Floating point accumulater with a single layer of shifters in the significand feedback
A floating-point accumulator circuit includes a floating-point input having an input significand field and a first shifter coupled to the input significand field and providing an output of the input significand field shifted by a first amount. A carry-save adder has a first, second, and third input and an output. The first input is coupled to the output of the first shifter and the output provides carry bits and sum bits representing a summation of the first input, the second input, and the third input as a significand of the accumulated value. Shifters are also coupled to the carry bits and the sum bits of the output of the carry-save adder to respectively provide the carry bits and the sum bits, both shifted by a second amount, to the second input and the third input of the carry-save adder.
1. A floating-point accumulator circuit comprising:
a floating-point input having an input significand field;
a first shifter coupled to the input significand field and providing an output of the input significand field shifted by a first amount;
a three-input carry-save adder having a first input coupled to the output of the first shifter, a second input, a third input, and an output providing carry bits and sum bits representing a summation of the first input, the second input, and the third input, as a significand of an accumulated value;
a second shifter coupled to the carry bits of the output of the three-input carry-save adder and providing an output of the carry bits shifted by a second amount to the second input of the three-input carry-save adder; and
a third shifter coupled to the sum bits of the output of the three-input carry-save adder and providing an output of the sum bits shifted by the second amount to the third input of the three-input carry-save adder.
2. The floating-point accumulator circuit of claim 1 , further comprising:
an input pipeline register clocked by a first clock and having an input coupled to the input significand field of the floating-point input and a registered significand output coupled to an input of the first shifter;
a fractional carry pipeline register clocked by the first clock and having an input coupled to the carry bits of the output of the three-input carry-save adder and a registered output coupled to an input of the second shifter; and
a fractional sum pipeline register clocked by the first clock and having an input coupled to the sum bits of the output of the three-input carry-save adder and a registered output coupled to an input of the third shifter.
3. The floating-point accumulator circuit of claim 2 , further comprising:
a first circuit path from the registered significand output of the input pipeline register to the input of the fractional sum pipeline register that passes through only one shifter and the three-input carry-save adder, the only one shifter being the first shifter.
4. The floating-point accumulator circuit of claim 2 , further comprising:
a second circuit path from the registered output of the fractional carry pipeline register to the input of the fractional carry pipeline register that passes through only one shifter and the three-input carry-save adder, the only one shifter being the second shifter; and
a third circuit path from the registered output of the fractional sum pipeline register to the input of the fractional sum pipeline register that passes through only one shifter and the three- input carry-save adder, the only one shifter being the third shifter.
5. The floating-point accumulator circuit of claim 4 , the third circuit path further comprising only one multiplexor.
6. The floating-point accumulator circuit of claim 2 , further comprising a carry-save conversion pipeline stage that includes:
an adder coupled to the registered output of the fractional carry pipeline register and the registered output of the fractional sum pipeline register and having an output to provide their sum; and
a pipeline register clocked by the first clock with an input coupled to the output of the adder and a registered output to provide a significand value of the accumulated value.
7. The floating-point accumulator circuit of claim 6 , wherein the significand value of the accumulated value includes a sign value and an unsigned magnitude value.
8. The floating-point accumulator circuit of claim 2 , further comprising:
an addend input having an addend signficand field; and
a multiplexor having a first input coupled to the addend signficand field of the addend input, a second input coupled to the registered output of the fractional sum pipeline register, and an output coupled to the input of the second shifter.
9. The floating-point accumulator circuit of claim 1 , wherein the floating-point input has one and only one input significand field to represent a magnitude of a significand of a floating-point value received.
10. The floating-point accumulator circuit of claim 1 , wherein the input significand field of the floating-point input uses a 2′s compliment representation of the significand of a floating-point value received.
11. The floating-point accumulator circuit of claim 1 , further comprising:
one and only one gate per bit of the output of the second shifter interposed between a bit of the output of the second shifter and a respective bit of the second input of the three-input carry-save adder; and
one and only one gate per bit of the output of the third shifter interposed between a bit of the output of the third shifter and a respective bit of the third input of the three-input carry-save adder.
12. The floating-point accumulator circuit of claim 11 , wherein the one and only one gate per bit of the output of the second shifter and the one and only one gate per bit of the output of the third shifter each consist of a two-input AND gate.
13. An integrated circuit comprising:
a floating-point input having an input significand field;
a first shifter coupled to the input significand field and providing an output of the input significand field shifted by a first amount;
a three-input carry-save adder having a first input coupled to the output of the first shifter, a second input, a third input, and an output providing carry bits and sum bits representing a summation of the first input, the second input, and the third input, as a significand of an accumulated value;
a second shifter coupled to the carry bits of the output of the three-input carry-save adder and providing an output of the carry bits shifted by a second amount to the second input of the three-input carry-save adder; and
a third shifter coupled to the sum bits of the output of the three-input carry-save adder and providing an output of the sum bits shifted by the second amount to the third input of the three-input carry-save adder.
14. The integrated circuit of claim 13 , further comprising:
an input pipeline register clocked by a first clock and having an input coupled to the floating-point input and a registered output coupled to an input of the first shifter;
a fractional carry pipeline register clocked by the first clock and having an input coupled to the carry bits of the output of the three-input carry-save adder and a registered output coupled to an input of the second shifter; and
a fractional sum pipeline register clocked by the first clock and having an input coupled to the sum bits of the output of the three-input carry-save adder and a registered output coupled to an input of the third shifter.
15. The integrated circuit of claim 14 , further comprising:
a first circuit path from the registered output of the input pipeline register to the input of the fractional sum pipeline register that passes through only one shifter and the three-input carry-save adder, the only one shifter being the first shifter.
16. The integrated circuit of claim 14 , further comprising:
a second circuit path from the registered output of the fractional carry pipeline register to the input of the fractional carry pipeline register that passes through only one shifter and the three-input carry-save adder, the only one shifter being the second shifter; and
a third circuit path from the registered output of the fractional sum pipeline register to the input of the fractional sum pipeline register that passes through only one shifter and the three-input carry-save adder, the only one shifter being the third shifter.
17. The integrated circuit of claim 16 , the third circuit path further comprising only one multiplexor.
18. The integrated circuit of claim 14 , further comprising a carry-save conversion pipeline stage that includes:
an adder coupled to the registered output of the fractional carry pipeline register and the registered output of the fractional sum pipeline register and having an output to provide their sum; and
a pipeline register clocked by the first clock with an input coupled to the output of the adder and a registered output to provide a significand value of the accumulated value.
19. The integrated circuit of claim 18 , wherein the significand value of the accumulated value includes a sign value and an unsigned magnitude value.
20. The integrated circuit of claim 14 , further comprising:
an addend input having an addend signficand field; and
a multiplexor having a first input coupled to the addend signficand field of the addend input, a second input coupled to the registered output of the fractional sum pipeline register, and an output coupled to the input of the second shifter.
21. The integrated circuit of claim 13 , wherein the input significand field of the floating-point input uses a 2′s compliment representation of a significand of a floating- point value received.
22. The integrated circuit of claim 13 , further comprising:
one and only one gate per bit of the output of the second shifter interposed between a bit of the output of the second shifter and a respective bit of the second input of the three-input carry-save adder; and
one and only one gate per bit of the output of the third shifter interposed between a bit of the output of the third shifter and a respective bit of the third input of the three-input carry-save adder.
23. The integrated circuit of claim 22 , wherein the one and only one gate per bit of the output of the second shifter and the one and only one gate per bit of the output of the third shifter each consist of a two-input AND gate.
24. A method of accumulating floating-point values in a computing device, the method comprising:
latching, in a first clock cycle, into a register of the computing device:
a floating-point input having a significand field to generate a latched input significand field,
accumulated carry bits from a three-input carry-save adder circuit of the computing device to generate latched accumulated carry bits, and
accumulated sum bits from the three-input carry-save adder circuit to generate latched accumulated sum bits;
shifting the latched input significand field by a first amount to generate a shifted input significand field;
shifting the latched accumulated carry bits by a second amount to generate shifted carry bits;
shifting the latched accumulated sum bits by the second amount to generate shifted sum bits; and
providing the shifted input significand field, the shifted carry bits, and the shifted sum bits to a first input, a second input, and a third input, respectively of the three-input carry-save adder circuit.
25. The method of claim 24 , wherein the significand field of the floating-point input uses a 2′s compliment representation of a significand of a floating-point value received.
26. The method of claim 24 , further comprising:
adding the latched accumulated carry bits and the latched accumulated sum bits to generate a significand value; and
latching, in a second clock cycle immediately following the first clock cycle, the significand value into a pipleline register of the computing device.
27. The method of claim 26 , wherein the significand value includes a sign value and an unsigned magnitude value.
28. The method of claim 24 , further comprising:
latching, in the first clock cycle, a second floating-point input having an addend signficand field; and
selecting between the addend signficand field of the second floating-point input, the latched accumulated sum bits to shift by the second amount and to provide to the third input of the three-input carry-save adder circuit.