[llvm] llvm-reduce: Add values to return reduction (PR #132686)

[Nikita Popov via llvm-commits]

================
@@ -0,0 +1,244 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Try to reduce a function by inserting new return instructions. Try to insert
+// an early return for each instruction value at that point. This requires
+// mutating the return type, or finding instructions with a compatible type.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "llvm-reduce"
+
+#include "ReduceValuesToReturn.h"
+
+#include "Delta.h"
+#include "Utils.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+
+using namespace llvm;
+
+/// Return true if it is legal to emit a copy of the function with a non-void
+/// return type.
+static bool canUseNonVoidReturnType(const Function &F) {
+  // Functions with sret arguments must return void.
+  return !F.hasStructRetAttr() &&
+         CallingConv::supportsNonVoidReturnType(F.getCallingConv());
+}
+
+/// Return true if it's legal to replace a function return type to use \p Ty.
+static bool isReallyValidReturnType(Type *Ty) {
+  return FunctionType::isValidReturnType(Ty) && !Ty->isTokenTy() &&
+         Ty->isFirstClassType();
+}
+
+/// Insert a ret inst after \p NewRetValue, which returns the value it produces.
+static void rewriteFuncWithReturnType(Function &OldF, Value *NewRetValue) {
+  Type *NewRetTy = NewRetValue->getType();
+  FunctionType *OldFuncTy = OldF.getFunctionType();
+
+  FunctionType *NewFuncTy =
+      FunctionType::get(NewRetTy, OldFuncTy->params(), OldFuncTy->isVarArg());
+
+  LLVMContext &Ctx = OldF.getContext();
+  Instruction *NewRetI = cast<Instruction>(NewRetValue);
+  BasicBlock *NewRetBlock = NewRetI->getParent();
+
+  BasicBlock::iterator NewValIt = NewRetI->getIterator();
+
+  // Hack up any return values in other blocks, we can't leave them as ret void.
+  if (OldFuncTy->getReturnType()->isVoidTy()) {
+    for (BasicBlock &OtherRetBB : OldF) {
+      if (&OtherRetBB != NewRetBlock) {
+        auto *OrigRI = dyn_cast<ReturnInst>(OtherRetBB.getTerminator());
+        if (!OrigRI)
+          continue;
+
+        OrigRI->eraseFromParent();
+        ReturnInst::Create(Ctx, getDefaultValue(NewRetTy), &OtherRetBB);
+      }
+    }
+  }
+
+  // Now prune any CFG edges we have to deal with.
+  //
+  // Use KeepOneInputPHIs in case the instruction we are using for the return is
+  // that phi.
+  // TODO: Could avoid this with fancier iterator management.
+  for (BasicBlock *Succ : successors(NewRetBlock))
+    Succ->removePredecessor(NewRetBlock, /*KeepOneInputPHIs=*/true);
+
+  // Now delete the tail of this block, in reverse to delete uses before defs.
+  for (Instruction &I : make_early_inc_range(
+           make_range(NewRetBlock->rbegin(), NewValIt.getReverse()))) {
+    Value *Replacement = getDefaultValue(I.getType());
+    I.replaceAllUsesWith(Replacement);
+    I.eraseFromParent();
+  }
+
+  ReturnInst::Create(Ctx, NewRetValue, NewRetBlock);
+
+  // TODO: We may be eliminating blocks that were originally unreachable. We
+  // probably ought to only be pruning blocks that became dead directly as a
+  // result of our pruning here.
+  EliminateUnreachableBlocks(OldF);
+
+  Function *NewF =
+      Function::Create(NewFuncTy, OldF.getLinkage(), OldF.getAddressSpace(), "",
+                       OldF.getParent());
+
+  NewF->removeFromParent();
+  OldF.getParent()->getFunctionList().insertAfter(OldF.getIterator(), NewF);
+  NewF->takeName(&OldF);
+  NewF->copyAttributesFrom(&OldF);
+
+  // Adjust the callsite uses to the new return type. We pre-filtered cases
+  // where the original call type was incorrectly non-void.
+  for (User *U : make_early_inc_range(OldF.users())) {
+    if (auto *CB = dyn_cast<CallBase>(U);
+        CB && CB->getCalledOperand() == &OldF) {
+      if (CB->getType()->isVoidTy()) {
+        FunctionType *CallType = CB->getFunctionType();
+
+        // The callsite may not match the new function type, in an undefined
+        // behavior way. Only mutate the local return type.
+        FunctionType *NewCallType = FunctionType::get(
+            NewRetTy, CallType->params(), CallType->isVarArg());
+
+        CB->mutateType(NewRetTy);
+        CB->setCalledFunction(NewCallType, NewF);
+      } else {
+        assert(CB->getType() == NewRetTy &&
+               "only handle exact return type match with non-void returns");
+      }
+    }
+  }
+
+  // Preserve the parameters of OldF.
+  ValueToValueMapTy VMap;
+  for (auto Z : zip_first(OldF.args(), NewF->args())) {
+    Argument &OldArg = std::get<0>(Z);
+    Argument &NewArg = std::get<1>(Z);
+
+    NewArg.setName(OldArg.getName()); // Copy the name over...
+    VMap[&OldArg] = &NewArg;          // Add mapping to VMap
+  }
+
+  SmallVector<ReturnInst *, 8> Returns; // Ignore returns cloned.
+  CloneFunctionInto(NewF, &OldF, VMap,
+                    CloneFunctionChangeType::LocalChangesOnly, Returns, "",
+                    /*CodeInfo=*/nullptr);
----------------
nikic wrote:

Actually, with opaque pointers, can we get away with just using mutateType() instead of doing the dance with copying into a new function?

https://github.com/llvm/llvm-project/pull/132686