[LLVMdev] Changes between 2.6 and 2.7: SSA Verifier and visitFreeInst

[Jianzhou Zhao]

On Sat, Mar 6, 2010 at 2:35 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
> Jianzhou Zhao wrote:
>>
>> Hi llvm,
>>
>> 1) The lib/VMCore/Verifier.cpp in 2.7 implements Verifier::VerifyType,
>> which is empty in 2.6. I noticed that it does not check all types,
>> for example, UnionTyID, OpaqueTyID, LabelTyID, MetadataTyID
>> and etc are ignored in the 'default' branch. Does it mean we dont
>> need to check them?
>
> We do need to check union. I'll add that. The others don't need a case
> because they don't contain subtypes; it's impossible to have an illegal
> label type. It's either a label type or it isn't. :)
>
>> Another question is: How much does Verifier.cpp check statically?
>> Can I take it as a type checker for SSA? Is there any static
>> semantics that has not been checked yet?
>
> There are things that it does not check yet. It doesn't verify constant
> expressions, so 'bitcast i32* @x to i32' will still escape detection, if you
> can manage to create one (ie., you're running with assertions off).
>
> Further, there are semantics of the program that the verifier will never
> verify. It is undefined behaviour to shift beyond the width of an integer.
> It is undefined behaviour to call a function with mismatching calling
> conventions. http://llvm.org/docs/FAQ.html#callconvwrong
>
>> 2) visitFreeInst has been removed from lib/ExecutionEngine/Interpreter in
>> 2.7
>> Will this have any back-compatible problem when interpreting any
>> *.bc from 2.6?
>
> The 'free' instruction has been removed from LLVM 2.7. Any .bc files using
> it will have a 'call @free' inserted in its place.

Thanks.

Another question is about 'ret'.
http://llvm.org/docs/LangRef.html#i_ret gives the syntax:
   ret <type> <value>       ; Return a value from a non-void function
   ret void                 ; Return from void function
In the case when the return type is not void, it returns
a single value. But if I am looking into the code,
the Verifier also allows 'multi returned values' if the return
type is of structure or array, while there is not interface
in 'ReturnInst' to create a multi-value 'ret'. Is it only for
the ll code that was generated by an old version LLVM
which had still supported 'multi-return'.

The comments in 'Verifier' say this 'multi-return' verification
feature may be removed from LLVM 3.0. So If I am generating
'ret' with multi values, will that be a verification error
when LLVM 3.0 releases?

>
> Nick
>



-- 
Jianzhou