[Dwarf-discuss] Proposal to support bundled instructions (e.g., Itanium)

[Chris Quenelle]

I think it looks good.

Shouldn't the new header fields go at the end
of the header so that old code might still work?
Of course, you would also want to update the version
number for that dwarf section.

Also, you could leave the minimum instruction length
set to something normal (like 1 or 16) and use a non-zero
value in the two new fields to indicate the new
style of information.

The question is whether we would like to promote the
earliest possible failure in non-coping tools
or whether we want to promote the ability to "limp along"
in some fashion.

--chris



Cary Coutant wrote:
> I'm tossing this idea out there just to see if anyone else likes it...
> 
> On the Itanium architecture, three instructions are packed into one  
> 16-byte "bundle". Only the bundle itself has a real (16-byte aligned)  
> address, leaving the compiler and debugger to agree on a convention  
> for how to represent the addresses of the instructions inside the  
> bundle. On HP-UX, we use the bundle address plus 0, 4, and 8 to  
> represent these; Linux has chosen the bundle address plus 0, 1, and  
> 2. In the line number program, we set minimum_instruction_length to  
> 4, and we then have to use increments of 1, 1, 2, 1, 1, 2, ....  
> Worse, Linux chose to set minimum_instruction_length to 1, and uses  
> increments of 1, 1, 14, 1, 1, 14, .... Ideally, I'd like to set  
> minimum_instruction_length to 16/3, so we can just use uniform  
> increments of 1.
> 
> I propose to extend the definition of the minimum_instruction_length  
> field of the line number program header, in Section 6.2.4, as follows:
> 
> "If minimum_instruction_length is zero, it is immediately followed by  
> two additional ubytes -- bundle_length and  
> instruction_slots_per_bundle. For architectures with instruction  
> bundles, the address register has two components: a bundle address  
> and a slot number. Line number program opcodes that alter the address  
> register first compute the new bundle address as (bundle_adress +  
> bundle_length * ((slot_number + operand) div  
> instruction_slots_per_bundle)), then compute the new slot number as  
> ((slot_number + operand) rem instruction_slots_per_bundle), where div  
> and rem are defined such that (a div b) is an integer and a = (a div  
> b) * b + (a rem b) and 0 <= (a rem b) < b."
> 
> For Itanium, we would set the minimum_instruction_length field to 0,  
> followed by the bundle_length and instruction_slots_per_bundle fields  
> of 16 and 3.
> 
> -cary
> 
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