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Remove op_b (rs2) register from muldiv_seq for modest LUT/FF savings

This commit is contained in:
Luke Wren 2022-10-08 18:22:08 +01:00
parent d3667769d2
commit aa438fc37c
1 changed files with 20 additions and 15 deletions
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35
hdl/arith/hazard3_muldiv_seq.v
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@ -3,14 +3,14 @@
| SPDX-License-Identifier: Apache-2.0 |
\*****************************************************************************/
// Combined multiply/divide/modulo circuit.
// All operations performed at 1 bit per clock; aiming for minimal resource usage.
// There are lots of opportunities for off-by-one errors here. See muldiv_model.py
// for a simple reference model of the mul/div/mod iterations.
// Combined multiply/divide/modulo circuit. All operations performed at 1 bit
// per clock; aiming for minimal resource usage on iCE40 FPGA. Optionally the
// circuit can be unrolled for slightly higher performance.
//
// When op_kill is high, the current calculation halts immediately. op_vld can be
// asserted on the same cycle, and the new calculation begins without delay, regardless
// of op_rdy. This may be used by the processor on e.g. mispredict or trap.
// When op_kill is high, the current calculation halts immediately. op_vld can
// be asserted on the same cycle, and the new calculation begins without
// delay, regardless of op_rdy. This may be used by the processor on e.g.
// mispredict or trap.
//
// The actual multiply/divide hardware is unsigned. We handle signedness at
// input/output.
@ -57,10 +57,11 @@ parameter W_CTR = $clog2(XLEN + 1);
reg [W_MULOP-1:0] op_r;
reg [2*XLEN-1:0] accum;
reg [XLEN-1:0] op_b_r;
reg op_a_neg_r;
reg op_b_neg_r;
reg prenegate_op_b;
wire op_a_signed =
op_r == M_OP_MULH ||
op_r == M_OP_MULHSU ||
@ -73,13 +74,16 @@ wire op_b_signed =
op_r == M_OP_REM;
wire op_a_neg = op_a_signed && accum[XLEN-1];
wire op_b_neg = op_b_signed && op_b_r[XLEN-1];
wire op_b_neg = op_b_signed && op_b[XLEN-1];
// Non-divide parts of the circuit should be constant-folded if all the MUL
// operations are handled by the fast multiplier
wire is_div = op_r[2] || (MUL_FAST && MULH_FAST);
wire [XLEN-1:0] op_b_sign_adj = prenegate_op_b ? -op_b : op_b;
// Controls for modifying sign of all/part of accumulator
wire accum_neg_l;
wire accum_inv_h;
@ -103,7 +107,7 @@ always @ (*) begin: alu
addsub_tmp = {2*XLEN{1'b0}};
neg_l_borrow = 1'b0;
for (i = 0; i < MULDIV_UNROLL; i = i + 1) begin
addend = {is_div && |op_b_r, op_b_r, {XLEN-1{1'b0}}};
addend = {is_div && |op_b_sign_adj, op_b_sign_adj, {XLEN-1{1'b0}}};
shift_tmp = is_div ? accum_next : accum_next >> 1;
addsub_tmp = shift_tmp + addend;
accum_next = (is_div ? !addsub_tmp[2 * XLEN - 1] : accum_next[0]) ?
@ -138,7 +142,7 @@ always @ (posedge clk or negedge rst_n) begin
op_r <= {W_MULOP{1'b0}};
op_a_neg_r <= 1'b0;
op_b_neg_r <= 1'b0;
op_b_r <= {XLEN{1'b0}};
prenegate_op_b <= 1'b0;
accum <= {XLEN*2{1'b0}};
end else if (op_kill || (op_vld && op_rdy)) begin
// Initialise circuit with operands + state
@ -146,8 +150,8 @@ always @ (posedge clk or negedge rst_n) begin
sign_preadj_done <= !op_vld;
sign_postadj_done <= !op_vld;
sign_postadj_carry <= 1'b0;
prenegate_op_b <= 1'b0;
op_r <= op;
op_b_r <= op_b;
accum <= {{XLEN{1'b0}}, op_a};
end else if (!sign_preadj_done) begin
// Pre-adjust sign if necessary, else perform first iteration immediately
@ -157,8 +161,9 @@ always @ (posedge clk or negedge rst_n) begin
if (accum_neg_l || (op_b_neg ^ is_div)) begin
if (accum_neg_l)
accum[0 +: XLEN] <= accum_next[0 +: XLEN];
if (op_b_neg ^ is_div)
op_b_r <= -op_b_r;
if (op_b_neg ^ is_div) begin
prenegate_op_b <= 1'b1;
end
end else begin
ctr <= ctr - MULDIV_UNROLL[W_CTR-1:0];
accum <= accum_next;
@ -206,7 +211,7 @@ wire op_signs_differ = op_a_neg_r ^ op_b_neg_r;
assign accum_neg_l =
!sign_preadj_done && op_a_neg ||
do_postadj && !sign_postadj_carry && op_signs_differ && !(is_div && ~|op_b_r);
do_postadj && !sign_postadj_carry && op_signs_differ && !(is_div && ~|op_b_sign_adj);
assign {accum_incr_h, accum_inv_h} =
do_postadj && is_div && op_a_neg_r ? 2'b11 :