[Bug] signed_mul_overflow bug

[JaewonHur]

in the code mor1kx_excute_alu.v

    // One signed overflow detection for all multiplication implmentations
    assign mul_signed_overflow = (FEATURE_MULTIPLIER=="NONE") || (FEATURE_MULTIPLIER=="PIPELINED") ? 1'b0 :
    // Same signs, check for negative result
    // (should be positive)
    ((a[OPTION_OPERAND_WIDTH-1] == b[OPTION_OPERAND_WIDTH-1]) && mul_result[OPTION_OPERAND_WIDTH-1]) ||
    // Differring signs, check for positive result
    // (should be negative)
    ((a[OPTION_OPERAND_WIDTH-1] ^ b[OPTION_OPERAND_WIDTH-1]) && !mul_result[OPTION_OPERAND_WIDTH-1]);

multiplying negative value with 0 will mistakenly assert the overflow.
Also, other cases such as '0x4 * 0x5fffffff' will not detect the overflow.

In the case of SERIAL multiplier, we can fix it by changing

assign mul_signed_overflow = (mul_prod_r[(OPTION_OPERAND_WIDTH*2)-1:OPTION_OPERAND_WIDTH] == 32'h0 || mul_prod_r[(OPTION_OPERAND_WIDTH*2)-1:OPTION_OPERAND_WIDTH] == 32'hffffffff) ? 0:1;

[JaewonHur]

Modifying the solution, I misunderstood multiply signed overflow.
The bug was that the original mul_signed_overflow code does not consider when one of the operands is 0 and the other is a negative value.
In this case, the original code will always set mul_signed_overflow.

My solution is

// One signed overflow detection for all multiplication implmentations
    assign mul_signed_overflow = (FEATURE_MULTIPLIER=="NONE") ||
 				(FEATURE_MULTIPLIER=="PIPELINED") ? 1'b0 :
                                // When either a or b is 0, result should not
                                // be negative
                                ((a == 0 || b == 0) && mul_result[OPTION_OPERAND_WIDTH-1]) ||
 				// Same signs, check for negative result
 				// (should be positive)
 				((a[OPTION_OPERAND_WIDTH-1] ==
 				  b[OPTION_OPERAND_WIDTH-1]) &&
 				 mul_result[OPTION_OPERAND_WIDTH-1]) ||
 				// Differring signs, check for positive result
 				// (should be negative)
 				((a[OPTION_OPERAND_WIDTH-1] ^
 				  b[OPTION_OPERAND_WIDTH-1]) &&
 				 !mul_result[OPTION_OPERAND_WIDTH-1]);