Small IR cleanups. (#2441)

3 files changed, 25 insertions, 1 deletions

diff --git a/source/slang/slang-emit.cpp b/source/slang/slang-emit.cpp
index a916d0d63..6d85e1ce0 100644
--- a/source/slang/slang-emit.cpp
+++ b/source/slang/slang-emit.cpp
@@ -830,6 +830,9 @@ Result linkAndOptimizeIR(
         }
     }
 
+    // Run a final round of DCE to clean up unused things after phi-elimination.
+    eliminateDeadCode(irModule);
+
     // We include one final step to (optionally) dump the IR and validate
     // it after all of the optimization passes are complete. This should
     // reflect the IR that code is generated from as closely as possible.
diff --git a/source/slang/slang-ir-eliminate-phis.cpp b/source/slang/slang-ir-eliminate-phis.cpp
index c4f0b8b9d..07d0e7374 100644
--- a/source/slang/slang-ir-eliminate-phis.cpp
+++ b/source/slang/slang-ir-eliminate-phis.cpp
@@ -807,7 +807,10 @@ struct PhiEliminationContext
             // so that any logic that might have moved another parameter
             // into a temporary will influence our result.
             //
-            m_builder.emitStore(dstParam.temp, *srcArg.currentValPtr);
+            if ((*srcArg.currentValPtr)->getOp() != kIROp_undefined)
+            {
+                m_builder.emitStore(dstParam.temp, *srcArg.currentValPtr);
+            }
 
             //
             // Once the store is emitted, the assignment has been performed,
diff --git a/source/slang/slang-ir-simplify-cfg.cpp b/source/slang/slang-ir-simplify-cfg.cpp
index db0274d32..a1bc38b64 100644
--- a/source/slang/slang-ir-simplify-cfg.cpp
+++ b/source/slang/slang-ir-simplify-cfg.cpp
@@ -12,6 +12,9 @@ bool processFunc(IRGlobalValueWithCode* func)
     if (!firstBlock)
         return false;
 
+    SharedIRBuilder sharedBuilder(func->getModule());
+    IRBuilder builder(&sharedBuilder);
+
     bool changed = false;
 
     List<IRBlock*> workList;
@@ -25,12 +28,27 @@ bool processFunc(IRGlobalValueWithCode* func)
         {
             if (auto loop = as<IRLoop>(block->getTerminator()))
             {
+                // If continue block is unreachable, remove it.
                 auto continueBlock = loop->getContinueBlock();
                 if (continueBlock && !continueBlock->hasMoreThanOneUse())
                 {
                     loop->continueBlock.set(loop->getTargetBlock());
                     continueBlock->removeAndDeallocate();
                 }
+                // If there isn't any actual back jumps into loop target, remove the header
+                // and turn it into a normal branch.
+                auto targetBlock = loop->getTargetBlock();
+                if (targetBlock->getPredecessors().getCount() == 1 && *targetBlock->getPredecessors().begin() == block)
+                {
+                    builder.setInsertBefore(loop);
+                    List<IRInst*> args;
+                    for (UInt i = 0; i < loop->getArgCount(); i++)
+                    {
+                        args.add(loop->getArg(i));
+                    }
+                    builder.emitBranch(targetBlock, args.getCount(), args.getBuffer());
+                    loop->removeAndDeallocate();
+                }
             }
             // If `block` does not end with an unconditional branch, bail.
             if (block->getTerminator()->getOp() != kIROp_unconditionalBranch)