llvm dev - Feb 2017 - LLVMBuildPhi() circular dependency problem

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170215/b48fc029/attachment.html>

I wrote earlier:
>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.
I think I just realized the correct solution: move LLVMAddIncoming() to
the  end of basic block?

Toshi



On Wed, Feb 15, 2017 at 5:08 PM, Toshiyasu Morita <toshi at tensyr.com>
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
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20170215/d38f6eae/attachment.html>

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
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev