[LLVMdev] Are there implicit rules or conventions for an llvm frontend to generate llvm IR?

[John Criswell]

On 10/10/13 10:43 AM, Hongxu Chen wrote:
> Hi, this question might be a bit silly: apart from the language
> reference(http://llvm.org/docs/LangRef.html#switch-instruction) page, are
> there additional rules for a regular llvm frontend to generate llvm IRs?
>
> There are a few cases that I got from clang/llvm-gcc/dragonegg when
> compiling *C* source code into llvm IR:
>
> 1. It seems that there is ONLY ONE ReturnInst(and NO InvokeInst) for such
> llvm IR; is it legal to add other *ReturnInst*s when transforming?

An LLVM function can have multiple ReturnInsts as long as each one 
terminates a basic block.  There is a transform (UnifyExitNodes, IIRC) 
that will take a function with multiple ReturnInsts and create one with 
a single ReturnInst.  Having a single ReturnInst (exit node) simplifies 
other analyses.

>
> 2. Is it possible for a frontend to generate a function whose CFG is
> something like:
>
>              bb0
>         /           \
>       bb1          bb2
>     /      \       /      \
> bb3         bb4        bb5
>     \           |          /
>       \         |        /
>         \       |      /
>                bb6
>
> (In this case, if I understand correctly, bb4 is control dependent on both
> bb1 and bb2.)
> I think it at least possible in theory, and there is a simple case:

Yes, that looks fine to me.  One of the LLVM passes might optimize that 
CFG or put it into some canonical form, but that CFG looks fine to me.

-- John T.

>
> int foo(int i) {
>    if (i < 0) {
>      if (i % 2 == 0) {
>        i += 1;
>      } else {
>        i += 2;
>      }
>    } else {
>      if (i % 2 == 0) {
>        i += 1;
>      } else {
>        i += 2;
>      }
>    }
> return 0;
> }
>
> However none of the frontends I used generate the basicblocks like
> that(there is always one or more basicblocks generated) /without any
> optimizations/. So is there any implicit rules for these frontends?
>
> And can I rely on these cases when I ONLY deal with C source code?
>
> Thanks!
>
>
>
> --
> View this message in context: http://llvm.1065342.n5.nabble.com/Are-there-implicit-rules-or-conventions-for-an-llvm-frontend-to-generate-llvm-IR-tp61938.html
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