Fix binding number calculation in desc sroa (#4095)

When there is an array of strutured buffers, desc sroa will only split
the array, but not a struct type in the structured buffer.  However,
the calcualtion of the number of binding a struct requires does not take
this into consideration.  This commit will fix that.

source/opt/desc_sroa.cpp

@@ -63,16 +63,7 @@ bool DescriptorScalarReplacement::IsCandidate(Instruction* var) {
 
   // All structures with descriptor assignments must be replaced by variables,
   // one for each of their members - with the exceptions of buffers.
-  // Buffers are represented as structures, but we shouldn't replace a buffer
-  // with its elements. All buffers have offset decorations for members of their
-  // structure types.
-  bool has_offset_decoration = false;
-  context()->get_decoration_mgr()->ForEachDecoration(
-      var_type_inst->result_id(), SpvDecorationOffset,
-      [&has_offset_decoration](const Instruction&) {
-        has_offset_decoration = true;
-      });
-  if (has_offset_decoration) {
+  if (IsTypeOfStructuredBuffer(var_type_inst)) {
     return false;
   }
 
@@ -99,6 +90,23 @@ bool DescriptorScalarReplacement::IsCandidate(Instruction* var) {
   return true;
 }
 
+bool DescriptorScalarReplacement::IsTypeOfStructuredBuffer(
+    const Instruction* type) const {
+  if (type->opcode() != SpvOpTypeStruct) {
+    return false;
+  }
+
+  // All buffers have offset decorations for members of their structure types.
+  // This is how we distinguish it from a structure of descriptors.
+  bool has_offset_decoration = false;
+  context()->get_decoration_mgr()->ForEachDecoration(
+      type->result_id(), SpvDecorationOffset,
+      [&has_offset_decoration](const Instruction&) {
+        has_offset_decoration = true;
+      });
+  return has_offset_decoration;
+}
+
 bool DescriptorScalarReplacement::ReplaceCandidate(Instruction* var) {
   std::vector<Instruction*> access_chain_work_list;
   std::vector<Instruction*> load_work_list;
@@ -368,7 +376,8 @@ uint32_t DescriptorScalarReplacement::GetNumBindingsUsedByType(
 
   // The number of bindings consumed by a structure is the sum of the bindings
   // used by its members.
-  if (type_inst->opcode() == SpvOpTypeStruct) {
+  if (type_inst->opcode() == SpvOpTypeStruct &&
+      !IsTypeOfStructuredBuffer(type_inst)) {
     uint32_t sum = 0;
     for (uint32_t i = 0; i < type_inst->NumInOperands(); i++)
       sum += GetNumBindingsUsedByType(type_inst->GetSingleWordInOperand(i));

source/opt/desc_sroa.h

@@ -93,6 +93,11 @@ class DescriptorScalarReplacement : public Pass {
   // bindings used by its members.
   uint32_t GetNumBindingsUsedByType(uint32_t type_id);
 
+  // Returns true if |type| is a type that could be used for a structured buffer
+  // as opposed to a type that would be used for a structure of resource
+  // descriptors.
+  bool IsTypeOfStructuredBuffer(const Instruction* type) const;
+
   // A map from an OpVariable instruction to the set of variables that will be
   // used to replace it. The entry |replacement_variables_[var][i]| is the id of
   // a variable that will be used in the place of the the ith element of the

test/opt/desc_sroa_test.cpp

@@ -729,6 +729,47 @@ TEST_F(DescriptorScalarReplacementTest, ResourceStructAsFunctionParam) {
   SinglePassRunAndMatch<DescriptorScalarReplacement>(checks + shader, true);
 }
 
+TEST_F(DescriptorScalarReplacementTest, BindingForResourceArrayOfStructs) {
+  // Check that correct binding numbers are given to an array of descriptors
+  // to structs.
+
+  const std::string shader = R"(
+; CHECK: OpDecorate {{%\w+}} Binding 0
+; CHECK: OpDecorate {{%\w+}} Binding 1
+               OpCapability Shader
+          %1 = OpExtInstImport "GLSL.std.450"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Fragment %2 "psmain"
+               OpExecutionMode %2 OriginUpperLeft
+               OpDecorate %5 DescriptorSet 0
+               OpDecorate %5 Binding 0
+               OpMemberDecorate %_struct_4 0 Offset 0
+               OpMemberDecorate %_struct_4 1 Offset 4
+               OpDecorate %_struct_4 Block
+      %float = OpTypeFloat 32
+        %int = OpTypeInt 32 1
+      %int_0 = OpConstant %int 0
+      %int_1 = OpConstant %int 1
+       %uint = OpTypeInt 32 0
+     %uint_2 = OpConstant %uint 2
+  %_struct_4 = OpTypeStruct %float %int
+%_arr__struct_4_uint_2 = OpTypeArray %_struct_4 %uint_2
+%_ptr_Uniform__arr__struct_4_uint_2 = OpTypePointer Uniform %_arr__struct_4_uint_2
+       %void = OpTypeVoid
+         %25 = OpTypeFunction %void
+%_ptr_Uniform_int = OpTypePointer Uniform %int
+          %5 = OpVariable %_ptr_Uniform__arr__struct_4_uint_2 Uniform
+          %2 = OpFunction %void None %25
+         %29 = OpLabel
+         %40 = OpAccessChain %_ptr_Uniform_int %5 %int_0 %int_1
+         %41 = OpAccessChain %_ptr_Uniform_int %5 %int_1 %int_1
+               OpReturn
+               OpFunctionEnd
+)";
+
+  SinglePassRunAndMatch<DescriptorScalarReplacement>(shader, true);
+}
+
 }  // namespace
 }  // namespace opt
 }  // namespace spvtools