[LLVMdev] Build Attributes Proposal

[Duncan Sands]

Hi Bob,

>>>> So, after long rounds to define how the build attributes are going to
>>>> be represented in IR, and after Jason has implemented build attributes
>>>> in MC, I have a proposal on how to represent this in IR.
>>>>
>>>> First, we need to distinguish between target dependent and independent
>>>> attributes. Generic things like optimization level, specific
>>>> optimizations, use of vector instructions, use of exceptions,
>>>> floating-point compatibility can all go in to the generic list of
>>>> attributes.
>>>
>>> I don't see why you would want to put any of these things in the module
>>> at all:
>>
>> In fact they are more suitable as function attributes. For example, we already have function attributes for -Os.
>
> I think some of this discussion is missing the context.  ARM has defined per-object file build attributes to record things like the optimization level, whether the code is ARM vs. Thumb, etc.  This is not something new nor is it specific to LLVM.  As far as I know, the linker is not obligated to do anything with this information, but compilers that follow ARM's specifications are expected to provide it.
>
> Putting the build attributes on individual functions is not a good match for the final result, which is a single set of attributes for the entire object file.  If you put separate attributes on the functions, then code gen will have to scan all the functions to see if they have the same attribute values.

thanks for the explanation.  Perhaps you can use global asm statements for this
(see http://llvm.org/docs/LangRef.html#moduleasm )?  These can be used to inject
arbitrary strings into the assembler.  Dragonegg uses this to provide an "ident"
string like this:

   $ gcc-4.5 -fplugin=dragonegg.so -S -o - -flto empty.c
   ; ModuleID = 'empty.c'
   target datalayout = 
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
   target triple = "x86_64-unknown-linux-gnu"

   module asm "\09.ident\09\22GCC: (GNU) 4.5.2 20101028 (prerelease) LLVM: 
118890\22"

In the assembler output this gives:
   $ gcc-4.5 -fplugin=dragonegg.so -S -o - empty.c
   	.file	"empty.c"

   	.ident	"GCC: (GNU) 4.5.2 20101028 (prerelease) LLVM: 118890"


   	.section	.note.GNU-stack,"", at progbits

Ciao,

Duncan.