[LLVMdev] Undoing SSA and Phi instructions

[Andrew Lenharth]

On Sun, Jan 23, 2011 at 12:45 AM, Nick Lewycky <nicholas at mxc.ca> wrote:
> On 01/22/2011 10:30 PM, Surinder wrote:
>> Hi,
>>
>> I am emitting llvm bit code using
>>
>> llvm-gcc -c -emit-llvm -O0 -o test.bc test.c
>>
>> and then optimizing it with
>>
>> opt -O3 -print-module test.bc
>>
>> in order to obtain a dump of generated IR.
>>
>>
>> The resulting code has Phi nodes and is perhaps in SSA form.  I want
>> to undo the SSA form while retaining all the other optimizations.  Is
>> mem2reg the right optimization to be added after -O3, i.e.,
>>
>> opt -O3 -mem2reg -print-module test.bc
>>
>> will print the llvm in non-SSA form with no Phi nodes.
>
> I think you're looking for "opt -O3 -reg2mem -S test.bc" though I don't
> think it *guarantees* that it will remove all phi nodes. I don't think
> we have anything besides codegen that does.

reg2mem inserts allocas to store values going into phis.  It doesn't
exactly undo SSA, but it should remove all phis.

Andrew