control: Fix forwarding when previous result write is suppressed

If we have two successive instructions that write the same result
register and then a third that uses the same register as an input, and
the second instruction suppresses the write of its result, we can
currently end up with the third instruction using the wrong value,
because it uses the register value from before the first instruction
rather than the result of the first instruction.  (An example of an
instruction suppressing the write of its result is a floating-point
instruction that generates an enabled invalid operation exception but
not an interrupt.)

To fix this, the control module now uses any forwarded value for the
register we want, not just the most recent value, but still stalls
until it has the most recent value, or the previous instruction
completes.  Thus in the case described above, decode2 will have
latched the value from the first instruction and so the third
instruction gets the correct value.

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

control.vhdl

@@ -36,6 +36,7 @@ entity control is
         execute_next_cr_tag : in instr_tag_t;
         execute2_next_tag    : in instr_tag_t;
         execute2_next_cr_tag : in instr_tag_t;
+        writeback_tag       : in instr_tag_t;
 
         cr_read_in          : in std_ulogic;
         cr_write_in         : in std_ulogic;
@@ -163,70 +164,109 @@ begin
         variable byp_cr : std_ulogic_vector(1 downto 0);
         variable tag_ov : instr_tag_t;
         variable tag_prev : instr_tag_t;
+        variable rma : std_ulogic_vector(TAG_COUNT-1 downto 0);
+        variable rmb : std_ulogic_vector(TAG_COUNT-1 downto 0);
+        variable rmc : std_ulogic_vector(TAG_COUNT-1 downto 0);
+        variable tag_a_stall : std_ulogic;
+        variable tag_b_stall : std_ulogic;
+        variable tag_c_stall : std_ulogic;
     begin
         tag_a := instr_tag_init;
+        tag_a_stall := '0';
+        rma := (others => '0');
         for i in tag_number_t loop
-            if tag_regs(i).wr_gpr = '1' and tag_regs(i).recent = '1' and tag_regs(i).reg = gpr_a_read_in then
-                tag_a.valid := '1';
-                tag_a.tag := i;
+            if tag_regs(i).valid = '1' and tag_regs(i).wr_gpr = '1' and
+                tag_regs(i).reg = gpr_a_read_in and gpr_a_read_valid_in = '1' then
+                rma(i) := '1';
+                if tag_regs(i).recent = '1' then
+                    tag_a_stall := '1';
                 end if;
-        end loop;
-        tag_b := instr_tag_init;
-        for i in tag_number_t loop
-            if tag_regs(i).wr_gpr = '1' and tag_regs(i).recent = '1' and tag_regs(i).reg = gpr_b_read_in then
-                tag_b.valid := '1';
-                tag_b.tag := i;
-            end if;
-        end loop;
-        tag_c := instr_tag_init;
-        for i in tag_number_t loop
-            if tag_regs(i).wr_gpr = '1' and tag_regs(i).recent = '1' and tag_regs(i).reg = gpr_c_read_in then
-                tag_c.valid := '1';
-                tag_c.tag := i;
             end if;
         end loop;
-
         byp_a := "0000";
-        if EX1_BYPASS and tag_match(execute_next_tag, tag_a) then
+        if EX1_BYPASS and execute_next_tag.valid = '1' and
+            rma(execute_next_tag.tag) = '1' then
             byp_a(1) := '1';
-        end if;
-        if EX1_BYPASS and tag_match(execute2_next_tag, tag_a) then
+            tag_a := execute_next_tag;
+        elsif EX1_BYPASS and execute2_next_tag.valid = '1' and
+            rma(execute2_next_tag.tag) = '1' then
             byp_a(2) := '1';
-        end if;
-        if tag_match(complete_in, tag_a) then
+            tag_a := execute2_next_tag;
+        elsif writeback_tag.valid = '1' and rma(writeback_tag.tag) = '1' then
             byp_a(3) := '1';
+            tag_a := writeback_tag;
         end if;
         byp_a(0) := gpr_a_read_valid_in and (byp_a(1) or byp_a(2) or byp_a(3));
+        if tag_a.valid = '1' and tag_regs(tag_a.tag).valid = '1' and
+            tag_regs(tag_a.tag).recent = '1' then
+            tag_a_stall := '0';
+        end if;
+
+        tag_b := instr_tag_init;
+        tag_b_stall := '0';
+        rmb := (others => '0');
+        for i in tag_number_t loop
+            if tag_regs(i).valid = '1' and tag_regs(i).wr_gpr = '1' and
+                tag_regs(i).reg = gpr_b_read_in and gpr_b_read_valid_in = '1' then
+                rmb(i) := '1';
+                if tag_regs(i).recent = '1' then
+                    tag_b_stall := '1';
+                end if;
+            end if;
+        end loop;
         byp_b := "0000";
-        if EX1_BYPASS and tag_match(execute_next_tag, tag_b) then
+        if EX1_BYPASS and execute_next_tag.valid = '1' and
+            rmb(execute_next_tag.tag) = '1' then
             byp_b(1) := '1';
-        end if;
-        if EX1_BYPASS and tag_match(execute2_next_tag, tag_b) then
+            tag_b := execute_next_tag;
+        elsif EX1_BYPASS and execute2_next_tag.valid = '1' and
+            rmb(execute2_next_tag.tag) = '1' then
             byp_b(2) := '1';
-        end if;
-        if tag_match(complete_in, tag_b) then
+            tag_b := execute2_next_tag;
+        elsif writeback_tag.valid = '1' and rmb(writeback_tag.tag) = '1' then
             byp_b(3) := '1';
+            tag_b := writeback_tag;
         end if;
         byp_b(0) := gpr_b_read_valid_in and (byp_b(1) or byp_b(2) or byp_b(3));
+        if tag_b.valid = '1' and tag_regs(tag_b.tag).valid = '1' and
+            tag_regs(tag_b.tag).recent = '1' then
+            tag_b_stall := '0';
+        end if;
+
+        tag_c := instr_tag_init;
+        tag_c_stall := '0';
+        rmc := (others => '0');
+        for i in tag_number_t loop
+            if tag_regs(i).valid = '1' and tag_regs(i).wr_gpr = '1' and
+                tag_regs(i).reg = gpr_c_read_in and gpr_c_read_valid_in = '1' then
+                rmc(i) := '1';
+                if tag_regs(i).recent = '1' then
+                    tag_c_stall := '1';
+                end if;
+            end if;
+        end loop;
         byp_c := "0000";
-        if EX1_BYPASS and tag_match(execute_next_tag, tag_c) then
+        if EX1_BYPASS and execute_next_tag.valid = '1' and rmc(execute_next_tag.tag) = '1' then
             byp_c(1) := '1';
-        end if;
-        if EX1_BYPASS and tag_match(execute2_next_tag, tag_c) then
+            tag_c := execute_next_tag;
+        elsif EX1_BYPASS and execute2_next_tag.valid = '1' and rmc(execute2_next_tag.tag) = '1' then
             byp_c(2) := '1';
-        end if;
-        if tag_match(complete_in, tag_c) then
+            tag_c := execute2_next_tag;
+        elsif writeback_tag.valid = '1' and rmc(writeback_tag.tag) = '1' then
             byp_c(3) := '1';
+            tag_c := writeback_tag;
         end if;
         byp_c(0) := gpr_c_read_valid_in and (byp_c(1) or byp_c(2) or byp_c(3));
+        if tag_c.valid = '1' and tag_regs(tag_c.tag).valid = '1' and
+            tag_regs(tag_c.tag).recent = '1' then
+            tag_c_stall := '0';
+        end if;
 
         gpr_bypass_a <= byp_a;
         gpr_bypass_b <= byp_b;
         gpr_bypass_c <= byp_c;
 
-        gpr_tag_stall <= (tag_a.valid and gpr_a_read_valid_in and not byp_a(0)) or
-                         (tag_b.valid and gpr_b_read_valid_in and not byp_b(0)) or
-                         (tag_c.valid and gpr_c_read_valid_in and not byp_c(0));
+        gpr_tag_stall <= tag_a_stall or tag_b_stall or tag_c_stall;
 
         incr_tag := curr_tag;
         instr_tag.tag <= curr_tag;

decode2.vhdl

@@ -283,6 +283,7 @@ begin
             execute_next_cr_tag  => execute_cr_bypass.tag,
             execute2_next_tag    => execute2_bypass.tag,
             execute2_next_cr_tag => execute2_cr_bypass.tag,
+            writeback_tag        => writeback_bypass.tag,
 
             cr_read_in           => cr_read_valid,
             cr_write_in          => cr_write_valid,