From 8317a3fbe57e917653f6982b3db3d9a256313f2d Mon Sep 17 00:00:00 2001
From: Hansung Kim <hansung_kim@berkeley.edu>
Date: Thu, 7 Mar 2024 17:39:18 -0800
Subject: [PATCH] Fix fence by disallowing x-initialization instead of all-0
 mask

Setting mem_req_mask to all-zero triggers an assertion error in
mem_scheduler.  Instead, disallow initialize-by-x in instruction decode
which is the source of x-propagation.  Since this seems to only happen
in VCS, define-gate it accordingly.

This reverts commit a15f4fd483591846ff7df0eb9ef3ca60f322b4b2.
---
 hw/rtl/core/VX_decode.sv   | 8 ++++++++
 hw/rtl/core/VX_lsu_unit.sv | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/hw/rtl/core/VX_decode.sv b/hw/rtl/core/VX_decode.sv
index e38b4c8c..aa18666f 100644
--- a/hw/rtl/core/VX_decode.sv
+++ b/hw/rtl/core/VX_decode.sv
@@ -148,13 +148,21 @@ module VX_decode  #(
     always @(*) begin
 
         ex_type   = '0;
+`ifdef VCS
+        op_type   = '0;
+`else
         op_type   = 'x;
+`endif
         op_mod    = '0;
         rd_r      = '0;
         rs1_r     = '0;
         rs2_r     = '0;
         rs3_r     = '0;
+`ifdef VCS
+        imm       = '0;
+`else
         imm       = 'x;
+`endif
         use_imm   = 0;
         use_PC    = 0;
         use_rd    = 0;
diff --git a/hw/rtl/core/VX_lsu_unit.sv b/hw/rtl/core/VX_lsu_unit.sv
index d823982c..5a57db4c 100644
--- a/hw/rtl/core/VX_lsu_unit.sv
+++ b/hw/rtl/core/VX_lsu_unit.sv
@@ -160,7 +160,7 @@ module VX_lsu_unit import VX_gpu_pkg::*; #(
                    && (~mem_req_rw || st_rsp_ready); // writes commit directly
 
     for (genvar i = 0; i < NUM_LANES; ++i) begin
-        assign mem_req_mask[i] = is_fence ? 1'b0 : (execute_if[0].data.tmask[i] && (~lsu_is_dup || (i == 0)));
+        assign mem_req_mask[i] = execute_if[0].data.tmask[i] && (~lsu_is_dup || (i == 0));
     end
 
     assign mem_req_rw = ~execute_if[0].data.wb;