[LLVMdev] clang branching using label

[Jim Grosbach]

Sometimes the anonymous numbering confuses things. Try running the .ll file through "opt -instnamer"  first.

-Jim

On Apr 27, 2012, at 2:17 PM, Nong Li <nongli at gmail.com> wrote:

> 
> How does the verifier deal with this?  I'm noticing that if I load a clang
> compiled function (with -emit-llvm) and run the verifier, there is no
> problem.  If I start mucking with the function body (adding an instruction),
> the verifier is unhappy saying the instructions that would have been in the
> commented out block aren't part of a basic block.  Does the clang compiled
> function have some special attributes?
> 
> 
> Duncan Sands wrote:
>> 
>> Hi Chris,
>> 
>>> Clang normally generates code that looks like
>>> 
>>> ....
>>> ;<label>:22                                      ; preds = %0
>>>   %23 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([24 x
>>> i8]*
>>> @.str, i32 0, i32 0))
>>>   br label %24
>>> 
>>> ;<label>:24                                      ; preds = %22, %0
>>>   %25 = load i32* %tmphigh, align 4
>>>   %26 = and i32 %25, 65535
>>>   store i32 %26, i32* %high1, align 4
>>> .....
>>> 
>>> 
>>> If I insert some instructions before %25 so it will look like:
>>> ....
>>> ;<label>:22                                      ; preds = %0
>>>   %23 = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([24 x
>>> i8]*
>>> @.str, i32 0, i32 0))
>>>   br label %24
>>> 
>>> ;<label>:24
>>>   %var = add i32 %7,   0          ; for testing
>>>   %25 = load i32* %tmphigh, align 4
>>>   %26 = and i32 %25, 65535
>>>   store i32 %26, i32* %high1, align 4
>>> .....
>>> 
>>> Will %var get evaluated or it will be skipped?
>> 
>> It will be evaluated.
>> 
>>> How does the branching work since the label is commented out?
>> 
>> Labels and most other names only exist for the convenience of human
>> readers.
>> LLVM knows that a basic block has ended because of "br label %24".  This
>> is
>> an example of a terminator instruction: an instruction that only occurs as
>> the last instruction of a basic block (every basic block must finish with
>> one).  Thus LLVM knows where all basic blocks start and end by just
>> looking
>> at the terminator instructions.  It doesn't need labels.
>> 
>>  Does it assume
>>> %24 is the nop before %25?
>> 
>> I didn't understand this question.  When printing human readable IR, LLVM
>> prints
>> a numerical name like %24 for things which have no name internally.  It
>> has to
>> put something since otherwise it wouldn't be able to read the IR back in
>> again.
>> The numbers it chooses just increment sequentially.  This makes it hard to
>> modify such human readable IR by hand since you have to preserve the
>> sequential
>> numbering.  (If you want everything to have a real name use opt's
>> -instnamer
>> pass).
>> 
>>> 
>>> How about the following case where a variable assignment does not occur?
>> 
>> I'm not sure what you mean by "a variable assignment does not occur".
>> An instruction like "%var = add i32 %7, 0" isn't really an assignment to
>> a variable "%var" since (thanks to SSA form) it isn't possible to "assign"
>> anything else to "%var" later.  Instead "%var" is just a name for the
>> value
>> of "add i32 %7, 0".  In any case it is exactly the same:
>> 
>>> 
>>> ....
>>>   %37 = icmp sle i32 %33, %36
>>>   br i1 %37, label %38, label %217
>> ^^  Terminator instruction, so basic block finishes here
>>> 
>>> ;38:
>>>   store i32 0, i32* %j, align 4
>> ^^ Thus this is the first instruction of the next basic block
>>>   br label %39
>>> 
>>> .....
>>> which is changed to
>>> 
>>> ......
>>>   %37 = icmp sle i32 %33, %36
>>>   br i1 %37, label %38, label %217
>> ^^  Terminator instruction, so basic block finishes here
>>> 
>>> ;38:
>>>   store i32 0, i32* %p, align 4
>> ^^ Thus this is the first instruction of the next basic block
>>>   store i32 0, i32* %j, align 4
>>>   br label %39
>>> .....
>>> 
>>> Will both store instructions get executed?
>> 
>> Yes.
>> 
>>> Does the empty line before the commented out label number have any real
>>> function?
>> 
>> No.
>> 
>> Ciao, Duncan.
>> 
>>> 
>>> 
>>> Thanks,
>>> 
>>> Chris
>>> 
>>> 
>> 
>> _______________________________________________
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>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> 
>> 
> 
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