[opt] Add struct-packing pass and unit test.

This pass allows to re-assign offset layout decorations
to tightly pack a struct according to its packing rules.

include/spirv-tools/optimizer.hpp

@@ -956,6 +956,18 @@ Optimizer::PassToken CreateFixFuncCallArgumentsPass();
 // the unknown capability interacts with one of the trimmed capabilities.
 Optimizer::PassToken CreateTrimCapabilitiesPass();
 
+// Creates a struct-packing pass.
+// This pass re-assigns all offset layout decorators to tightly pack
+// the specified struct according to its packing rules.
+// This is especially useful when spirv-opt removes lots of unused fields
+// in HLSL's "$Globals" struct after which this pass can reduce its memory
+// footprint. Accepted packing rules are:
+// std140, std140EnhancedLayout, std430,
+// std430EnhancedLayout, hlslCbuffer, hlslCbufferPackOffset, scalar,
+// scalarEnhancedLayout.
+Optimizer::PassToken CreateStructPackingPass(const char* structToPack,
+                                             const char* packingRule);
+
 // Creates a switch-descriptorset pass.
 // This pass changes any DescriptorSet decorations with the value |ds_from| to
 // use the new value |ds_to|.

source/opt/CMakeLists.txt

@@ -240,6 +240,7 @@ set(SPIRV_TOOLS_OPT_SOURCES
   strip_debug_info_pass.cpp
   strip_nonsemantic_info_pass.cpp
   struct_cfg_analysis.cpp
+  struct_packing_pass.cpp
   switch_descriptorset_pass.cpp
   trim_capabilities_pass.cpp
   type_manager.cpp

source/opt/optimizer.cpp

@@ -571,6 +571,25 @@ bool Optimizer::RegisterPassFromFlag(const std::string& flag,
              pass_args.c_str());
       return false;
     }
+  } else if (pass_name == "struct-packing") {
+    if (pass_args.size() == 0) {
+      Error(consumer(), nullptr, {},
+            "--struct-packing requires a name:rule argument.");
+      return false;
+    }
+
+    auto separator_pos = pass_args.find(':');
+    if (separator_pos == std::string::npos || separator_pos == 0 ||
+        separator_pos + 1 == pass_args.size()) {
+      Errorf(consumer(), nullptr, {},
+             "Invalid argument for --struct-packing: %s", pass_args.c_str());
+      return false;
+    }
+
+    const std::string struct_name = pass_args.substr(0, separator_pos);
+    const std::string rule_name = pass_args.substr(separator_pos + 1);
+
+    RegisterPass(CreateStructPackingPass(struct_name.c_str(), rule_name.c_str()));
   } else if (pass_name == "switch-descriptorset") {
     if (pass_args.size() == 0) {
       Error(consumer(), nullptr, {},
@@ -1152,6 +1171,14 @@ Optimizer::PassToken CreateTrimCapabilitiesPass() {
       MakeUnique<opt::TrimCapabilitiesPass>());
 }
 
+Optimizer::PassToken CreateStructPackingPass(const char* structToPack,
+                                             const char* packingRule) {
+  return MakeUnique<Optimizer::PassToken::Impl>(
+      MakeUnique<opt::StructPackingPass>(
+          structToPack,
+          opt::StructPackingPass::ParsePackingRuleFromString(packingRule)));
+}
+
 Optimizer::PassToken CreateSwitchDescriptorSetPass(uint32_t from, uint32_t to) {
   return MakeUnique<Optimizer::PassToken::Impl>(
       MakeUnique<opt::SwitchDescriptorSetPass>(from, to));

source/opt/passes.h

@@ -83,6 +83,7 @@
 #include "source/opt/strength_reduction_pass.h"
 #include "source/opt/strip_debug_info_pass.h"
 #include "source/opt/strip_nonsemantic_info_pass.h"
+#include "source/opt/struct_packing_pass.h"
 #include "source/opt/switch_descriptorset_pass.h"
 #include "source/opt/trim_capabilities_pass.h"
 #include "source/opt/unify_const_pass.h"