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HLSL doesn't have a runtime loader model that supports global
construction by a loader or runtime initializer. To allow us to leverage
global constructors with minimal code generation impact we put calls to
the global constructors inside the generated entry function.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D132977
Files:
clang/docs/HLSL/EntryFunctions.rst
clang/lib/CodeGen/CGHLSLRuntime.cpp
clang/lib/CodeGen/CGHLSLRuntime.h
clang/test/CodeGenHLSL/GlobalConstructors.hlsl
Index: clang/test/CodeGenHLSL/GlobalConstructors.hlsl
===================================================================
--- /dev/null
+++ clang/test/CodeGenHLSL/GlobalConstructors.hlsl
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -x hlsl -S -emit-llvm -disable-llvm-passes %s -o - | FileCheck %s
+
+RWBuffer<float> Buffer;
+
+[numthreads(1,1,1)]
+void main(unsigned GI : SV_GroupIndex) {}
+
+//CHECK: define void @main()
+//CHECK-NEXT: entry:
+//CHECK-NEXT: call void @_GLOBAL__sub_I_GlobalConstructors.hlsl()
+//CHECK-NEXT: %0 = call i32 @llvm.dx.flattened.thread.id.in.group()
+//CHECK-NEXT: call void @"?main@@YAXI@Z"(i32 %0)
+//CHECK-NEXT: ret void
Index: clang/lib/CodeGen/CGHLSLRuntime.h
===================================================================
--- clang/lib/CodeGen/CGHLSLRuntime.h
+++ clang/lib/CodeGen/CGHLSLRuntime.h
@@ -46,6 +46,7 @@
virtual ~CGHLSLRuntime() {}
void annotateHLSLResource(const VarDecl *D, llvm::GlobalVariable *GV);
+ void generateGlobalCtorCalls();
void finishCodeGen();
Index: clang/lib/CodeGen/CGHLSLRuntime.cpp
===================================================================
--- clang/lib/CodeGen/CGHLSLRuntime.cpp
+++ clang/lib/CodeGen/CGHLSLRuntime.cpp
@@ -54,6 +54,8 @@
Triple T(M.getTargetTriple());
if (T.getArch() == Triple::ArchType::dxil)
addDxilValVersion(TargetOpts.DxilValidatorVersion, M);
+
+ generateGlobalCtorCalls();
}
void CGHLSLRuntime::annotateHLSLResource(const VarDecl *D, GlobalVariable *GV) {
@@ -143,3 +145,40 @@
// FIXME: Handle codegen for return type semantics.
B.CreateRetVoid();
}
+
+void CGHLSLRuntime::generateGlobalCtorCalls() {
+ llvm::Module &M = CGM.getModule();
+ const auto *GlobalCtors = M.getNamedGlobal("llvm.global_ctors");
+ if (!GlobalCtors)
+ return;
+ const auto *CA = dyn_cast<ConstantArray>(GlobalCtors->getInitializer());
+ if (!CA)
+ return;
+ // The global_ctor array elements are a struct [Priority, Fn *, COMDat].
+ // HLSL neither supports priorities or COMDat values, so we will check those
+ // in an assert but not handle them.
+
+ llvm::SmallVector<Function *> CtorFns;
+ for (const auto &Ctor : CA->operands()) {
+ if (isa<ConstantAggregateZero>(Ctor))
+ continue;
+ ConstantStruct *CS = cast<ConstantStruct>(Ctor);
+
+ assert(cast<ConstantInt>(CS->getOperand(0))->getValue() == 65535 &&
+ "HLSL doesn't support setting priority for global ctors.");
+ assert(isa<ConstantPointerNull>(CS->getOperand(2)) &&
+ "HLSL doesn't support COMDat for global ctors.");
+ CtorFns.push_back(cast<Function>(CS->getOperand(1)));
+ }
+
+ // Insert a call to the global constructor at the beginning of the entry block
+ // to externally exported functions. This is a bit of a hack, but HLSL allows
+ // global constructors, but doesn't support driver initialization of globals.
+ for (auto &F : M.functions()) {
+ if (!F.hasFnAttribute("hlsl.shader"))
+ continue;
+ IRBuilder<> B(&F.getEntryBlock(), F.getEntryBlock().begin());
+ for (auto *Fn : CtorFns)
+ B.CreateCall(FunctionCallee(Fn));
+ }
+}
Index: clang/docs/HLSL/EntryFunctions.rst
===================================================================
--- clang/docs/HLSL/EntryFunctions.rst
+++ clang/docs/HLSL/EntryFunctions.rst
@@ -37,10 +37,16 @@
The actual exported entry function which can be called by the GPU driver is a
``void(void)`` function that isn't name mangled. In code generation we generate
-the unmangled entry function, instantiations of the parameters with their
-semantic values populated, and a call to the user-defined function. After the
-call instruction the return value (if any) is saved using a target-appropriate
-intrinsic for storing outputs (for DirectX, the ``llvm.dx.store.output``).
+the unmangled entry function to serve as the actual shader entry. The shader
+entry function is annotated with the ``hlsl.shader`` function attribute
+identifying the entry's pipeline stage.
+
+The body of the unmangled entry function contains first a call to execute global
+constructors, then instantiations of the user-defined entry parameters with
+their semantic values populated, and a call to the user-defined function.
+After the call instruction the return value (if any) is saved using a
+target-appropriate intrinsic for storing outputs (for DirectX, the
+``llvm.dx.store.output``). Global destructors are not supported in HLSL.
.. note::
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