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

[Hongxu Chen]

On Fri, Oct 11, 2013 at 12:06 AM, John Criswell <criswell at illinois.edu>wrote:

> 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<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.
>
> Thanks so much, John; especially for pointing out 'UnifyExitNodes' pass!

>
>
>> 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.
>

Got it!

And can I say : as long as it is not explicitly in llvm language reference,
there are generally no restrictions for frontends/transformations to
generate IR(of course, they pass the verifier)?


>
>
>> 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<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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>>
>
>
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