[clang] [llvm] [WIP] Expand variadic functions in IR (PR #89007)

[Matt Arsenault 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 varidic function into a va_list
+// and fix up the call sites. This is completely effective if the calling
+// convention can declare that to be the right thing, e.g. on GPUs or where
+// the application is wholly statically linked. In the usual case, it will
+// replace known calls to known variadic functions with calls that are amenable
+// to inlining and other optimisations.
+//
+// The target-dependent parts are in class VariadicABIInfo. Enabling a new
+// target means adding a case to VariadicABIInfo::create() along with tests.
+// This will be especially simple if the va_list representation is a char*.
+//
+// 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. The target specific
+// part is packing arguments into a contiguous buffer that the clang expansion
+// of va_arg will do the right thing with.
+//
+// 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.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO/ExpandVariadics.h"
+#include "llvm/ADT/SmallSet.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 <cstdio>
+
+#define DEBUG_TYPE "expand-variadics"
+
+using namespace llvm;
+
+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")));
+
+namespace {
+
+// Module implements getFunction() which returns nullptr on missing declaration
+// and getOrInsertFunction which creates one when absent. Intrinsics.h
+// implements getDeclaration which creates one when missing. This should be
+// changed to be consistent with Module()'s naming. Implementing as a local
+// function here in the meantime to decouple from that process.
+Function *getPreexistingDeclaration(Module *M, Intrinsic::ID id,
+                                    ArrayRef<Type *> Tys = std::nullopt) {
----------------
arsenm wrote:

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

https://github.com/llvm/llvm-project/pull/89007