[LLVMdev] SSA in the Front End

[Chris Lattner]

On Fri, 27 May 2005, Ricardo wrote:
> Thanks for the explanation. It's more clear now
>
> The only thing that seems strange is that in the function llvm_expand_shortcircuit_truth_expr in
> the front end, there is the creation of a PHI instruction. If there is no SSA yet, why do you do
> that?

That is just a shortcut.  We could introduce an alloca for every 
shortcircuit expression, but as a minor compile time efficiency issue, we 
don't.

-Chris


> --- John Criswell <criswell at cs.uiuc.edu> wrote:
>
>> Ricardo wrote:
>>> Hi,
>>>
>>> I have been looking into the code that generates the LLVM assembly in the LLVM front end, but
>> I am
>>> not very sure if at the time that the llvm_c_expand_body_1 function is called, the SSA form
>> was
>>> already constructed (each definition dominates all the uses). Can somebody please tell me?
>>
>> The LLVM GCC frontend does not translate variables directly into LLVM
>> virtual regsiters (which must be in SSA form).
>>
>> Instead, the LLVM GCC frontend simply uses a memory location for each
>> variable (e.g. a local variable is allocated via the LLVM alloca
>> instruction).  If you use llvm-gcc -S -o file.ll file.c, you can see the
>> code that llvm-gcc generates.
>>
>> Next, gccas (after assembling the LLVM assembly file) runs a pass called
>> mem2reg.  This pass finds memory locations that can be promoted to LLVM
>> virtual registers and promotes them into registers.  The following
>> commands will allow you to see the change:
>>
>> llvm-as -o file.bc file.ll
>> opt -mem2reg -o newfile.bc file.bc
>> llvm-dis -o newfile.ll newfile.bc
>>
>> If you compare newfile.ll to file.ll, you'll see what the mem2reg
>> transformation does.
>>
>> Using this technique, language frontends don't need to worry about
>> maintaining SSA for variables that it generates.  It simply generates
>> them as memory locations and the LLVM optimizer fixes it up into more
>> efficient code.
>>
>> -- John T.
>>
>>>
>>> Thanks
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>> --
>> John T. Criswell
>> Research Programmer
>> University of Illinois at Urbana-Champaign
>> "It's today!" said Piglet. "My favorite day," said Pooh.
>>
>>
>
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-Chris

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