Run SSA pass to clean up temporary variables during generics lowering. (#1447)

* Run SSA pass to clean up generic temporary variables during lowering.

* Fix `undefined` emitting logic.

* revert dumpir control flag

* Defer fold decision of `undefined`  values after special case logic for GLSL and HLSL.

* Update expected test result.

* Manually update raygen.slang.glsl to minimize change.

* fix formatting

Co-authored-by: Tim Foley <tfoleyNV@users.noreply.github.com>

4 files changed, 26 insertions, 1 deletions

diff --git a/source/slang/slang-emit-c-like.cpp b/source/slang/slang-emit-c-like.cpp
index 733811183..a3b11a908 100644
--- a/source/slang/slang-emit-c-like.cpp
+++ b/source/slang/slang-emit-c-like.cpp
@@ -1074,6 +1074,15 @@ bool CLikeSourceEmitter::shouldFoldInstIntoUseSites(IRInst* inst)
     if(inst->findDecoration<IRPreciseDecoration>())
         return false;
 
+    // In general, undefined value should be emitted as an uninitialized
+    // variable, so we shouldn't fold it.
+    // However, we cannot emit all undefined values a separate variable
+    // definition for certain types on certain targets (e.g. `out TriangleStream<T>`
+    // for GLSL), so we check this only after all those special cases are
+    // considered.
+    if (inst->op == kIROp_undefined)
+        return false;
+
     // Okay, at this point we know our instruction must have a single use.
     auto use = inst->firstUse;
     SLANG_ASSERT(use);
diff --git a/source/slang/slang-ir-lower-generic-var.cpp b/source/slang/slang-ir-lower-generic-var.cpp
index 8799dea6b..936ed017f 100644
--- a/source/slang/slang-ir-lower-generic-var.cpp
+++ b/source/slang/slang-ir-lower-generic-var.cpp
@@ -79,6 +79,12 @@ namespace Slang
                 // have already been replaced with the pointer to that variable by now.
                 // So we don't need to do anything here.
             }
+            else if (valPtr->op == kIROp_undefined)
+            {
+                // We don't need to store an undef value.
+                storeInst->removeAndDeallocate();
+                return;
+            }
             else
             {
                 // If value does not come from another generic variable, then it must be
@@ -138,7 +144,7 @@ namespace Slang
 
         void processInst(IRInst* inst)
         {
-            if (inst->op == kIROp_Var || inst->op == kIROp_undefined)
+            if (inst->op == kIROp_Var)
             {
                 processVarInst(inst);
             }
diff --git a/source/slang/slang-ir-lower-generics.cpp b/source/slang/slang-ir-lower-generics.cpp
index 61fa8ad17..c60b5386a 100644
--- a/source/slang/slang-ir-lower-generics.cpp
+++ b/source/slang/slang-ir-lower-generics.cpp
@@ -6,6 +6,8 @@
 #include "slang-ir-lower-generic-call.h"
 #include "slang-ir-lower-generic-var.h"
 #include "slang-ir-witness-table-wrapper.h"
+#include "slang-ir-ssa.h"
+#include "slang-ir-dce.h"
 
 namespace Slang
 {
@@ -16,7 +18,14 @@ namespace Slang
         sharedContext.module = module;
         lowerGenericFunctions(&sharedContext);
         lowerGenericCalls(&sharedContext);
+        // We might have generated new temporary variables during lowering.
+        // An SSA pass can clean up unncessary load/stores.
+        constructSSA(module);
+        eliminateDeadCode(module);
         lowerGenericVar(&sharedContext);
+        // After lowerGenericVar, there could be some unused `undef` values.
+        // We eliminate them in a DCE pass.
+        eliminateDeadCode(module);
         generateWitnessTableWrapperFunctions(&sharedContext);
     }
 } // namespace Slang
diff --git a/source/slang/slang-ir.cpp b/source/slang/slang-ir.cpp
index fc4b71138..f551dcbba 100644
--- a/source/slang/slang-ir.cpp
+++ b/source/slang/slang-ir.cpp
@@ -5151,6 +5151,7 @@ namespace Slang
             return false;
 
         case kIROp_Nop:
+        case kIROp_undefined:
         case kIROp_Specialize:
         case kIROp_lookup_interface_method:
         case kIROp_getAddr: