[LLVMdev] Basic block with two return instructions

[Kenneth Uildriks]

On Fri, Feb 5, 2010 at 11:30 AM, Russell Wallace
<russell.wallace at gmail.com> wrote:
> Ah! I didn't know about verifyFunction; it does indeed catch it,
> thanks! I'll leave that call in my code for all cases for the moment,
> should help identify problems like that.
>
> Is there a recommended way to avoid this problem when compiling a
> language that has an explicit and optional return statement?
>
> On Fri, Feb 5, 2010 at 5:25 PM, Garrison Venn <gvenn.cfe.dev at gmail.com> wrote:
>> Did your test include running llvm::verifyFunction(...) on the function in question?
>> I guess I should test this, but I would have thought this would catch the issue.
>>
>> Garrison
>>
>> On Feb 5, 2010, at 12:13, Russell Wallace wrote:
>>
>>> Fair enough. In that case, is there an elegant way to test whether a
>>> basic block already has a terminator instruction?
>>>
>>> (I can think of several ways to do it in the front-end, but all of
>>> them are fairly inelegant. The problem I'm trying to solve is things
>>> like 'a return instruction needs to be added to the end of a function,
>>> if and only if the programmer didn't already end the function with an
>>> explicit return statement', 'if the programmer did write an explicit
>>> return statement everything after it must be ignored' etc.)
>>>
>>> On Fri, Feb 5, 2010 at 4:58 PM, Kenneth Uildriks <kennethuil at gmail.com> wrote:
>>>> On Fri, Feb 5, 2010 at 10:56 AM, Russell Wallace
>>>> <russell.wallace at gmail.com> wrote:
>>>>> When I generate a simple function with a single basic block with two
>>>>> return instructions, I had expected that if it did anything at all, it
>>>>> would use the first one, but it actually seems to ignore the first one
>>>>> and take the value of the second one.
>>>>>
>>>>> I'm guessing what's going on here is something like "a basic block
>>>>> must end in exactly one terminator instruction, having two of them is
>>>>> undefined behavior and the code generator is entitled to do anything
>>>>> including make demons fly out of your nose"; is this correct?
>>>>> _______________________________________________
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>>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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>>>>>
>>>>
>>>> That's pretty much been my experience.  Usually, something crashes
>>>> when I try it though.
>>>>
>>>
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>>
>>
>

Since terminators are only allowed at the end of a block, you could
always check the last statement of the block to see if it's a
terminator.  For that matter, BasicBlock::getTerminator returns the
terminator, or NULL if it hasn't got one yet.