[LLVMdev] convert LLVM IR to another IR without SSA

[Xiangyang Guo]

Hi, Diego,

Thanks for your quick reply. Inserting a copy instruction may not work here
because I have a limitation of virtual register number. I need to assign
registers with ssa form to registers without ssa form. I will look the
source code you point out.

Thanks

Xiangyang

On Fri, Apr 24, 2015 at 4:19 PM, Diego Novillo <dnovillo at google.com> wrote:

>
>
> On Fri, Apr 24, 2015 at 3:17 PM, Xiangyang Guo <xguo6 at ncsu.edu> wrote:
>
>> Hi,
>>
>> I want to convert LLVM IR to another type of IR, which has no SSA form.
>> So the most important thing I need to handle is Phi node and register
>> allocation because another type of IR has a limitation of virtual register
>> number. The first thing I can think about is to learn how LLVM Backend
>> works because LLVM Backend handles these things. But I'm not familiar with
>> Backend. After reading some source code and online tutorials, I think a
>> Backend is too much for my purpose. I really appreciate that if someone can
>> give me hints.
>>
>
> The easiest way to think about PHI nodes is to consider them copies on CFG
> edges.  If you do the naive translation of inserting a copy instruction on
> the corresponding edge for each PHI argument, you'll get a rough
> approximation for the corresponding normal form.
>
> This is, of course, very inefficient, but it's the main idea.
>
> If you can look at GCC's source code, take a look at the algorithm in file
> gcc/tree-outof-ssa.c.  That implements an SSA->Normal transformation. I'm
> not really sure where in the LLVM's backend this is done.
>
>
> Diego.
>
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