fix: do not wipe EX on FP-induced WAW stalls (#649)

Fixes #304.

* fix: do not wipe EX on FP-induced WAW stalls

Similar to #305 (which originally fixed #304), add a check on whether
EX is empty before inserting a BUBBLE instruction.

Also add some more verbose logging to help reason about the flow
without having to use a debugger.

* test: add a regression test for issue 304

This test would have exposed the second reoccurrence of issue #304.

Thanks to @hugmanrique for this code.

* fix: fix merge conflict resolution error

src/main/java/org/edumips64/core/CPU.java

@@ -445,32 +445,41 @@ public void step() throws AddressErrorException, HaltException, IrregularWriteOp
       pc.writeDoubleWord((pc.getValue()) + 4);
 
     } catch (RAWException ex) {
+      logger.info("RAW - Read-After-Write");
       if (currentPipeStage == Pipeline.Stage.ID && pipe.EX() == null) {
+        logger.info("Adding a BUBBLE instruction into EX.");
         pipe.setEX(bubble);
       }
       RAWStalls++;
 
     } catch (WAWException ex) {
+      logger.info("WAW - Write-After-Write");
       logger.info(fpPipe.toString());
 
-      if (currentPipeStage == Pipeline.Stage.ID) {
+      if (currentPipeStage == Pipeline.Stage.ID && pipe.EX() == null) {
+        logger.info("Adding a BUBBLE instruction into EX.");
         pipe.setEX(bubble);
       }
       WAWStalls++;
 
     } catch (FPDividerNotAvailableException ex) {
+      logger.info("Structural Stall - FP Divider Unavailable");
       if (currentPipeStage == Pipeline.Stage.ID) {
+        logger.info("Adding a BUBBLE instruction into EX.");
         pipe.setEX(bubble);
       }
       dividerStalls++;
 
     } catch (FPFunctionalUnitNotAvailableException ex) {
+      logger.info("Structural Stall - FP Unavailable");
       if (currentPipeStage == Pipeline.Stage.ID) {
+        logger.info("Adding a BUBBLE instruction into EX.");
         pipe.setEX(bubble);
       }
       funcUnitStalls++;
 
     } catch (EXNotAvailableException ex) {
+      logger.info("Structural Stall - EX Unavailable");
       exStalls++;
 
     } catch (SynchronousException ex) {

src/test/java/org/edumips64/EndToEndTests.java

@@ -684,6 +684,13 @@ public void testIssue304UI() throws Exception {
     runMipsTest("infinite-bug-304.s", false);
   }
 
+  /* Issue #304: Infinite RAW stall in floating-point due to instruction overwrite in the EX stage.
+  */
+  @Test(timeout=2000)
+  public void testIssue304EX() throws Exception {
+    runMipsTest("issue-304-ex.s", false);
+  }
+
   /* Issue #304: Missing MEM/WB in Cycles UI for some FPU instructions.
   */
   @Test(timeout=2000)

src/test/resources/issue-304-ex.s

@@ -0,0 +1,40 @@
+; Thanks to @hugmanrique for this code.
+
+.data
+v: .space 256
+w: .space 256
+x: .double 2.0
+y: .double 1.5
+z: .double 0.0
+
+.text
+daddi R1, R0, v
+daddi R2, R0, w
+daddi R3, R0, 256
+
+ldc1 F2, x(R0)
+ldc1 F4, y(R0)
+daddi R4, R0, 0
+
+loop1:
+dmtc1 R4, F6
+daddi R4, R4, 8
+cvt.d.l F6, F6
+mul.d F8, F2, F6
+mul.d F10, F4, F6
+dmtc1 R4, F12
+daddi R4, R4, 8
+cvt.d.l F12, F12
+mul.d F14, F2, F12
+daddi R1, R1, 16
+mul.d F8, F8, F8
+daddi R2, R2, 16
+
+sdc1 F10, -16(R2)
+mul.d F14, F14, F14
+mul.d F16, F4, F12
+sdc1 F8, -16(R1) ; here 
+sdc1 F14, -8(R1)
+sdc1 F16, -8(R2)
+bne R4, R3, loop1
+syscall 0