[LLVMdev] LLVM Assembly code

[John Criswell]

Manvi Agarwal wrote:
> Hi,
> 
> I have generated llvm assembly file using following commands:
> llvm-ld main.o fir.o
> llvm-dis a.out.bc
> 
> I get a.out.ll as the llvm assembly file. When i go through this assembly
> file I come across the following code
> fir.entry:		; preds = %loopexit.1.i
> 	call void %print( sbyte* getelementptr ([2 x sbyte]* %.str_3, int
> 0, int 0), float* getelementptr ([97 x float]* %y, int 0, int 0), int 97 )
> 	call void %exit( int 0 )
> 	unreachable
> 
> 
> Can anybody please tell me what is meant by "unreachable"? The output is
> coming fine.

The unreachable instruction tells the LLVM compiler that the statement 
cannot be reached.  It allows LLVM to optimize the program more 
aggressively.

In this case, the unreachable instruction was probably inserted by the 
LLVM GCC frontend because it knows that the call to exit() will never 
return.

-- John T.

> 
> Regards,
> manvi
> 
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-- 
John T. Criswell
Research Programmer
University of Illinois at Urbana-Champaign
"It's today!" said Piglet. "My favorite day," said Pooh.