[llvm-dev] LLVMBuildPhi() circular dependency problem

[Manuel Jacob via llvm-dev]

Hi Toshi,

You can add the incoming value later, after you actually created it.

     // Here you build the PHI and add the first incoming value
     counter = LLVMBuildPhi(builder, LLVMInt64Type(), tmp_name);
     phi_values[0] = counter_init;
     phi_basic_blocks[0] = loop_init;
     LLVMAddIncoming(counter, phi_values, phi_basic_blocks, 1);

     // When you created the loop body and the "counter_next" value, you 
can add another incoming value
     phi_values[0] = counter_next;
     phi_basic_blocks[0] = loop_body;
     LLVMAddIncoming(counter, phi_values, phi_basic_blocks, 1);

-Manuel

On 2017-02-16 02:08, Toshiyasu Morita via llvm-dev wrote:
> I'm having a circular dependency problem with LLVMBuildPhi.
> 
> The short explanation:
> 
> The compiler is complaining when I build the LLVMValueRef array for
> LLVMAddIncoming that one of the values is undefined, because it's 
> actually
> assigned later in the basic block.
> 
> The long explanation:
> 
> I have a loop init basic block which contains:
> 
> // Initialize loop counter
>         counter_init = LLVMConstInt(LLVMInt64Type(), count, 0);
> ..
> The loop body basic block contains:
> 
> // Set the counter
>         sprintf(tmp_name, "tmp%d", *tmp_num++);
>         counter = LLVMBuildPhi(builder, LLVMInt64Type(), tmp_name);
>         phi_values[0] = counter_init;
>         phi_values[1] = counter_next;
>         phi_basic_blocks[0] = loop_init;
>         phi_basic_blocks[1] = loop_body;
>         LLVMAddIncoming(data3_pntr, phi_values, phi_basic_blocks, 2);
> 
> // Decrement the counter
>         sprintf(tmp_name, "tmp%d", *tmp_num++);
>         counter_next = LLVMBuildAdd(builder, counter,
> LLVMConstInt(LLVMInt64Type(), -1, 1), tmp_name);
> 
> When this code is compiled, a warning is issued:
> 
> capi_test.c:1520:16: error: ‘counter_next’ is used uninitialized in 
> this
> function [-Werror=uninitialized]
>   phi_values[1] = counter_next;
> 
> What is the correct way to break this circular dependency?
> 
> Toshi
> 
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