[llvm-dev] LLVMBuildPhi() circular dependency problem

Toshiyasu Morita via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 15 17:08:51 PST 2017

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?

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