[HLSL, SPIRV_1_3] Hoist OpSelect returning a composite into `if`/`else` (#7594)

* emit var and hoist out OpSelect if Composite

* cleanup comment

* address review

check for version in spv context

use phi node instead of using var

move inst's using a list (not in-place modification)

* format code

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

6 files changed, 188 insertions, 44 deletions

diff --git a/source/slang/slang-compiler.h b/source/slang/slang-compiler.h
index 57c20aed2..60f6cc92f 100644
--- a/source/slang/slang-compiler.h
+++ b/source/slang/slang-compiler.h
@@ -2947,6 +2947,34 @@ class ExtensionTracker : public RefObject
 public:
 };
 
+struct RequiredLoweringPassSet
+{
+    bool debugInfo;
+    bool resultType;
+    bool optionalType;
+    bool enumType;
+    bool combinedTextureSamplers;
+    bool reinterpret;
+    bool generics;
+    bool bindExistential;
+    bool autodiff;
+    bool derivativePyBindWrapper;
+    bool bitcast;
+    bool existentialTypeLayout;
+    bool bindingQuery;
+    bool meshOutput;
+    bool higherOrderFunc;
+    bool globalVaryingVar;
+    bool glslSSBO;
+    bool byteAddressBuffer;
+    bool dynamicResource;
+    bool dynamicResourceHeap;
+    bool resolveVaryingInputRef;
+    bool specializeStageSwitch;
+    bool missingReturn;
+    bool nonVectorCompositeSelect;
+};
+
 /// A context for code generation in the compiler back-end
 struct CodeGenContext
 {
@@ -3076,11 +3104,24 @@ public:
     // This is a no-op if modules are not precompiled.
     bool shouldSkipDownstreamLinking();
 
+    RequiredLoweringPassSet& getRequiredLoweringPassSet() { return m_requiredLoweringPassSet; }
+
 protected:
     CodeGenTarget m_targetFormat = CodeGenTarget::Unknown;
     Profile m_targetProfile;
     ExtensionTracker* m_extensionTracker = nullptr;
 
+    // To improve the performance of our backend, we will try to avoid running
+    // passes related to features not used in the user code.
+    // To do so, we will scan the IR module once, and determine which passes are needed
+    // based on the instructions used in the IR module.
+    // This will allow us to skip running passes that are not needed, without having to
+    // run all the passes only to find out that no work is needed.
+    // This is especially important for the performance of the backend, as some passes
+    // have an initialization cost (such as building reference graphs or DOM trees) that
+    // can be expensive.
+    RequiredLoweringPassSet m_requiredLoweringPassSet;
+
     /// Will output assembly as well as the artifact if appropriate for the artifact type for
     /// assembly output and conversion is possible
     void _dumpIntermediateMaybeWithAssembly(IArtifact* artifact);
diff --git a/source/slang/slang-emit.cpp b/source/slang/slang-emit.cpp
index db8a9ba61..aaefdaee0 100644
--- a/source/slang/slang-emit.cpp
+++ b/source/slang/slang-emit.cpp
@@ -56,6 +56,7 @@
 #include "slang-ir-layout.h"
 #include "slang-ir-legalize-array-return-type.h"
 #include "slang-ir-legalize-binary-operator.h"
+#include "slang-ir-legalize-composite-select.h"
 #include "slang-ir-legalize-empty-array.h"
 #include "slang-ir-legalize-global-values.h"
 #include "slang-ir-legalize-image-subscript.h"
@@ -300,43 +301,6 @@ struct LinkingAndOptimizationOptions
     CLikeSourceEmitter* sourceEmitter = nullptr;
 };
 
-// To improve the performance of our backend, we will try to avoid running
-// passes related to features not used in the user code.
-// To do so, we will scan the IR module once, and determine which passes are needed
-// based on the instructions used in the IR module.
-// This will allow us to skip running passes that are not needed, without having to
-// run all the passes only to find out that no work is needed.
-// This is especially important for the performance of the backend, as some passes
-// have an initialization cost (such as building reference graphs or DOM trees) that
-// can be expensive.
-//
-struct RequiredLoweringPassSet
-{
-    bool debugInfo;
-    bool resultType;
-    bool optionalType;
-    bool enumType;
-    bool combinedTextureSamplers;
-    bool reinterpret;
-    bool generics;
-    bool bindExistential;
-    bool autodiff;
-    bool derivativePyBindWrapper;
-    bool bitcast;
-    bool existentialTypeLayout;
-    bool bindingQuery;
-    bool meshOutput;
-    bool higherOrderFunc;
-    bool globalVaryingVar;
-    bool glslSSBO;
-    bool byteAddressBuffer;
-    bool dynamicResource;
-    bool dynamicResourceHeap;
-    bool resolveVaryingInputRef;
-    bool specializeStageSwitch;
-    bool missingReturn;
-};
-
 // Scan the IR module and determine which lowering/legalization passes are needed based
 // on the instructions we see.
 //
@@ -471,6 +435,10 @@ void calcRequiredLoweringPassSet(
     case kIROp_MissingReturn:
         result.missingReturn = true;
         break;
+    case kIROp_Select:
+        if (!isScalarOrVectorType(inst->getFullType()))
+            result.nonVectorCompositeSelect = true;
+        break;
     }
     if (!result.generics || !result.existentialTypeLayout)
     {
@@ -492,7 +460,10 @@ void calcRequiredLoweringPassSet(
     }
     for (auto child : inst->getDecorationsAndChildren())
     {
-        calcRequiredLoweringPassSet(result, codeGenContext, child);
+        calcRequiredLoweringPassSet(
+            codeGenContext->getRequiredLoweringPassSet(),
+            codeGenContext,
+            child);
     }
 }
 
@@ -764,7 +735,8 @@ Result linkAndOptimizeIR(
     dumpIRIfEnabled(codeGenContext, irModule, "POST IR VALIDATION");
 
     // Scan the IR module and determine which lowering/legalization passes are needed.
-    RequiredLoweringPassSet requiredLoweringPassSet = {};
+    RequiredLoweringPassSet& requiredLoweringPassSet = codeGenContext->getRequiredLoweringPassSet();
+    requiredLoweringPassSet = {};
     calcRequiredLoweringPassSet(requiredLoweringPassSet, codeGenContext, irModule->getModuleInst());
 
     // Debug info is added by the front-end, and therefore needs to be stripped out by targets that
@@ -1083,6 +1055,18 @@ Result linkAndOptimizeIR(
     if (requiredLoweringPassSet.optionalType)
         lowerOptionalType(irModule, sink);
 
+    if (requiredLoweringPassSet.nonVectorCompositeSelect)
+    {
+        switch (target)
+        {
+        case CodeGenTarget::HLSL:
+            legalizeNonVectorCompositeSelect(irModule);
+            break;
+        default:
+            break;
+        }
+    }
+
     switch (target)
     {
     case CodeGenTarget::CPPSource:
@@ -1097,7 +1081,6 @@ Result linkAndOptimizeIR(
         break;
     }
 
-    requiredLoweringPassSet = {};
     calcRequiredLoweringPassSet(requiredLoweringPassSet, codeGenContext, irModule->getModuleInst());
 
     switch (target)
diff --git a/source/slang/slang-ir-insts.h b/source/slang/slang-ir-insts.h
index afe06f9a1..b0a0c74f9 100644
--- a/source/slang/slang-ir-insts.h
+++ b/source/slang/slang-ir-insts.h
@@ -2423,6 +2423,17 @@ struct IRImageStore : IRInst
 // Terminators
 
 FIDDLE()
+struct IRSelect : IRInst
+{
+    FIDDLE(leafInst());
+
+    IRInst* getCondition() { return getOperand(0); }
+    IRInst* getTrueResult() { return getOperand(1); }
+    IRInst* getFalseResult() { return getOperand(2); }
+};
+
+
+FIDDLE()
 struct IRReturn : IRTerminatorInst
 {
     FIDDLE(leafInst())
diff --git a/source/slang/slang-ir-legalize-composite-select.cpp b/source/slang/slang-ir-legalize-composite-select.cpp
new file mode 100644
index 000000000..1b2ba0670
--- /dev/null
+++ b/source/slang/slang-ir-legalize-composite-select.cpp
@@ -0,0 +1,79 @@
+#include "slang-ir-legalize-composite-select.h"
+
+#include "slang-ir-clone.h"
+#include "slang-ir-insts.h"
+#include "slang-ir-legalize-varying-params.h"
+#include "slang-ir-specialize-address-space.h"
+#include "slang-ir-util.h"
+#include "slang-ir.h"
+
+namespace Slang
+{
+void legalizeASingleNonVectorCompositeSelect(IRBuilder& builder, IRSelect* selectInst)
+{
+    SLANG_ASSERT(selectInst);
+
+    auto resultType = selectInst->getFullType();
+    auto trueResult = selectInst->getTrueResult();
+    auto falseResult = selectInst->getFalseResult();
+
+    IRBlock* trueBlock;
+    IRBlock* falseBlock;
+    IRBlock* afterBlock;
+    builder.emitIfElseWithBlocks(selectInst->getCondition(), trueBlock, falseBlock, afterBlock);
+
+    // Generate if-select-true and else-select-false clause
+    builder.setInsertInto(trueBlock);
+    builder.emitBranch(afterBlock, 1, &trueResult);
+
+    builder.setInsertInto(falseBlock);
+    builder.emitBranch(afterBlock, 1, &falseResult);
+
+    // Move everything after the OpSelect into the "after" block
+    List<IRInst*> instsToMove;
+    instsToMove.reserve(15);
+    IRInst* nextInst = selectInst;
+    while (nextInst)
+    {
+        instsToMove.add(nextInst);
+        nextInst = nextInst->getNextInst();
+    }
+    for (auto i : instsToMove)
+        afterBlock->insertAtEnd(i);
+
+    // Merge result of branches into param
+    builder.setInsertInto(afterBlock);
+    auto param = builder.emitParam(resultType);
+    selectInst->replaceUsesWith(param);
+
+    // Clean up
+    selectInst->removeAndDeallocate();
+}
+void legalizeNonVectorCompositeSelect(IRModule* module)
+{
+    IRBuilder builder(module);
+    for (auto globalInst : module->getModuleInst()->getChildren())
+    {
+        auto func = as<IRFunc>(globalInst);
+        if (!func)
+            continue;
+        for (auto block : func->getBlocks())
+        {
+            auto inst = block->getFirstInst();
+            IRInst* next;
+            for (; inst; inst = next)
+            {
+                next = inst->getNextInst();
+                switch (inst->getOp())
+                {
+                case kIROp_Select:
+                    // Replace OpSelect with if/else branch (same process as glslang)
+                    if (!isScalarOrVectorType(inst->getFullType()))
+                        legalizeASingleNonVectorCompositeSelect(builder, as<IRSelect>(inst));
+                    continue;
+                }
+            }
+        }
+    }
+}
+} // namespace Slang
diff --git a/source/slang/slang-ir-legalize-composite-select.h b/source/slang/slang-ir-legalize-composite-select.h
new file mode 100644
index 000000000..dc11ccb21
--- /dev/null
+++ b/source/slang/slang-ir-legalize-composite-select.h
@@ -0,0 +1,11 @@
+#pragma once
+
+#include "slang-compiler.h"
+#include "slang-ir.h"
+
+namespace Slang
+{
+class DiagnosticSink;
+
+void legalizeNonVectorCompositeSelect(IRModule* module);
+} // namespace Slang
diff --git a/source/slang/slang-ir-spirv-legalize.cpp b/source/slang/slang-ir-spirv-legalize.cpp
index 2a5701e33..20b721a20 100644
--- a/source/slang/slang-ir-spirv-legalize.cpp
+++ b/source/slang/slang-ir-spirv-legalize.cpp
@@ -12,6 +12,7 @@
 #include "slang-ir-inline.h"
 #include "slang-ir-insts.h"
 #include "slang-ir-layout.h"
+#include "slang-ir-legalize-composite-select.h"
 #include "slang-ir-legalize-global-values.h"
 #include "slang-ir-legalize-mesh-outputs.h"
 #include "slang-ir-loop-unroll.h"
@@ -39,6 +40,8 @@ struct SPIRVLegalizationContext : public SourceEmitterBase
 
     IRModule* m_module;
 
+    CodeGenContext* m_codeGenContext;
+
     DiagnosticSink* m_sink;
 
     struct LoweredStructuredBufferTypeInfo
@@ -202,8 +205,12 @@ struct SPIRVLegalizationContext : public SourceEmitterBase
     SPIRVLegalizationContext(
         SPIRVEmitSharedContext* sharedContext,
         IRModule* module,
+        CodeGenContext* codeGenContext,
         DiagnosticSink* sink)
-        : m_sharedContext(sharedContext), m_module(module), m_sink(sink)
+        : m_sharedContext(sharedContext)
+        , m_module(module)
+        , m_codeGenContext(codeGenContext)
+        , m_sink(sink)
     {
     }
 
@@ -2208,6 +2215,11 @@ struct SPIRVLegalizationContext : public SourceEmitterBase
         // invalid SPIR-V.
         bool skipFuncParamValidation = false;
         validateAtomicOperations(skipFuncParamValidation, m_sink, m_module->getModuleInst());
+
+        // If older than spirv 1.4, legalize OpSelect returning non-vector-composites
+        if (m_codeGenContext->getRequiredLoweringPassSet().nonVectorCompositeSelect &&
+            !m_sharedContext->isSpirv14OrLater())
+            legalizeNonVectorCompositeSelect(m_module);
     }
 
     void updateFunctionTypes()
@@ -2281,9 +2293,16 @@ SpvSnippet* SPIRVEmitSharedContext::getParsedSpvSnippet(IRTargetIntrinsicDecorat
     return snippet;
 }
 
-void legalizeSPIRV(SPIRVEmitSharedContext* sharedContext, IRModule* module, DiagnosticSink* sink)
+void legalizeSPIRV(
+    SPIRVEmitSharedContext* sharedContext,
+    IRModule* module,
+    CodeGenContext* codeGenContext)
 {
-    SPIRVLegalizationContext context(sharedContext, module, sink);
+    SPIRVLegalizationContext context(
+        sharedContext,
+        module,
+        codeGenContext,
+        codeGenContext->getSink());
     context.processModule();
 }
 
@@ -2424,7 +2443,7 @@ void legalizeIRForSPIRV(
     CodeGenContext* codeGenContext)
 {
     SLANG_UNUSED(entryPoints);
-    legalizeSPIRV(context, module, codeGenContext->getSink());
+    legalizeSPIRV(context, module, codeGenContext);
     simplifyIRForSpirvLegalization(context->m_targetProgram, codeGenContext->getSink(), module);
     buildEntryPointReferenceGraph(context->m_referencingEntryPoints, module);
     insertFragmentShaderInterlock(context, module);