[HLSL][SPIR-V] Add -fspv-enable-maximal-reconvergence flag to clang dxc. (#163474)

Implement the frontend change to support maximal reconvergence feature.

The next work is to generate the corresponding SPIR-V instructions
(`OpExtension` and `OpExecutionMode`) based on the llvm ir added in this
CL `"enable-maximal-reconvergence"="true"`.


Addresses llvm/llvm-project#136930

Author: luciechoi

clang/include/clang/Basic/LangOptions.def

@@ -243,6 +243,7 @@ ENUM_LANGOPT(HLSLVersion, HLSLLangStd, 16, HLSL_Unset, NotCompatible, "HLSL Vers
 LANGOPT(HLSLStrictAvailability, 1, 0, NotCompatible,
         "Strict availability diagnostic mode for HLSL built-in functions.")
 LANGOPT(HLSLSpvUseUnknownImageFormat, 1, 0, NotCompatible, "For storage images and texel buffers, sets the default format to 'Unknown' when not specified via the `vk::image_format` attribute. If this option is not used, the format is inferred from the resource's data type.")
+LANGOPT(HLSLSpvEnableMaximalReconvergence, 1, 0, NotCompatible, "Enables the MaximallyReconvergesKHR execution mode for this module. This ensures that control flow reconverges at well-defined merge points as defined by the Vulkan spec.")
 
 LANGOPT(CUDAIsDevice      , 1, 0, NotCompatible, "compiling for CUDA device")
 LANGOPT(CUDAHostDeviceConstexpr, 1, 1, NotCompatible, "treating unattributed constexpr functions as __host__ __device__")

clang/include/clang/Driver/Options.td

@@ -9601,6 +9601,15 @@ def fhlsl_spv_use_unknown_image_format
                "from the resource's data type.">,
       MarshallingInfoFlag<LangOpts<"HLSLSpvUseUnknownImageFormat">>;
 
+def fhlsl_spv_enable_maximal_reconvergence
+    : Flag<["-"], "fspv-enable-maximal-reconvergence">,
+      Group<dxc_Group>,
+      Visibility<[CC1Option, DXCOption]>,
+      HelpText<"Enables the MaximallyReconvergesKHR execution mode for this "
+               "module. This ensures that control flow reconverges at "
+               "well-defined merge points as defined by the Vulkan spec.">,
+      MarshallingInfoFlag<LangOpts<"HLSLSpvEnableMaximalReconvergence">>;
+
 def no_wasm_opt : Flag<["--"], "no-wasm-opt">,
   Group<m_Group>,
   HelpText<"Disable the wasm-opt optimizer">,

clang/lib/CodeGen/CGHLSLRuntime.cpp

@@ -519,6 +519,10 @@ void clang::CodeGen::CGHLSLRuntime::setHLSLEntryAttributes(
   if (CGM.getCodeGenOpts().OptimizationLevel == 0)
     Fn->addFnAttr(llvm::Attribute::OptimizeNone);
   Fn->addFnAttr(llvm::Attribute::NoInline);
+
+  if (CGM.getLangOpts().HLSLSpvEnableMaximalReconvergence) {
+    Fn->addFnAttr("enable-maximal-reconvergence", "true");
+  }
 }
 
 static Value *buildVectorInput(IRBuilder<> &B, Function *F, llvm::Type *Ty) {

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3755,7 +3755,8 @@ static void RenderHLSLOptions(const ArgList &Args, ArgStringList &CmdArgs,
       options::OPT_hlsl_entrypoint,
       options::OPT_fdx_rootsignature_define,
       options::OPT_fdx_rootsignature_version,
-      options::OPT_fhlsl_spv_use_unknown_image_format};
+      options::OPT_fhlsl_spv_use_unknown_image_format,
+      options::OPT_fhlsl_spv_enable_maximal_reconvergence};
   if (!types::isHLSL(InputType))
     return;
   for (const auto &Arg : ForwardedArguments)

clang/test/CodeGenHLSL/vk-features/maximal_reconvergence.hlsl

@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -triple spirv1.6-unknown-vulkan1.3-compute -fspv-enable-maximal-reconvergence -emit-llvm -o - -O0 %s | FileCheck %s --check-prefixes=CHECK
+// RUN: %clang_cc1 -triple spirv1.6-unknown-vulkan1.3-compute -hlsl-entry test -fspv-enable-maximal-reconvergence -emit-llvm -o - -O0 %s | FileCheck %s --check-prefixes=CHECK-ENTRY
+
+[numthreads(1,1,1)]
+void main() {
+// CHECK: define void @main() [[attributeNumber:#[0-9]+]] {
+}
+
+// CHECK: attributes [[attributeNumber]] = {{.*}} "enable-maximal-reconvergence"="true" {{.*}}
+
+
+[numthreads(1,1,1)]
+void test() {
+// CHECK-ENTRY: define void @test() [[attributeNumber:#[0-9]+]] {
+}
+    
+// CHECK-ENTRY: attributes [[attributeNumber]] = {{.*}} "enable-maximal-reconvergence"="true" {{.*}}