[LLVMdev] clang branching using label

[Nong Li]

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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> 

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