[LLVMdev] IR in SSA form?

[John Criswell]

On 3/29/11 12:26 PM, George Baah wrote:
> Hi All,
>   When I run the following command
> llvm-gcc -03 -emit-llvm test.cpp -c -o test.bc or llvm-gcc -emit-llvm 
> test.cpp -c -o test.bc
>
> on the program test.cpp, the IR representation is not in SSA form.
> I do not see any phi functions.

Actually, it is in SSA form (or more precisely, the virtual registers 
are in SSA form; LLVM only puts virtual registers into SSA form).  
However, all program variables are code-generated to alloca instructions 
and then accessed using load and store instructions.

To lift stack-allocated variables into virtual registers (essentially 
putting them into SSA form), run the mem2reg pass (opt -mem2reg test.bc 
-f -o test.opt.bc).  This will take non-address taken alloca's, convert 
them into virtual registers, and introduce phi-nodes where needed.

-- John T.

>
> program: test.cpp
> int main(int argc, char **argv)
> {
>   int a[2],i,j;
>   for(i=0;i<2;i++)
>   {
>     a[i] = i;
>   }
>   return a[1];
> }
>
> Any clarifications will be greatly appreciated. Thanks.
>
> George