[LLVMdev] overflow + saturation stuff

[Nick Lewycky]

Jonas Maebe wrote:
> On 08 Feb 2009, at 14:41, Gordon Henriksen wrote:
> 
>> On 2009-02-08, at 05:59, Jonas Maebe wrote:
>>
>>> The proposal suggests to change/split the existing sub/add/mul
>>> opcodes. This makes me wonder to what extent it is (currently, or
>>> ever) advisable for an external compiler to generate LLVM IR. Is
>>> there a plan to stabilise at some point and guarantee backwards
>>> compatibility to a certain extent, or should compilers that are not
>>> integrated in the LLVM infrastructure always target one particular
>>> release of LLVM?
>> LLVM does guarantee backwards compatibility with compiled bitcode. The
>> C++ interfaces are not frozen, so you may need to upgrade code
>> targeting LLVM when upgrading; reasonable efforts are made to avoid
>> making this process painful.
> 
> Sorry for being unclear: I did not mean the C++ interface nor compiled  
> bitcode, but the LLVM IR "assembler" interface (i.e., the .s files  
> that llvm-gcc generats with "-emit-llvm -S").
> 
>> Of course, what code is contributed to
>> the project will be maintained through these changes.
> 
> In our case, I doubt that would happen since it's a self-hosting  
> Pascal compiler (with several other code generators besides the under- 
> development LLVM backend).
> 
> But basically, if I understand you correctly: the correct interface  
> would be compiled bitcode rather than the "assembler" level interface?

The textual IR (generally .ll files, not .s) are run through an 
auto-upgrader, the same as bitcode. Once we reach LLVM 3.0, we may break 
support for 2.x series .ll and .bc files.

Of course, we might get to LLVM 3.0 before implementing this feature. 
What really happens is that once we've changed the .ll/.bc format enough 
that backwards compatibility is difficult to maintain, we'll declare 
LLVM 3.0.

Nick

> 
> Jonas
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