Reland the WA that emits a NOP after branch instruction to support single stepping in debugger.

The WA is enabled only when compiling with -O0 currently.

Author: petechou

IGC/Compiler/CISACodeGen/CISABuilder.cpp

@@ -5102,6 +5102,12 @@ namespace IGC
         if (IGC_IS_FLAG_ENABLED(deadLoopForFloatException)) {
           SaveOption(vISA_AddIEEEExceptionTrap, true);
         }
+
+        // WA to support single stepping in debugger. Currently only enabled it
+        // when compiling with -O0.
+        if (isOptDisabled && m_program->m_Platform->WaAddNopAfterBranchInst()) {
+            SaveOption(vISA_GenerateNopAfterCFInst, true);
+        }
     } // InitVISABuilderOptions
 
     // Get a unqiue label for inline asm instruction blocks at the module level.

IGC/Compiler/CISACodeGen/Platform.hpp

@@ -1490,5 +1490,18 @@ unsigned int roundUpTgsmSize(DWORD size) const
     return iSTD::RoundPower2(size) * blockSize;
 }
 
+bool WaAddNopAfterBranchInst() const
+{
+    if (m_platformInfo.eProductFamily == IGFX_PVC)
+        return true;
+
+    if (m_platformInfo.eProductFamily == IGFX_DG2 &&
+        (GFX_IS_DG2_G11_CONFIG(m_platformInfo.usDeviceID) ||
+         GFX_IS_DG2_G12_CONFIG(m_platformInfo.usDeviceID)))
+        return true;
+
+    return false;
+}
+
 };
 }//namespace IGC

visa/Optimizer.h

@@ -260,6 +260,7 @@ class Optimizer {
   void applyNamedBarrierWA(INST_LIST_ITER it, G4_BB *bb);
   void insertIEEEExceptionTrap();
   void expandIEEEExceptionTrap(INST_LIST_ITER it, G4_BB *bb);
+  void insertNopAfterCFInst(BB_LIST_ITER ib, INST_LIST_ITER it);
 
   typedef std::vector<vISA::G4_INST *> InstListType;
   // create instruction sequence to calculate call offset from ip

visa/SWWA.cpp

@@ -3337,6 +3337,36 @@ void Optimizer::expandIEEEExceptionTrap(INST_LIST_ITER it, G4_BB *bb) {
   *it = restoreFlag;
 }
 
+void Optimizer::insertNopAfterCFInst(BB_LIST_ITER ib, INST_LIST_ITER ii) {
+  // If the CF inst is not the last inst of current BB, simply insert
+  // a nop after it. Otherwise, insert a dummy BB to hold a nop between
+  // the current BB and the fall-through BB.
+  G4_BB *bb = *ib;
+  G4_INST *inst = *ii;
+  if (inst != bb->back()) {
+    bb->insertAfter(ii, builder.createNop(InstOpt_NoOpt));
+  } else {
+    G4_BB *dummyBB = fg.createNewBBWithLabel("CFInstWA_BB");
+    dummyBB->push_back(builder.createNop(InstOpt_NoOpt));
+    BB_LIST_ITER nextBI = std::next(ib);
+    fg.insert(nextBI, dummyBB);
+    // Update the pred/succ edges accordingly.
+    bb->setPhysicalSucc(dummyBB);
+    dummyBB->setPhysicalPred(bb);
+    if (nextBI != fg.end()) {
+      G4_BB *nextBB = *nextBI;
+      dummyBB->setPhysicalSucc(nextBB);
+      nextBB->setPhysicalPred(dummyBB);
+      if (std::any_of(bb->Succs.begin(), bb->Succs.end(),
+                      [=](G4_BB *succ) { return succ == nextBB; } )) {
+        fg.removePredSuccEdges(bb, nextBB);
+        fg.addPredSuccEdges(bb, dummyBB);
+        fg.addPredSuccEdges(dummyBB, nextBB);
+      }
+    }
+  }
+}
+
 // For a subroutine, insert a dummy move with {Switch} option immediately
 // before the first non-label instruction in BB. Otherwie, for a following
 // basic block, insert a dummy move before *any* instruction to ensure that
@@ -3767,6 +3797,9 @@ void Optimizer::HWWorkaround() {
       if (inst->isIEEEExceptionTrap())
         expandIEEEExceptionTrap(ii, bb);
 
+      if (inst->isCFInst() && builder.getOption(vISA_GenerateNopAfterCFInst))
+        insertNopAfterCFInst(ib, ii);
+
       ii++;
     }
   }

visa/include/VISAOptionsDefs.h

@@ -641,6 +641,8 @@ DEF_VISA_OPTION(vISA_noIndirectSrcForCompressedInstWA, ET_BOOL,
 DEF_VISA_OPTION(vISA_GenerateDebugInfo, ET_BOOL, "-generateDebugInfo", UNUSED,
                 false)
 DEF_VISA_OPTION(vISA_setStartBreakPoint, ET_BOOL, "-setstartbp", UNUSED, false)
+DEF_VISA_OPTION(vISA_GenerateNopAfterCFInst, ET_BOOL,
+                "-generateNopAfterCFInst", UNUSED, false)
 DEF_VISA_OPTION(vISA_InsertHashMovs, ET_BOOL, NULLSTR, UNUSED, false)
 DEF_VISA_OPTION(vISA_InsertDummyMovForHWRSWA, ET_BOOL, "-insertRSDummyMov",
                 UNUSED, false)