Make the "hack" sampler explicit for now

- We use this to work around the fact that, e.g., `Texture2D.Load` doesn't take a sampler, but the equivalent GLSL operation `texelFetch` requires one

- Previously we tried to hide the sampler from the user, hoping that glslang would drop it and we could just ignore it, but that doesn't work

- For now we'll go ahead and explicitly show the sampler in the reflection info so that an app can react appropriately

- We also generate a unique binding for the sampler, instead of the old behavior that fixed it with `binding = 0`
  - We still fix it with `set = 0`, so it might still surprise users

source/slang/emit.cpp

@@ -3848,7 +3848,10 @@ String emitEntryPoint(
 
     if (sharedContext.needHackSamplerForTexelFetch)
     {
-        finalResultBuilder << "layout(set = 0, binding = 0) uniform sampler SLANG_hack_samplerForTexelFetch;\n";
+        finalResultBuilder
+            << "layout(set = 0, binding = "
+            << programLayout->bindingForHackSampler
+            << ") uniform sampler SLANG_hack_samplerForTexelFetch;\n";
     }
 
     finalResultBuilder << code;

source/slang/parameter-binding.cpp

@@ -1501,6 +1501,31 @@ static void collectParameters(
     }
 }
 
+static bool isGLSLCrossCompilerNeeded(CompileRequest* request)
+{
+    switch (request->Target)
+    {
+    default:
+        return false;
+
+    case CodeGenTarget::GLSL:
+    case CodeGenTarget::SPIRV:
+    case CodeGenTarget::SPIRVAssembly:
+        break;
+    }
+
+    if (request->loadedModulesList.Count() != 0)
+        return true;
+
+    for (auto tu : request->translationUnits)
+    {
+        if (tu->sourceLanguage != SourceLanguage::GLSL)
+            return true;
+    }
+
+    return false;
+}
+
 void generateParameterBindings(
     CompileRequest*                 request)
 {
@@ -1649,6 +1674,39 @@ void generateParameterBindings(
         globalScopeLayout = globalConstantBufferLayout;
     }
 
+    // Final final step: pick a binding for the "hack sampler", if needed...
+    //
+    // We only want to do this if the GLSL cross-compilation support is
+    // being invoked, so that we don't gum up other shaders.
+    if(isGLSLCrossCompilerNeeded(request))
+    {
+        UInt space = 0;
+        auto hackSamplerUsedRanges = findUsedRangeSetForSpace(&context, space);
+
+        UInt binding = hackSamplerUsedRanges->usedResourceRanges[(int)LayoutResourceKind::DescriptorTableSlot].Allocate(nullptr, 1);
+
+        programLayout->bindingForHackSampler = (int)binding;
+
+        RefPtr<Variable> var = new Variable();
+        var->Name.Content = "SLANG_hack_samplerForTexelFetch";
+        var->Type.type = new SamplerStateType();
+
+        auto typeLayout = new TypeLayout();
+        typeLayout->type = var->Type.type;
+        typeLayout->addResourceUsage(LayoutResourceKind::DescriptorTableSlot, 1);
+
+        auto varLayout = new VarLayout();
+        varLayout->varDecl = makeDeclRef(var.Ptr());
+        varLayout->typeLayout = typeLayout;
+        auto resInfo = varLayout->AddResourceInfo(LayoutResourceKind::DescriptorTableSlot);
+        resInfo->index = binding;
+        resInfo->space = space;
+
+        programLayout->hackSamplerVar = var;
+
+        globalScopeStructLayout->fields.Add(varLayout);
+    }
+
     // We now have a bunch of layout information, which we should
     // record into a suitable object that represents the program
     programLayout->globalScopeLayout = globalScopeLayout;

source/slang/type-layout.h

@@ -383,6 +383,11 @@ class ProgramLayout : public Layout
     // and any entry-point-specific parameter data
     // will (eventually) belong there...
     List<RefPtr<EntryPointLayout>> entryPoints;
+
+    // HACK: binding to use when we have to create
+    // a dummy sampler just to appease glslang
+    int bindingForHackSampler = 0;
+    RefPtr<VarDeclBase> hackSamplerVar;
 };
 
 struct LayoutRulesFamilyImpl;

tests/reflection/cross-compile.slang.expected

@@ -42,6 +42,13 @@ standard output = {
                     ]
                 }
             }
+        },
+        {
+            "name": "SLANG_hack_samplerForTexelFetch",
+            "binding": {"kind": "descriptorTableSlot", "index": 3},
+            "type": {
+                "kind": "samplerState"
+            }
         }
     ]
 }