Minimal support of floating-point operand bundles

This is a lite version of llvm#109798,
where code changes are minimized to facilitate discussion about the
implementation. The motivations and ideas behind the new floating-point
operation support are described in that PR and in the discussion
https://discourse.llvm.org/t/rfc-change-of-strict-fp-operation-representation-in-ir/85021.
There are concerns that the proposed changes are too invasive and a new
approach is required to make the transition smoother.

This implementation is essentially a subset of PR109798, where
everything beyond the minimum is removed. It tries to build eventually
the same implementation as that PR but in different steps.

The patch does not attempt to modify the existing implementation based
on the constrained intrinsics. Instead it introduces a new one using
operand bundles. This new implementation initially has very limited
functionality, which latter will be extended and finally can replace the
existing one.

This PR introduces the notion of floating-point operation, this is an
intrinsic, that is listed in the file "FloatingPointOps.def". These have
two additional properties:

1. In the strict environment (a function with strictfp attribute) calls
   to these operations acquire side effect, now it is read/write access
   to inaccessible memory, just as constrained intrinsics do.

2. Calls to these operations may have floating-point operand bundles.
   There are two kinds of such bundles, tagged with "fp.control" and
   "fp.except", which are used to carry additional information about
   control modes and exception handling. Initially the set of control
   modes consists of rounding mode only.

The set of operations enlisted in "FloatingPointOps.def" and in
"ConstrainedOps.def" are completely independent, an intrinsic may be in
one list or in both. The set of floating-point operations is expected to
grow and finally all FP intrinsics will be available in the new
implementation. In this patch set of intrinsics in
"FloatingPointOps.def" is minimum necessary for tests.

Author: spavloff

llvm/docs/LangRef.rst

@@ -3199,6 +3199,51 @@ A "convergencectrl" operand bundle is only valid on a ``convergent`` operation.
 When present, the operand bundle must contain exactly one value of token type.
 See the :doc:`ConvergentOperations` document for details.
 
+.. _ob_fp:
+
+Floating-point Operand Bundles
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+These operand bundles are used for calls that involve floating-point
+operations and interact with :ref:`floating-point environment <floatenv>` or
+depend on floating-point options, such as rounding mode, denormal modes, etc.
+There are two kinds of such operand bundles, which represent the value of
+floating-point control modes and the treatment of status bits respectively.
+
+An operand bundle tagged with "fp.control" contains information about the
+control modes used for the operation execution. Operands specified in this
+bundle represent particular options. Currently, only rounding mode is supported.
+It is represented by a metadata string value, which specifies the rounding mode
+to be used for the operation evaluation. Possible values are:
+
+::
+
+    "rtz"  - toward zero
+    "rte"  - to nearest, ties to even
+    "rtp"  - toward positive infinity
+    "rtn"  - toward negative infinity
+    "rmm"  - to nearest, ties away from zero
+    "dyn"  - rounding mode is taken from control register
+
+If "fp.control" is absent, the default rounding rounding mode is taken from the
+control register (dynamic rounding). In the particular case of
+:ref:`default floating-point environment <floatenv>`, it must be rounding to
+nearest, ties to even.
+
+An operand bundle tagged with "fp.except" may be associated with operations
+that can read or write floating-point exception flags. It contains a single
+metadata string value, which can have one of the following values:
+
+::
+
+    "ignore"
+    "strict"
+    "maytrap"
+
+It has the same meaning as the corresponding argument in
+:ref:`constrained intrinsics <constrainedfp>`.
+
+
 .. _moduleasm:
 
 Module-Level Inline Assembly
@@ -3949,9 +3994,9 @@ round-to-nearest rounding mode, and subnormals are assumed to be preserved.
 Running LLVM code in an environment where these assumptions are not met
 typically leads to undefined behavior. The ``strictfp`` and ``denormal-fp-math``
 attributes as well as :ref:`Constrained Floating-Point Intrinsics
-<constrainedfp>` can be used to weaken LLVM's assumptions and ensure defined
-behavior in non-default floating-point environments; see their respective
-documentation for details.
+<constrainedfp>` or :ref:`floating-point operand bundles<ob_fp>` can be used to
+weaken LLVM's assumptions and ensure defined behavior in non-default
+floating-point environments; see their respective documentation for details.
 
 .. _floatnan:
 

llvm/docs/ReleaseNotes.md

@@ -67,6 +67,7 @@ Changes to the LLVM IR
   Instead, the `align` attribute should be placed on the pointer (or vector of
   pointers) argument.
 * A `load atomic` may now be used with vector types on x86.
+* Floating-point operand bundles have been added.
 
 Changes to LLVM infrastructure
 ------------------------------

llvm/include/llvm/IR/FPEnv.h

@@ -49,20 +49,38 @@ enum ExceptionBehavior : uint8_t {
 /// metadata.
 LLVM_ABI std::optional<RoundingMode> convertStrToRoundingMode(StringRef);
 
+/// Returns a valid RoundingMode enumerator given a string that is used as
+/// rounding mode specifier in operand bundles.
+std::optional<RoundingMode> convertBundleToRoundingMode(StringRef);
+
 /// For any RoundingMode enumerator, returns a string valid as input in
 /// constrained intrinsic rounding mode metadata.
 LLVM_ABI std::optional<StringRef> convertRoundingModeToStr(RoundingMode);
 
+/// For any RoundingMode enumerator, returns a string to be used in operand
+/// bundles.
+std::optional<StringRef> convertRoundingModeToBundle(RoundingMode);
+
 /// Returns a valid ExceptionBehavior enumerator when given a string
 /// valid as input in constrained intrinsic exception behavior metadata.
 LLVM_ABI std::optional<fp::ExceptionBehavior>
     convertStrToExceptionBehavior(StringRef);
 
+/// Returns a valid ExceptionBehavior enumerator given a string from the operand
+/// bundle argument.
+std::optional<fp::ExceptionBehavior>
+    convertBundleToExceptionBehavior(StringRef);
+
 /// For any ExceptionBehavior enumerator, returns a string valid as
 /// input in constrained intrinsic exception behavior metadata.
 LLVM_ABI std::optional<StringRef>
     convertExceptionBehaviorToStr(fp::ExceptionBehavior);
 
+/// Return string representing the given exception behavior for use in operand
+/// bundles
+std::optional<StringRef>
+    convertExceptionBehaviorToBundle(fp::ExceptionBehavior);
+
 /// Returns true if the exception handling behavior and rounding mode
 /// match what is used in the default floating point environment.
 inline bool isDefaultFPEnvironment(fp::ExceptionBehavior EB, RoundingMode RM) {

llvm/include/llvm/IR/FloatingPointOps.def

@@ -0,0 +1,24 @@
+//===- llvm/IR/FloatingPointOps.def - FP intrinsics -------------*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines set of intrinsics, which are classified as floating-point operations.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef FUNCTION
+#define FUNCTION(N,D)
+#endif
+
+// Arguments of the entries are:
+// - intrinsic function name,
+// - DAG node corresponding to the intrinsic.
+
+FUNCTION(nearbyint,                          FNEARBYINT)
+FUNCTION(trunc,                              FTRUNC)
+
+#undef FUNCTION

llvm/include/llvm/IR/IRBuilder.h

@@ -1000,6 +1000,16 @@ class IRBuilderBase {
                                      FMFSource FMFSource = {},
                                      const Twine &Name = "");
 
+  /// Create a call to intrinsic \p ID with \p Args, mangled using \p Types and
+  /// with operand bundles.
+  /// If \p FMFSource is provided, copy fast-math-flags from that instruction to
+  /// the intrinsic.
+  CallInst *CreateIntrinsic(Intrinsic::ID ID, ArrayRef<Type *> Types,
+                            ArrayRef<Value *> Args,
+                            ArrayRef<OperandBundleDef> OpBundles,
+                            Instruction *FMFSource = nullptr,
+                            const Twine &Name = "");
+
   /// Create a call to non-overloaded intrinsic \p ID with \p Args. If
   /// \p FMFSource is provided, copy fast-math-flags from that instruction to
   /// the intrinsic.
@@ -2511,24 +2521,13 @@ class IRBuilderBase {
   CallInst *CreateCall(FunctionType *FTy, Value *Callee,
                        ArrayRef<Value *> Args = {}, const Twine &Name = "",
                        MDNode *FPMathTag = nullptr) {
-    CallInst *CI = CallInst::Create(FTy, Callee, Args, DefaultOperandBundles);
-    if (IsFPConstrained)
-      setConstrainedFPCallAttr(CI);
-    if (isa<FPMathOperator>(CI))
-      setFPAttrs(CI, FPMathTag, FMF);
-    return Insert(CI, Name);
+    return CreateCall(FTy, Callee, Args, DefaultOperandBundles, Name,
+                      FPMathTag);
   }
 
   CallInst *CreateCall(FunctionType *FTy, Value *Callee, ArrayRef<Value *> Args,
                        ArrayRef<OperandBundleDef> OpBundles,
-                       const Twine &Name = "", MDNode *FPMathTag = nullptr) {
-    CallInst *CI = CallInst::Create(FTy, Callee, Args, OpBundles);
-    if (IsFPConstrained)
-      setConstrainedFPCallAttr(CI);
-    if (isa<FPMathOperator>(CI))
-      setFPAttrs(CI, FPMathTag, FMF);
-    return Insert(CI, Name);
-  }
+                       const Twine &Name = "", MDNode *FPMathTag = nullptr);
 
   CallInst *CreateCall(FunctionCallee Callee, ArrayRef<Value *> Args = {},
                        const Twine &Name = "", MDNode *FPMathTag = nullptr) {
@@ -2767,6 +2766,24 @@ class IRBuilderBase {
   /// assumption on the provided pointer.
   LLVM_ABI CallInst *CreateDereferenceableAssumption(Value *PtrValue,
                                                      Value *SizeValue);
+
+  /// Create an operand bundle in the provided bundle set to represent given FP
+  /// rounding mode.
+  ///
+  /// If the rounding mode is not defined, adds the default rounding mode,
+  /// stored in this builder object.
+  void
+  createFPRoundingBundle(SmallVectorImpl<OperandBundleDef> &Bundles,
+                         std::optional<RoundingMode> Rounding = std::nullopt);
+
+  /// Create an operand bundle in the provided bundle set to represent FP
+  /// exception behavior.
+  ///
+  /// If the exception behavior is not defined, adds the default behavior,
+  /// stored in this builder object.
+  void createFPExceptionBundle(
+      SmallVectorImpl<OperandBundleDef> &Bundles,
+      std::optional<fp::ExceptionBehavior> Except = std::nullopt);
 };
 
 /// This provides a uniform API for creating instructions and inserting

llvm/include/llvm/IR/InstrTypes.h

@@ -25,6 +25,7 @@
 #include "llvm/IR/CallingConv.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/FMF.h"
+#include "llvm/IR/FPEnv.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/LLVMContext.h"
@@ -1093,6 +1094,13 @@ template <typename InputTy> class OperandBundleDefT {
 using OperandBundleDef = OperandBundleDefT<Value *>;
 using ConstOperandBundleDef = OperandBundleDefT<const Value *>;
 
+void addFPRoundingBundle(LLVMContext &Ctx,
+                         SmallVectorImpl<OperandBundleDef> &Bundles,
+                         RoundingMode Rounding);
+void addFPExceptionBundle(LLVMContext &Ctx,
+                          SmallVectorImpl<OperandBundleDef> &Bundles,
+                          fp::ExceptionBehavior Except);
+
 //===----------------------------------------------------------------------===//
 //                               CallBase Class
 //===----------------------------------------------------------------------===//
@@ -1152,6 +1160,8 @@ class CallBase : public Instruction {
   /// number of extra operands.
   LLVM_ABI unsigned getNumSubclassExtraOperandsDynamic() const;
 
+  MemoryEffects getFloatingPointMemoryEffects() const;
+
 public:
   using Instruction::getContext;
 
@@ -2162,6 +2172,12 @@ class CallBase : public Instruction {
     return false;
   }
 
+  /// Return rounding mode specified by operand bundles.
+  RoundingMode getRoundingMode() const;
+
+  /// Return exception behavior specified by operand bundles.
+  std::optional<fp::ExceptionBehavior> getExceptionBehavior() const;
+
   /// Used to keep track of an operand bundle.  See the main comment on
   /// OperandBundleUser above.
   struct BundleOpInfo {

llvm/include/llvm/IR/IntrinsicInst.h

@@ -129,6 +129,14 @@ class IntrinsicInst : public CallInst {
   /// course of IR transformations
   LLVM_ABI static bool mayLowerToFunctionCall(Intrinsic::ID IID);
 
+  /// Check if \p ID represents a function that may access FP environment and
+  /// may have FP operand bundles.
+  ///
+  /// Access to FP environment means that in the strict FP environment the
+  /// function has read/write memory effect, which is used to maintain proper
+  /// instructions ordering.
+  static bool isFloatingPointOperation(Intrinsic::ID IID);
+
   /// Methods for support type inquiry through isa, cast, and dyn_cast:
   static bool classof(const CallInst *I) {
     auto *F = dyn_cast_or_null<Function>(I->getCalledOperand());

llvm/include/llvm/IR/LLVMContext.h

@@ -98,8 +98,9 @@ class LLVMContext {
     OB_kcfi = 8,                   // "kcfi"
     OB_convergencectrl = 9,        // "convergencectrl"
     OB_align = 10,                 // "align"
-    OB_LastBundleID = OB_align     // Marker for last bundle ID
-  };
+    OB_fp_control = 11,            // "fp.control"
+    OB_fp_except = 12,             // "fp.except"
+    OB_LastBundleID = OB_fp_except // Marker for last bundle ID  };
 
   /// getMDKindID - Return a unique non-zero ID for the specified metadata kind.
   /// This ID is uniqued across modules in the current LLVMContext.

llvm/include/llvm/Support/ModRef.h

@@ -235,6 +235,11 @@ template <typename LocationEnum> class MemoryEffectsBase {
     return getWithoutLoc(Location::InaccessibleMem).doesNotAccessMemory();
   }
 
+  /// Whether this function accesses inaccessible memory.
+  bool doesAccessInaccessibleMem() const {
+    return isModOrRefSet(getModRef(Location::InaccessibleMem));
+  }
+
   /// Whether this function only (at most) accesses errno memory.
   bool onlyAccessesErrnoMem() const {
     return getWithoutLoc(Location::ErrnoMem).doesNotAccessMemory();

llvm/lib/IR/FPEnv.cpp

@@ -34,6 +34,17 @@ std::optional<RoundingMode> convertStrToRoundingMode(StringRef RoundingArg) {
       .Default(std::nullopt);
 }
 
+std::optional<RoundingMode> convertBundleToRoundingMode(StringRef RoundingArg) {
+  return StringSwitch<std::optional<RoundingMode>>(RoundingArg)
+      .Case("dyn", RoundingMode::Dynamic)
+      .Case("rte", RoundingMode::NearestTiesToEven)
+      .Case("rmm", RoundingMode::NearestTiesToAway)
+      .Case("rtn", RoundingMode::TowardNegative)
+      .Case("rtp", RoundingMode::TowardPositive)
+      .Case("rtz", RoundingMode::TowardZero)
+      .Default(std::nullopt);
+}
+
 std::optional<StringRef> convertRoundingModeToStr(RoundingMode UseRounding) {
   std::optional<StringRef> RoundingStr;
   switch (UseRounding) {
@@ -61,6 +72,33 @@ std::optional<StringRef> convertRoundingModeToStr(RoundingMode UseRounding) {
   return RoundingStr;
 }
 
+std::optional<StringRef> convertRoundingModeToBundle(RoundingMode UseRounding) {
+  std::optional<StringRef> RoundingStr;
+  switch (UseRounding) {
+  case RoundingMode::Dynamic:
+    RoundingStr = "dyn";
+    break;
+  case RoundingMode::NearestTiesToEven:
+    RoundingStr = "rte";
+    break;
+  case RoundingMode::NearestTiesToAway:
+    RoundingStr = "rmm";
+    break;
+  case RoundingMode::TowardNegative:
+    RoundingStr = "rtn";
+    break;
+  case RoundingMode::TowardPositive:
+    RoundingStr = "rtp";
+    break;
+  case RoundingMode::TowardZero:
+    RoundingStr = "rtz";
+    break;
+  default:
+    break;
+  }
+  return RoundingStr;
+}
+
 std::optional<fp::ExceptionBehavior>
 convertStrToExceptionBehavior(StringRef ExceptionArg) {
   return StringSwitch<std::optional<fp::ExceptionBehavior>>(ExceptionArg)
@@ -70,6 +108,15 @@ convertStrToExceptionBehavior(StringRef ExceptionArg) {
       .Default(std::nullopt);
 }
 
+std::optional<fp::ExceptionBehavior>
+convertBundleToExceptionBehavior(StringRef ExceptionArg) {
+  return StringSwitch<std::optional<fp::ExceptionBehavior>>(ExceptionArg)
+      .Case("ignore", fp::ebIgnore)
+      .Case("maytrap", fp::ebMayTrap)
+      .Case("strict", fp::ebStrict)
+      .Default(std::nullopt);
+}
+
 std::optional<StringRef>
 convertExceptionBehaviorToStr(fp::ExceptionBehavior UseExcept) {
   std::optional<StringRef> ExceptStr;
@@ -87,6 +134,23 @@ convertExceptionBehaviorToStr(fp::ExceptionBehavior UseExcept) {
   return ExceptStr;
 }
 
+std::optional<StringRef>
+convertExceptionBehaviorToBundle(fp::ExceptionBehavior UseExcept) {
+  std::optional<StringRef> ExceptStr;
+  switch (UseExcept) {
+  case fp::ebStrict:
+    ExceptStr = "strict";
+    break;
+  case fp::ebIgnore:
+    ExceptStr = "ignore";
+    break;
+  case fp::ebMayTrap:
+    ExceptStr = "maytrap";
+    break;
+  }
+  return ExceptStr;
+}
+
 Intrinsic::ID getConstrainedIntrinsicID(const Instruction &Instr) {
   Intrinsic::ID IID = Intrinsic::not_intrinsic;
   switch (Instr.getOpcode()) {