Allowing RDV to call getArgOperandsMutable() (#160415)

## Problem

`RemoveDeadValues` can legally drop dead function arguments on private
`func.func` callees. But call-sites to such functions aren't fixed if
the call operation keeps its call arguments in a **segmented operand
group** (i.ie, uses `AttrSizedOperandSegments`), unless the call op
implements `getArgOperandsMutable` and the RDV pass actually uses it.

## Fix
When RDV decides to drop callee function args, it should, for each
call-site that implements `CallOpInterface`, **shrink the call's
argument segment** via `getArgOperandsMutable()` using the same dead-arg
indices. This keeps both the flat operand list and the
`operand_segment_sizes` attribute in sync (that's what
`MutableOperandRange` does when bound to the segment).

## Note
This change is a no-op for:
* call ops without segment operands (they still get their flat operands
erased via the generic path)
* call ops whose calle args weren't dropped (public, external,
non-`func-func`, unresolved symbol, etc)
* `llvm.call`/`llvm.invoke` (RDV doesn't drop `llvm.func` args

---------

Co-authored-by: Mehdi Amini <joker.eph@gmail.com>

mlir/lib/Transforms/RemoveDeadValues.cpp

@@ -88,6 +88,8 @@ struct FunctionToCleanUp {
 struct OperationToCleanup {
   Operation *op;
   BitVector nonLive;
+  Operation *callee =
+      nullptr; // Optional: For CallOpInterface ops, stores the callee function
 };
 
 struct BlockArgsToCleanup {
@@ -306,19 +308,19 @@ static void processFuncOp(FunctionOpInterface funcOp, Operation *module,
       nonLiveSet.insert(arg);
     }
 
-  // Do (2).
+  // Do (2). (Skip creating generic operand cleanup entries for call ops.
+  // Call arguments will be removed in the call-site specific segment-aware
+  // cleanup, avoiding generic eraseOperands bitvector mechanics.)
   SymbolTable::UseRange uses = *funcOp.getSymbolUses(module);
   for (SymbolTable::SymbolUse use : uses) {
     Operation *callOp = use.getUser();
     assert(isa<CallOpInterface>(callOp) && "expected a call-like user");
-    // The number of operands in the call op may not match the number of
-    // arguments in the func op.
-    BitVector nonLiveCallOperands(callOp->getNumOperands(), false);
-    SmallVector<OpOperand *> callOpOperands =
-        operandsToOpOperands(cast<CallOpInterface>(callOp).getArgOperands());
-    for (int index : nonLiveArgs.set_bits())
-      nonLiveCallOperands.set(callOpOperands[index]->getOperandNumber());
-    cl.operands.push_back({callOp, nonLiveCallOperands});
+    // Push an empty operand cleanup entry so that call-site specific logic in
+    // cleanUpDeadVals runs (it keys off CallOpInterface). The BitVector is
+    // intentionally all false to avoid generic erasure.
+    // Store the funcOp as the callee to avoid expensive symbol lookup later.
+    cl.operands.push_back({callOp, BitVector(callOp->getNumOperands(), false),
+                           funcOp.getOperation()});
   }
 
   // Do (3).
@@ -746,6 +748,10 @@ static void cleanUpDeadVals(RDVFinalCleanupList &list) {
 
   // 3. Functions
   LDBG() << "Cleaning up " << list.functions.size() << " functions";
+  // Record which function arguments were erased so we can shrink call-site
+  // argument segments for CallOpInterface operations (e.g. ops using
+  // AttrSizedOperandSegments) in the next phase.
+  DenseMap<Operation *, BitVector> erasedFuncArgs;
   for (auto &f : list.functions) {
     LDBG() << "Cleaning up function: " << f.funcOp.getOperation()->getName();
     LDBG() << "  Erasing " << f.nonLiveArgs.count() << " non-live arguments";
@@ -754,17 +760,52 @@ static void cleanUpDeadVals(RDVFinalCleanupList &list) {
     // Some functions may not allow erasing arguments or results. These calls
     // return failure in such cases without modifying the function, so it's okay
     // to proceed.
-    (void)f.funcOp.eraseArguments(f.nonLiveArgs);
+    if (succeeded(f.funcOp.eraseArguments(f.nonLiveArgs))) {
+      // Record only if we actually erased something.
+      if (f.nonLiveArgs.any())
+        erasedFuncArgs.try_emplace(f.funcOp.getOperation(), f.nonLiveArgs);
+    }
     (void)f.funcOp.eraseResults(f.nonLiveRets);
   }
 
   // 4. Operands
   LDBG() << "Cleaning up " << list.operands.size() << " operand lists";
   for (OperationToCleanup &o : list.operands) {
-    if (o.op->getNumOperands() > 0) {
-      LDBG() << "Erasing " << o.nonLive.count()
-             << " non-live operands from operation: "
-             << OpWithFlags(o.op, OpPrintingFlags().skipRegions());
+    // Handle call-specific cleanup only when we have a cached callee reference.
+    // This avoids expensive symbol lookup and is defensive against future
+    // changes.
+    bool handledAsCall = false;
+    if (o.callee && isa<CallOpInterface>(o.op)) {
+      auto call = cast<CallOpInterface>(o.op);
+      auto it = erasedFuncArgs.find(o.callee);
+      if (it != erasedFuncArgs.end()) {
+        const BitVector &deadArgIdxs = it->second;
+        MutableOperandRange args = call.getArgOperandsMutable();
+        // First, erase the call arguments corresponding to erased callee
+        // args. We iterate backwards to preserve indices.
+        for (unsigned argIdx : llvm::reverse(deadArgIdxs.set_bits()))
+          args.erase(argIdx);
+        // If this operand cleanup entry also has a generic nonLive bitvector,
+        // clear bits for call arguments we already erased above to avoid
+        // double-erasing (which could impact other segments of ops with
+        // AttrSizedOperandSegments).
+        if (o.nonLive.any()) {
+          // Map the argument logical index to the operand number(s) recorded.
+          int operandOffset = call.getArgOperands().getBeginOperandIndex();
+          for (int argIdx : deadArgIdxs.set_bits()) {
+            int operandNumber = operandOffset + argIdx;
+            if (operandNumber < static_cast<int>(o.nonLive.size()))
+              o.nonLive.reset(operandNumber);
+          }
+        }
+        handledAsCall = true;
+      }
+    }
+    // Perform generic operand erasure for:
+    // - Non-call operations
+    // - Call operations without cached callee (where handledAsCall is false)
+    // But skip call operations that were already handled via segment-aware path
+    if (!handledAsCall && o.nonLive.any()) {
       o.op->eraseOperands(o.nonLive);
     }
   }

mlir/test/Transforms/remove-dead-values-call-segments.mlir

@@ -0,0 +1,23 @@
+// RUN: mlir-opt --split-input-file --remove-dead-values --mlir-print-op-generic %s | FileCheck %s --check-prefix=GEN
+
+// -----
+// Private callee: both args become dead after internal DCE; RDV drops callee
+// args and shrinks the *args* segment on the call-site to zero; sizes kept in
+// sync.
+
+module {
+  func.func private @callee(%x: i32, %y: i32) {
+    %u = arith.addi %x, %x : i32   // %y is dead
+    return
+  }
+
+  func.func @caller(%a: i32, %b: i32) {
+    // args segment initially has 2 operands.
+    "test.call_with_segments"(%a, %b) { callee = @callee,
+      operandSegmentSizes = array<i32: 0, 2, 0> } : (i32, i32) -> ()
+    return
+  }
+}
+
+// GEN: "test.call_with_segments"() <{callee = @callee, operandSegmentSizes = array<i32: 0, 0, 0>}> : () -> ()
+//       ^ args shrank from 2 -> 0

mlir/test/lib/Dialect/Test/TestDialect.cpp

@@ -431,3 +431,47 @@ void TestDialect::getCanonicalizationPatterns(
     RewritePatternSet &results) const {
   results.add(&dialectCanonicalizationPattern);
 }
+
+//===----------------------------------------------------------------------===//
+// TestCallWithSegmentsOp
+//===----------------------------------------------------------------------===//
+// The op `test.call_with_segments` models a call-like operation whose operands
+// are divided into 3 variadic segments: `prefix`, `args`, and `suffix`.
+// Only the middle segment represents the actual call arguments. The op uses
+// the AttrSizedOperandSegments trait, so we can derive segment boundaries from
+// the generated `operandSegmentSizes` attribute. We provide custom helpers to
+// expose the logical call arguments as both a read-only range and a mutable
+// range bound to the proper segment so that insertion/erasure updates the
+// attribute automatically.
+
+// Segment layout indices in the DenseI32ArrayAttr: [prefix, args, suffix].
+static constexpr unsigned kTestCallWithSegmentsArgsSegIndex = 1;
+
+Operation::operand_range CallWithSegmentsOp::getArgOperands() {
+  // Leverage generated getters for segment sizes: slice between prefix and
+  // suffix using current operand list.
+  return getOperation()->getOperands().slice(getPrefix().size(),
+                                             getArgs().size());
+}
+
+MutableOperandRange CallWithSegmentsOp::getArgOperandsMutable() {
+  Operation *op = getOperation();
+
+  // Obtain the canonical segment size attribute name for this op.
+  auto segName =
+      CallWithSegmentsOp::getOperandSegmentSizesAttrName(op->getName());
+  auto sizesAttr = op->getAttrOfType<DenseI32ArrayAttr>(segName);
+  assert(sizesAttr && "missing operandSegmentSizes attribute on op");
+
+  // Compute the start and length of the args segment from the prefix size and
+  // args size stored in the attribute.
+  auto sizes = sizesAttr.asArrayRef();
+  unsigned start = static_cast<unsigned>(sizes[0]); // prefix size
+  unsigned len = static_cast<unsigned>(sizes[1]);   // args size
+
+  NamedAttribute segNamed(segName, sizesAttr);
+  MutableOperandRange::OperandSegment binding{kTestCallWithSegmentsArgsSegIndex,
+                                              segNamed};
+
+  return MutableOperandRange(op, start, len, {binding});
+}

mlir/test/lib/Dialect/Test/TestOps.td

@@ -3746,4 +3746,47 @@ def TestOpWithSuccessorRef : TEST_Op<"dummy_op_with_successor_ref"> {
   }];
 }
 
+def CallWithSegmentsOp : TEST_Op<"call_with_segments",
+    [AttrSizedOperandSegments,
+  DeclareOpInterfaceMethods<CallOpInterface>]> {
+  let summary = "test call op with segmented args";
+  let arguments = (ins
+    FlatSymbolRefAttr:$callee,
+    Variadic<AnyType>:$prefix,   // non-arg segment (e.g., 'in')
+    Variadic<AnyType>:$args,     // <-- the call *arguments* segment
+    Variadic<AnyType>:$suffix    // non-arg segment (e.g., 'out')
+  );
+  let results = (outs);
+  let assemblyFormat = [{
+    $callee `(` $prefix `:` type($prefix) `)`
+            `(` $args `:` type($args) `)`
+            `(` $suffix `:` type($suffix) `)` attr-dict
+  }];
+
+  // Provide stub implementations for the ArgAndResultAttrsOpInterface.
+  let extraClassDeclaration = [{
+    ::mlir::ArrayAttr getArgAttrsAttr() { return {}; }
+    ::mlir::ArrayAttr getResAttrsAttr() { return {}; }
+    void setArgAttrsAttr(::mlir::ArrayAttr) {}
+    void setResAttrsAttr(::mlir::ArrayAttr) {}
+    ::mlir::Attribute removeArgAttrsAttr() { return {}; }
+    ::mlir::Attribute removeResAttrsAttr() { return {}; }
+  }];
+
+  let extraClassDefinition = [{
+    ::mlir::CallInterfaceCallable $cppClass::getCallableForCallee() {
+      if (auto sym = (*this)->getAttrOfType<::mlir::SymbolRefAttr>("callee"))
+        return ::mlir::CallInterfaceCallable(sym);
+      return ::mlir::CallInterfaceCallable();
+    }
+    void $cppClass::setCalleeFromCallable(::mlir::CallInterfaceCallable callee) {
+      if (auto sym = callee.dyn_cast<::mlir::SymbolRefAttr>())
+        (*this)->setAttr("callee", sym);
+      else
+        (*this)->removeAttr("callee");
+    }
+  }];
+}
+
+
 #endif // TEST_OPS