From: Paul Mackerras <paulus@ozlabs.org>
Date: Sat, 13 Jun 2020 13:00:13 +0000 (+1000)
Subject: dcache: Reduce back-to-back store latency from 3 cycles to 2
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=a4500c63a281a57752edbfdd4d9033974a98c8c8;p=microwatt.git

dcache: Reduce back-to-back store latency from 3 cycles to 2

This uses the machinery we already had for comparing the real address
of a new request with the tag of a previous request (r1.reload_tag)
to get better timing on comparing the address of a second store with
the one in progress.  The comparison is now on the set size rather
than the page size, but since set size can't be larger than the page
size (and usually will equal the page size), that is OK.

The same comparison can also be used to tell when we can satisfy
a load miss during a cache line refill.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
---

diff --git a/dcache.vhdl b/dcache.vhdl
index bc351b0..9ecb6a9 100644
--- a/dcache.vhdl
+++ b/dcache.vhdl
@@ -232,6 +232,7 @@ architecture rtl of dcache is
         byte_sel  : std_ulogic_vector(7 downto 0);
         hit_way   : way_t;
         repl_way  : way_t;
+        same_tag  : std_ulogic;
     end record;
 
     -- First stage register, contains state for stage 1 of load hits
@@ -301,6 +302,7 @@ architecture rtl of dcache is
     signal req_tag     : cache_tag_t;
     signal req_op      : op_t;
     signal req_data    : std_ulogic_vector(63 downto 0);
+    signal req_same_tag : std_ulogic;
 
     signal early_req_row  : row_t;
 
@@ -777,6 +779,7 @@ begin
                 rel_match := '1';
             end if;
         end if;
+        req_same_tag <= rel_match;
 
         -- See if the request matches the line currently being reloaded
         if r1.state = RELOAD_WAIT_ACK and req_index = r1.store_index and
@@ -1222,6 +1225,7 @@ begin
                     req.byte_sel := r0.req.byte_sel;
                     req.hit_way := req_hit_way;
                     req.repl_way := replace_way;
+                    req.same_tag := req_same_tag;
 
                     -- Store the incoming request from r0, if it is a slow request
                     -- Note that r1.full = 1 implies req_op = OP_NONE
@@ -1243,6 +1247,7 @@ begin
                     r1.store_row <= get_row(req.real_addr);
                     r1.end_row_ix <= get_row_of_line(get_row(req.real_addr)) - 1;
                     r1.reload_tag <= get_tag(req.real_addr);
+                    r1.req.same_tag <= '1';
 
                     if req.op = OP_STORE_HIT then
                         r1.store_way <= req.hit_way;
@@ -1346,11 +1351,10 @@ begin
                         -- complete the request next cycle.
                         -- Compare the whole address in case the request in
                         -- r1.req is not the one that started this refill.
-			if r1.full = '1' and
+			if r1.full = '1' and r1.req.same_tag = '1' and
                             ((r1.dcbz = '1' and r1.req.dcbz = '1') or
                              (r1.dcbz = '0' and r1.req.op = OP_LOAD_MISS)) and
-                            r1.store_row = get_row(r1.req.real_addr) and
-                            r1.reload_tag = get_tag(r1.req.real_addr) then
+                            r1.store_row = get_row(r1.req.real_addr) then
                             r1.full <= '0';
                             r1.slow_valid <= '1';
                             r1.forward_sel <= (others => '1');
@@ -1379,19 +1383,14 @@ begin
                     if wishbone_in.stall = '0' then
                         -- See if there is another store waiting to be done
                         -- which is in the same real page.
-                        -- Using r1.req rather than req here limits us to one
-                        -- store every two cycles, but helps timing in that we
-                        -- don't depend on req_op or ra.
-                        if r1.full = '1' and acks < 7 and
-                            (r1.req.op = OP_STORE_MISS or r1.req.op = OP_STORE_HIT) and
-                            (r1.req.real_addr(r1.wb.adr'left downto TLB_LG_PGSZ) =
-                             r1.wb.adr(r1.wb.adr'left downto TLB_LG_PGSZ)) then
-                            r1.wb.adr <= r1.req.real_addr(r1.wb.adr'left downto 0);
-                            r1.wb.dat <= r1.req.data;
-                            r1.wb.sel <= r1.req.byte_sel;
+                        if acks < 7 and req.same_tag = '1' and
+                            (req.op = OP_STORE_MISS or req.op = OP_STORE_HIT) then
+                            r1.wb.adr <= req.real_addr(r1.wb.adr'left downto 0);
+                            r1.wb.dat <= req.data;
+                            r1.wb.sel <= req.byte_sel;
                             r1.wb.stb <= '1';
                             stbs_done := false;
-                            if r1.req.op = OP_STORE_HIT then
+                            if req.op = OP_STORE_HIT then
                                 r1.write_bram <= '1';
                             end if;
                             r1.full <= '0';