[clang] [libc] [llvm] [AMDGPU] Implement variadic functions by IR lowering (PR #93362)

[Pierre van Houtryve via cfe-commits]

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+//===-- ExpandVariadicsPass.cpp --------------------------------*- C++ -*-=//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This is an optimization pass for variadic functions. If called from codegen,
+// it can serve as the implementation of variadic functions for a given target.
+//
+// The strategy is to turn the ... part of a variadic function into a va_list
+// and fix up the call sites. The majority of the pass is target independent.
+// The exceptions are the va_list type itself and the rules for where to store
+// variables in memory such that va_arg can iterate over them given a va_list.
+//
+// The majority of the plumbing is splitting the variadic function into a
+// single basic block that packs the variadic arguments into a va_list and
+// a second function that does the work of the original. That packing is
+// exactly what is done by va_start. Further, the transform from ... to va_list
+// replaced va_start with an operation to copy a va_list from the new argument,
+// which is exactly a va_copy. This is useful for reducing target-dependence.
+//
+// A va_list instance is a forward iterator, where the primary operation va_arg
+// is dereference-then-increment. This interface forces significant convergent
+// evolution between target specific implementations. The variation in runtime
+// data layout is limited to that representable by the iterator, parameterised
+// by the type passed to the va_arg instruction.
+//
+// Therefore the majority of the target specific subtlety is packing arguments
+// into a stack allocated buffer such that a va_list can be initialised with it
+// and the va_arg expansion for the target will find the arguments at runtime.
+//
+// The aggregate effect is to unblock other transforms, most critically the
+// general purpose inliner. Known calls to variadic functions become zero cost.
+//
+// Consistency with clang is primarily tested by emitting va_arg using clang
+// then expanding the variadic functions using this pass, followed by trying
+// to constant fold the functions to no-ops.
+//
+// Target specific behaviour is tested in IR - mainly checking that values are
+// put into positions in call frames that make sense for that particular target.
+//
+// There is one "clever" invariant in use. va_start intrinsics that are not
+// within a varidic functions are an error in the IR verifier. When this
+// transform moves blocks from a variadic function into a fixed arity one, it
+// moves va_start intrinsics along with everything else. That means that the
+// va_start intrinsics that need to be rewritten to use the trailing argument
+// are exactly those that are in non-variadic functions so no further state
+// is needed to distinguish those that need to be rewritten.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO/ExpandVariadics.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/PassManager.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+#include "llvm/Passes/OptimizationLevel.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/TargetParser/Triple.h"
+#include "llvm/Transforms/Utils/ModuleUtils.h"
+
+#define DEBUG_TYPE "expand-variadics"
+
+using namespace llvm;
+
+namespace {
+
+cl::opt<ExpandVariadicsMode> ExpandVariadicsModeOption(
+    DEBUG_TYPE "-override", cl::desc("Override the behaviour of " DEBUG_TYPE),
+    cl::init(ExpandVariadicsMode::Unspecified),
+    cl::values(clEnumValN(ExpandVariadicsMode::Unspecified, "unspecified",
+                          "Use the implementation defaults"),
+               clEnumValN(ExpandVariadicsMode::Disable, "disable",
+                          "Disable the pass entirely"),
+               clEnumValN(ExpandVariadicsMode::Optimize, "optimize",
+                          "Optimise without changing ABI"),
+               clEnumValN(ExpandVariadicsMode::Lowering, "lowering",
+                          "Change variadic calling convention")));
+
+bool commandLineOverride() {
+  return ExpandVariadicsModeOption != ExpandVariadicsMode::Unspecified;
+}
+
+// Instances of this class encapsulate the target-dependant behaviour as a
+// function of triple. Implementing a new ABI is adding a case to the switch
+// in create(llvm::Triple) at the end of this file.
+class VariadicABIInfo {
+protected:
+  VariadicABIInfo() {}
+
+public:
+  static std::unique_ptr<VariadicABIInfo> create(llvm::Triple const &Triple);
+
+  // Allow overriding whether the pass runs on a per-target basis
+  virtual bool enableForTarget() = 0;
+
+  // Whether a valist instance is passed by value or by address
+  // I.e. does it need to be alloca'ed and stored into, or can
+  // it be passed directly in a SSA register
+  virtual bool vaListPassedInSSARegister() = 0;
+
+  // The type of a va_list iterator object
+  virtual Type *vaListType(LLVMContext &Ctx) = 0;
+
+  // The type of a va_list as a function argument as lowered by C
+  virtual Type *vaListParameterType(Module &M) = 0;
+
+  // Initialize an allocated va_list object to point to an already
+  // initialized contiguous memory region.
+  // Return the value to pass as the va_list argument
+  virtual Value *initializeVaList(Module &M, LLVMContext &Ctx,
+                                  IRBuilder<> &Builder, AllocaInst *VaList,
+                                  Value *Buffer) = 0;
+
+  struct VAArgSlotInfo {
+    Align DataAlign; // With respect to the call frame
+    bool Indirect;   // Passed via a pointer
+  };
+  virtual VAArgSlotInfo slotInfo(const DataLayout &DL, Type *Parameter) = 0;
+
+  // Targets implemented so far all have the same trivial lowering for these
+  bool vaEndIsNop() { return true; }
+  bool vaCopyIsMemcpy() { return true; }
+
+  virtual ~VariadicABIInfo() {}
+};
+
+// Module implements getFunction() which returns nullptr on missing declaration
+// and getOrInsertFunction which creates one when absent. Intrinsics.h only
+// implements getDeclaration which creates one when missing. Checking whether
+// an intrinsic exists thus inserts it in the module and it then needs to be
+// deleted again to clean up.
+// The right name for the two functions on intrinsics would match Module::,
+// but doing that in a single change would introduce nullptr dereferences
+// where currently there are none. The minimal collateral damage approach
+// would split the change over a release to help downstream branches. As it
+// is unclear what approach will be preferred, implementing the trivial
+// function here in the meantime to decouple from that discussion.
+Function *getPreexistingDeclaration(Module *M, Intrinsic::ID Id,
+                                    ArrayRef<Type *> Tys = std::nullopt) {
----------------
Pierre-vh wrote:

```suggestion
                                    ArrayRef<Type *> Tys = {}) {
```

https://github.com/llvm/llvm-project/pull/93362