[LLVMdev] How to prevent an instruction to be executed more than once?

[Alistair Lynn]

Hello-

Doing control-flow (jumps and/or labels) in inline asm is a really bad idea. What are you trying to achieve?

Alistair

On 24 Jun 2010, at 12:54, Xinfinity wrote:

> 
> Hi LLVM-devs,
> 
> I am looking for a way to prevent an instruction to be executed more than
> once, or duplicated .. I know that by default everything can be duplicated,
> but is there a work-around?
> 
> The structure of the code is 
> BB1    BB2
>   |          |
>   |          |
>   |          |
>   |----------|
>        |
>        v
>       BB3
> 
> where the first instruction in BB3 is inserting an inline assembly which
> contains a newly defined label:
> 
>            call void asm "BB4_label:\0A", ""()
> 
> I want to find a way to have this instruction executed only once. The
> problem is that in the optimisation steps, LLVM duplicates BB3 and
> concatenates it with BB1 and with BB2, therefore the label is defined twice,
> which gives an error.
> 
> Is it possible to do this, without having to modify the back-end of LLVM ?
> 
> Thanks,
> Alexandra
> 
> -- 
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> 
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