[LLVMdev] not to break 'for' statement into basic blocks

[Seung Jae Lee]

According to the instruction manual of this target machine, 'goto' should not be used in C code. :-/

Could you tell me a little more about your advice as to using 'reg2mem', if you're fine?

Thank you so much, Anton.

Best,
Seung J. Lee


---- Original message ----
>Date: Sun, 15 Jul 2007 02:23:27 +0400
>From: Anton Korobeynikov <asl at math.spbu.ru>  
>Subject: Re: [LLVMdev] not to break 'for' statement into basic blocks  
>To: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
>
>Hello, Seung J. Lee
>
>> LLVM optimization and other tools are really fantastic. 
>> However I don't want LLVM breaks my 'for' statement in C code into
>> basic blocks during compiling. 
>> I'm sure this sounds really strange but there is a reason for me.
>LLVM is 'low-level'. It doesn't contain any special instruction for
>loops at all.
>
>> Furthermore, this assembly does not have the notion of
>> 'br'instruction, phi-node and so on. Therefore, I have no choice
>> without making 'for' in the assembly. 
>Maybe you can look, how code operates on phi-nodes in C & MSIL backends.
>
>> 1) First, I tried to re-unite basic blocks which llvm-gcc spits out to make 'for' again. 
>> But this is quite tricky. Generalizing it for the optimzed llvm bytecode is not easy.
>I'd say 'is not possible at all'.
>
>In general, loop contain induction variable, which is being assigned
>each iteration of loop. As LLVM IR is in SSA mode, there are only two
>possibilities to operate on induction variable so:
>
>1. Use phi node
>2. Use memory (memory is never in SSA form)
>
>So, if you don't want to deal with phi-node, you have to lower
>everything to memory. 'reg2mem' pass is your friend in this case. This
>will eliminate all phi nodes, but you have to resolve all branches.
>
>However, I really don't see, how you can transform C code (in general)
>to your target, because C *has* explicit goto statement. Maybe you can
>split the whole CFG into "cycles" and "paths" and try to "lower" them.
>But this can be pretty complicated.
>
>-- 
>With best regards, Anton Korobeynikov.
>
>Faculty of Mathematics & Mechanics, Saint Petersburg State University.
>
>
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