, i32 0, i32 0,
i32 0
  %gc_savedhead = load %gc_stackentry** %gc_frame.next1
  store %gc_stackentry* %gc_savedhead, %gc_stackentry** @llvm_gc_root_chain
  ret i32 0

gc_cleanup:                                       ; preds = %entry
  %gc_frame.next2 = getelementptr %gc_stackentry.main* %gc_frame, i32 0, i32 0,
i32 0
  %gc_savedhead3 = load %gc_stackentry** %gc_frame.next2
  store %gc_stackentry* %gc_savedhead3, %gc_stackentry** @llvm_gc_root_chain
  unwind
}

However, there isn't a personality function associated here. This conflicts
with the fact that there's an LSDA associated with the function. It's
not really feasible that there would be an LSDA but no personality function.
Also note that the landing pad of the 'invoke' doesn't include any
of the exception handling intrinsics. During the DwarfEHPrepare pass, an
@llvm.eh.exception() call is added (thus marking that as a "landing
pad" in code-gen), but there isn't an equivalent @llvm.eh.selector
call.

I don't know enough about GC stuff to say what should happen here. Though it
looks like we at least need the gc_cleanup to generate an llvm.eh.selector call
at the very least...The question becomes which personality function it should
call, because that's language-specific.

-bw

On Apr 11, 2011, at 9:42 AM, Nick Kledzik wrote:
> The linker is going off in the weeds trying to parse the dwarf unwind info.
The CIE has:
> 
> 
> Leh_frame_common_begin0:
> 	.long	0                       ## CIE Identifier Tag
> 	.byte	1                       ## DW_CIE_VERSION
> 	.asciz	 "zLR"                  ## CIE Augmentation
> 	.byte	1                       ## CIE Code Alignment Factor
> 	.byte	120                     ## CIE Data Alignment Factor
> 	.byte	16                      ## CIE Return Address Column
> 	.byte	2                       ## Augmentation Size
> 	.byte	16                      ## LSDA Encoding = pcrel
> 	.byte	16                      ## FDE Encoding = pcrel
> 
> 
> But the FDE has:
> 
> 	.byte	 0                       ## Augmentation size
> 
> The linker is expecting that since the CIE said there was an LSDA in each
FDE that is pcrel encoded, that there would in fact be an LSDA in each FDE.  But
in this case, the compiler is trying to omit the LSDA by marking that the FDE
has a zero sized augmentation data.
> 
> I'll have to research if this is a valid dwarf unwind optimization. 
Even if it is, it is tripping up the existing linker.
> 
> -Nick
> 
> 
> On Apr 10, 2011, at 6:06 AM, Nicolas Ojeda Bar wrote:
>> Hi,
>> 
>> If I type
>> 
>> define i32 @main() gc "shadow-stack" {
>> entry:
>> %0 = alloca i8*
>> %1 = call i8* @malloc(i64 1)
>> store i8* %1, i8** %0
>> call void @llvm.gcroot(i8** %0, i8* null)
>> ret i32 0
>> }
>> 
>> declare i8* @malloc(i64)
>> 
>> declare void @llvm.gcroot(i8**, i8*) nounwind
>> 
>> in test.ll
>> 
>> and then do
>> 
>>> llc test.ll
>>> gcc test.s
>> 
>> I get the error
>> 
>> ld: in /var/folders/Ea/EakEx6b+GBSo6cUn2AnFZk+++TI/-Tmp-//cc5d3RCx.o,
section not found for address 0x64200E00000004CD
>> 
>> If I remove line containing malloc and the line after that, no such
error happens. If I remove
>> the gcroot line then no such error happens.
>> 
>> Any hints as to what this error means would be great!
>> 
>> Cheers,
>> N
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev