llvm dev - Feb 2010 - [LLVMdev] Basic block with two return instructions

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?

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?
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
That's pretty much been my experience.  Usually, something crashes
when I try it though.

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?
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
> That's pretty much been my experience.  Usually, something crashes
> when I try it though.
>