[AMDGPU] Fix WMM Entry SCC preservation

[critson]

Summary

SCC was not correctly preserved when entering WWM.
Current lit test was unable to detect this as entry block is
handled differently.
Additionally fix an issue where SCC was unnecessarily preserved
when exiting from WWM to Exact mode.

[foad]

llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp

843

// for comments please.

847

All three SaveSCC = calculations could do with an explanatory comment.

856

llvm_unreachable

llvm/test/CodeGen/AMDGPU/wqm.mir

63

Remove most of this? Usually you only need name, tracksRegLiveness and body.

[critson]

llvm/test/CodeGen/AMDGPU/wqm.mir

63

registers are needed if types are not embedded.
I have switch the new test to embedded types.

[critson]

Address comments.

[foad]

Looks good.

llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp

if (!(Needs & State)) {
       if (State == StateWWM || Needs == StateWWM) {
         // We must switch to or from WWM
         First = FirstWWM;
       } else {
         // We only need to switch to/from WQM, so we can use FirstWQM
         First = FirstWQM;
       }
 
+      // Whether we need to save SCC depends on start and end states
+      bool SaveSCC = false;
+      switch (State) {
+      case StateExact:
+      case StateWWM:
+        // Exact/WWM -> WWM: save SCC
+        // Exact/WWM -> WQM: save SCC if WQM mask is generated from exec
+        // Exact/WWM -> Exact: no save
+        SaveSCC = (Needs & StateWWM) || ((Needs & StateWQM) && WQMFromExec);
+        break;
+      case StateWQM:
+        // WQM -> Exact/WMM: save SCC
+        SaveSCC = !(Needs & StateWQM);
+        break;
+      default:
+        llvm_unreachable("Unknown state");
+        break;
+      }
       MachineBasicBlock::iterator Before =
-          prepareInsertion(MBB, First, II, Needs == StateWQM,
-                           Needs == StateExact || WQMFromExec);
+          prepareInsertion(MBB, First, II, Needs == StateWQM, SaveSCC);
 
       if (State == StateWWM) {
         assert(SavedNonWWMReg);
         fromWWM(MBB, Before, SavedNonWWMReg);
         LIS->createAndComputeVirtRegInterval(SavedNonWWMReg);
         SavedNonWWMReg = 0;
         State = NonWWMState;
       }

llvm/test/CodeGen/AMDGPU/wqm.mir

bb.0:
     %5 = S_CSELECT_B32 %2, %1, implicit $scc
     %11 = V_ADD_CO_U32_e32 %5, %12, implicit-def $vcc, implicit $exec
     $vgpr0 = WWM %11, implicit $exec
     SI_RETURN_TO_EPILOG $vgpr0
 
 ...
 
 ---
+# Second test for awareness that s_or_saveexec_b64 clobbers SCC
+# Because entry block is treated differently.
+#
+#CHECK: %bb.1
+#CHECK: S_CMP_LT_I32
+#CHECK: COPY $scc
+#CHECK: ENTER_WWM
+#CHECK: $scc = COPY
+#CHECK: S_CSELECT_B32
+name:            test_wwm_scc2
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $sgpr0, $sgpr1, $sgpr2, $vgpr0
+
+    %3:vgpr_32 = COPY $vgpr0
+    %2:sgpr_32 = COPY $sgpr2
+    %1:sgpr_32 = COPY $sgpr1
+    %0:sgpr_32 = COPY $sgpr0
+    %13:sgpr_128 = IMPLICIT_DEF
+
+  bb.1:
+    S_CMP_LT_I32 0, %0:sgpr_32, implicit-def $scc
+    %10:vgpr_32 = BUFFER_LOAD_DWORD_OFFEN %3:vgpr_32, %13:sgpr_128, 0, 0, 0, 0, 0, 0, 0, implicit $exec
+    %12:vgpr_32 = V_ADD_CO_U32_e32 %3:vgpr_32, %3:vgpr_32, implicit-def $vcc, implicit $exec
+    %5:sgpr_32 = S_CSELECT_B32 %2:sgpr_32, %1:sgpr_32, implicit $scc
+    %11:vgpr_32 = V_ADD_CO_U32_e32 %5:sgpr_32, %12:vgpr_32, implicit-def $vcc, implicit $exec
+    $vgpr0 = WWM %11:vgpr_32, implicit $exec
+    $vgpr1 = COPY %10:vgpr_32
+    SI_RETURN_TO_EPILOG $vgpr0, $vgpr1
+
+...
+
+---
 # V_SET_INACTIVE, when its second operand is undef, is replaced by a
 # COPY by si-wqm. Ensure the instruction is removed.
 #CHECK-NOT: V_SET_INACTIVE
 name:            no_cfg
 alignment:       1
 exposesReturnsTwice: false
 legalized:       false
 regBankSelected: false