[LLVMdev] RFC: Supporting ELF symbol aliases via GlobalAlias GEPs

[Dan Gohman]

Hello David,

This feature sounds reasonable to me. Please update the patch to
include a feature test, and LangRef.html changes as necessary,
including a mention that the feature depends on the target.

Here are a few misc. comments on the patch itself:

+  /// isTargetElf - returns true if the target is ELF.
+  virtual bool isTargetElf() const { return true; }

This does not belong in TargetMachine.h.

+      if ((GEP = dyn_cast<ConstantExpr>(I->getAliasee()))
+          && (GEP->getOpcode() == Instruction::GetElementPtr)) {

Please follow LLVM style.

+        assert(PrintGlobalOffsetAlias(cast<GlobalAlias>(I)) &&
+                "Target doesn't support offset aliases.");

This won't call PrintGlobalOffsetAlias when asserts are disabled.

Thanks,

Dan

On Aug 25, 2009, at 9:07 AM, David Chisnall wrote:

> I've attached a less-hackish implementation of this.  This includes  
> the following modifications:
>
> - getSupportsOverlappingAliases() method on TargetMachine which  
> returns whether the target supports multiple symbols to the same  
> object.  This returns false in the superclass and needs to be  
> explicitly overridden for each target to enable it.
>
> - An implementation of this method in X86TargetMachine which returns  
> true for ELF targets.
>
> - printObjectType() in AsmPrinter.  Currently all of the subclasses  
> hard-code this (e.g. ".type " + name + ", @object") when emitting  
> global variables.  I've implemented this for X86ATTAsmWPrinter, and  
> will add the same for other classes as required.
>
> - PrintGlobalOffsetAlias(), also in AsmPrinter.  This outputs a  
> GlobalAlias which is a GEP to a GlobalVariable, if the target  
> supports it.
>
> This is a proof-of-concept implementation which, pending review, I'd  
> like to commit as a work-in-progress and then work on adding support  
> for more targets.  It should be relatively trivial to add this for  
> other ELF targets; just override getSupportsOverlappingAliases() in  
> the relevant TargetMachine subclass to return true and copy the code  
> out of the PrintGlobalVariable() method in the AsmPrinter subclass  
> to implement printObjectType().
>
> David
>
> On 23 Aug 2009, at 23:41, David Chisnall wrote:
>
>> Hi Everyone,
>>
>> Chris suggested[1] I should ask for feedback as to whether this is  
>> a desired feature before I put too much effort into it, so here goes:
>>
>> I would like to be able to export a symbol that is inside an LLVM  
>> structure.  This is possible on ELF targets[2], and the attached  
>> proof-of-concept patch to AsmWriter makes it work (although in a  
>> hackish way that I am NOT suggesting be committed as-is).
>>
>> With this patch, the you can compile this:
>>
>> %0 = type { i32, i32 }
>> @structure = global %0 { i32 0, i32 1 }
>> @element1 = alias getelementptr( %0* @structure, i32 0, i32 1)
>>
>> To this:
>>
>>   .subsections_via_symbols
>>   .section    __DATA,__data
>>   .align  3
>> _structure:                                                 ##  
>> @structure
>>   .space  4
>>   .long   1                                           ## 0x1
>>
>>   .globl  _element1
>>   .set _element1, _structure+4
>>   .size   _element1, 4
>>   .type   _element1, at object
>>
>> The element1 symbol is an i32* pointing to element 1 in the  
>> structure (the one emitted by .long 1).
>>
>> There are really two questions here:
>>
>> 1) Do we want to be able to generate this kind of output at all (I  
>> do!)
>> 2) If we do, do we want to use the global alias initialised with a  
>> constant GEP to do it, or provide some other mechanism?
>>
>> David
>>
>> [1] http://llvm.org/bugs/show_bug.cgi?id=4739#c24
>> [2] I know it's not possible on Mach-O (well, it is for internal  
>> symbols, just not for ones exported via the symbol table) - does  
>> anyone know if PE allows it?
>> <llvm.diff>
>
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