From a59bf2fbede4994c9b93fe6698ef590b15a48689 Mon Sep 17 00:00:00 2001
From: Alban Gruin <alban.gruin@irit.fr>
Date: Mon, 13 Dec 2021 15:32:37 +0100
Subject: [PATCH] Revert "frontend: remove the RAS"
This reverts commit 04a4e12b90ada078a9e97cf9856a6a5f7e0b0109.
---
src/frontend/frontend.sv | 26 ++++++++++++++++++++------
1 file changed, 20 insertions(+), 6 deletions(-)
diff --git a/src/frontend/frontend.sv b/src/frontend/frontend.sv
index e14fe45a..7300c59a 100644
--- a/src/frontend/frontend.sv
+++ b/src/frontend/frontend.sv
@@ -159,7 +159,7 @@ module frontend import ariane_pkg::*; #(
// unconditional jumps with known target -> immediately resolved
assign is_jump[i] = instruction_valid[i] & (rvi_jump[i] | rvc_jump[i]);
// unconditional jumps with unknown target -> BTB
- assign is_jalr[i] = instruction_valid[i] & (rvi_jalr[i] | rvc_jalr[i] | rvc_jr[i]);
+ assign is_jalr[i] = instruction_valid[i] & ~is_return[i] & ~is_call[i] & (rvi_jalr[i] | rvc_jalr[i] | rvc_jr[i]);
// cf that needs a prediction
assign is_cf[i] = instruction_valid[i] & (rvi_branch[i] | rvc_branch[i] | rvi_jalr[i] | rvc_jalr[i] | rvc_jr[i]);
@@ -181,10 +181,10 @@ module frontend import ariane_pkg::*; #(
// lower most prediction gets precedence
for (int i = INSTR_PER_FETCH - 1; i >= 0 ; i--) begin
- unique case ({is_branch[i], is_jump[i], is_jalr[i]})
- 3'b000:; // regular instruction e.g.: no branch
+ unique case ({is_branch[i], is_return[i], is_jump[i], is_jalr[i]})
+ 4'b0000:; // regular instruction e.g.: no branch
// unconditional jump to register, we need the BTB to resolve this
- 3'b001: begin
+ 4'b0001: begin
ras_pop = 1'b0;
ras_push = 1'b0;
if (btb_prediction_shifted[i].valid) begin
@@ -193,15 +193,23 @@ module frontend import ariane_pkg::*; #(
end
end
// its an unconditional jump to an immediate
- 3'b010: begin
+ 4'b0010: begin
ras_pop = 1'b0;
ras_push = 1'b0;
taken_rvi_cf[i] = rvi_jump[i];
taken_rvc_cf[i] = rvc_jump[i];
cf_type[i] = ariane_pkg::Jump;
end
+ // return
+ 4'b0100: begin
+ // make sure to only alter the RAS if we actually consumed the instruction
+ ras_pop = ras_predict.valid & instr_queue_consumed[i];
+ ras_push = 1'b0;
+ predict_address = ras_predict.ra;
+ cf_type[i] = ariane_pkg::Return;
+ end
// branch prediction
- 3'b100: begin
+ 4'b1000: begin
ras_pop = 1'b0;
ras_push = 1'b0;
// if we have a valid dynamic prediction use it
@@ -219,6 +227,12 @@ module frontend import ariane_pkg::*; #(
default:;
// default: $error("Decoded more than one control flow");
endcase
+ // if this instruction, in addition, is a call, save the resulting address
+ // but only if we actually consumed the address
+ if (is_call[i]) begin
+ ras_push = instr_queue_consumed[i];
+ ras_update = addr[i] + (rvc_call[i] ? 2 : 4);
+ end
// calculate the jump target address
if (taken_rvc_cf[i] || taken_rvi_cf[i]) begin
predict_address = addr[i] + (taken_rvc_cf[i] ? rvc_imm[i] : rvi_imm[i]);
--
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