[LLVMdev] Questions about LLVM IR encoding

[Duncan Sands]

Hi 张乐,

> Hi all, I am new to LLVM (even the field of compiler) and currently I am engaged
> in the work of adapting LLVM IR to M5 simulator to observe the enhancement of
> the novel architecture we design.
> Simply speaking if you know little about M5, my aim is to know how LLVM IR is
> interpreted and encoded, then try to implement it in the framework of M5.
> I have read the LLVM documents, yet I still have some questions as follows:
> 1. As IR is target-independent, how can we encode them into bit-code executable
> files without specific targets' information?

I don't know what you mean by a "bit-code executable".  However if you want to
run bitcode (eg: by compiling to a native program using llc, or using the JIT or
interpreter lli) then at that moment you need to specify the target if you did
not specify it before (the target can be specified in the bitcode).

> 2. The "bitstream container format" as the document refers, is XML-like, and I
> wonder how does LLVM translate it into executable format such as bit-code.

You should have no need to deal directly with the on-disk bitcode format.  Again
I am not sure what you mean when you call bit-code an "executable format".

Ciao, Duncan.

>      I have made several tests as the document suggests, but I conclude with
> nothing helpful to my work.
>      The result of my experiment is attached.
>      It is a simple add program, and from the bc file generated by
> llvm-bcanalyzer I cannot relate it to the original IR.
>      For example, the expressions of
> <INST_STORE2 op0=6 op1=1 op2=3 op3=0/>
> <INST_STORE2 op0=5 op1=3 op2=3 op3=0/>
>      actually represent those of
>      store i32 1, i32* %a, align 4
>      store i32 2, i32* %b, align 4
>      in the .ll file.
>      Despite the explicit opcode matches in the two forms, I am confused of the
> information of op0, op1 etc in the .bc file.
>      What does it mean? This is not very clearly clarified in the document, or
> may I omit something?
>
> Can anybody please lend me a hand?
> Thanks very much sincerely!
>
> --
> /with best regards /
> Zhang  Le
>
>
>
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