Add intrinsics for constrained floating point operations

This commit introduces a set of experimental intrinsics intended to prevent
optimizations that make assumptions about the rounding mode and floating point
exception behavior.  These intrinsics will later be extended to specify
flush-to-zero behavior.  More work is also required to model instruction
dependencies in machine code and to generate these instructions from clang
(when required by pragmas and/or command line options that are not currently
supported).

Differential Revision: https://reviews.llvm.org/D27028



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293226 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Andrew Kaylor

docs/LangRef.rst

@@ -12064,6 +12064,277 @@ Semantics:
 Returns another pointer that aliases its argument but which is considered different 
 for the purposes of ``load``/``store`` ``invariant.group`` metadata.
 
+Constrained Floating Point Intrinsics
+-------------------------------------
+
+These intrinsics are used to provide special handling of floating point
+operations when specific rounding mode or floating point exception behavior is
+required.  By default, LLVM optimization passes assume that the rounding mode is
+round-to-nearest and that floating point exceptions will not be monitored.
+Constrained FP intrinsics are used to support non-default rounding modes and
+accurately preserve exception behavior without compromising LLVM's ability to
+optimize FP code when the default behavior is used.
+
+Each of these intrinsics corresponds to a normal floating point operation.  The
+first two arguments and the return value are the same as the corresponding FP
+operation.
+
+The third argument is a metadata argument specifying the rounding mode to be
+assumed. This argument must be one of the following strings:
+
+::
+      "round.dynamic"
+      "round.tonearest"
+      "round.downward"
+      "round.upward"
+      "round.towardzero"
+
+If this argument is "round.dynamic" optimization passes must assume that the
+rounding mode is unknown and may change at runtime.  No transformations that
+depend on rounding mode may be performed in this case.
+
+The other possible values for the rounding mode argument correspond to the
+similarly named IEEE rounding modes.  If the argument is any of these values
+optimization passes may perform transformations as long as they are consistent
+with the specified rounding mode.
+
+For example, 'x-0'->'x' is not a valid transformation if the rounding mode is
+"round.downward" or "round.dynamic" because if the value of 'x' is +0 then
+'x-0' should evaluate to '-0' when rounding downward.  However, this
+transformation is legal for all other rounding modes.
+
+For values other than "round.dynamic" optimization passes may assume that the
+actual runtime rounding mode (as defined in a target-specific manner) matches
+the specified rounding mode, but this is not guaranteed.  Using a specific
+non-dynamic rounding mode which does not match the actual rounding mode at
+runtime results in undefined behavior.
+
+The fourth argument to the constrained floating point intrinsics specifies the
+required exception behavior.  This argument must be one of the following
+strings:
+
+::
+      "fpexcept.ignore"
+      "fpexcept.maytrap"
+      "fpexcept.strict"
+
+If this argument is "fpexcept.ignore" optimization passes may assume that the
+exception status flags will not be read and that floating point exceptions will
+be masked.  This allows transformations to be performed that may change the
+exception semantics of the original code.  For example, FP operations may be
+speculatively executed in this case whereas they must not be for either of the
+other possible values of this argument.
+
+If the exception behavior argument is "fpexcept.maytrap" optimization passes
+must avoid transformations that may raise exceptions that would not have been
+raised by the original code (such as speculatively executing FP operations), but
+passes are not required to preserve all exceptions that are implied by the
+original code.  For example, exceptions may be potentially hidden by constant
+folding.
+
+If the exception behavior argument is "fpexcept.strict" all transformations must
+strictly preserve the floating point exception semantics of the original code.
+Any FP exception that would have been raised by the original code must be raised
+by the transformed code, and the transformed code must not raise any FP
+exceptions that would not have been raised by the original code.  This is the
+exception behavior argument that will be used if the code being compiled reads 
+the FP exception status flags, but this mode can also be used with code that
+unmasks FP exceptions.
+
+The number and order of floating point exceptions is NOT guaranteed.  For
+example, a series of FP operations that each may raise exceptions may be
+vectorized into a single instruction that raises each unique exception a single
+time.
+
+
+'``llvm.experimental.constrained.fadd``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare <type> 
+      @llvm.experimental.constrained.fadd(<type> <op1>, <type> <op2>,
+                                          metadata <rounding mode>,
+                                          metadata  <exception behavior>)
+
+Overview:
+"""""""""
+
+The '``llvm.experimental.constrained.fadd``' intrinsic returns the sum of its
+two operands.
+
+
+Arguments:
+""""""""""
+
+The first two arguments to the '``llvm.experimental.constrained.fadd``'
+intrinsic must be :ref:`floating point <t_floating>` or :ref:`vector <t_vector>`
+of floating point values. Both arguments must have identical types.
+
+The third and fourth arguments specify the rounding mode and exception
+behavior as described above.
+
+Semantics:
+""""""""""
+
+The value produced is the floating point sum of the two value operands and has
+the same type as the operands.
+
+
+'``llvm.experimental.constrained.fsub``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare <type> 
+      @llvm.experimental.constrained.fsub(<type> <op1>, <type> <op2>,
+                                          metadata <rounding mode>,
+                                          metadata  <exception behavior>)
+
+Overview:
+"""""""""
+
+The '``llvm.experimental.constrained.fsub``' intrinsic returns the difference
+of its two operands.
+
+
+Arguments:
+""""""""""
+
+The first two arguments to the '``llvm.experimental.constrained.fsub``'
+intrinsic must be :ref:`floating point <t_floating>` or :ref:`vector <t_vector>`
+of floating point values. Both arguments must have identical types.
+
+The third and fourth arguments specify the rounding mode and exception
+behavior as described above.
+
+Semantics:
+""""""""""
+
+The value produced is the floating point difference of the two value operands
+and has the same type as the operands.
+
+
+'``llvm.experimental.constrained.fmul``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare <type> 
+      @llvm.experimental.constrained.fmul(<type> <op1>, <type> <op2>,
+                                          metadata <rounding mode>,
+                                          metadata  <exception behavior>)
+
+Overview:
+"""""""""
+
+The '``llvm.experimental.constrained.fmul``' intrinsic returns the product of
+its two operands.
+
+
+Arguments:
+""""""""""
+
+The first two arguments to the '``llvm.experimental.constrained.fmul``'
+intrinsic must be :ref:`floating point <t_floating>` or :ref:`vector <t_vector>`
+of floating point values. Both arguments must have identical types.
+
+The third and fourth arguments specify the rounding mode and exception
+behavior as described above.
+
+Semantics:
+""""""""""
+
+The value produced is the floating point product of the two value operands and
+has the same type as the operands.
+
+
+'``llvm.experimental.constrained.fdiv``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare <type> 
+      @llvm.experimental.constrained.fdiv(<type> <op1>, <type> <op2>,
+                                          metadata <rounding mode>,
+                                          metadata  <exception behavior>)
+
+Overview:
+"""""""""
+
+The '``llvm.experimental.constrained.fdiv``' intrinsic returns the quotient of
+its two operands.
+
+
+Arguments:
+""""""""""
+
+The first two arguments to the '``llvm.experimental.constrained.fdiv``'
+intrinsic must be :ref:`floating point <t_floating>` or :ref:`vector <t_vector>`
+of floating point values. Both arguments must have identical types.
+
+The third and fourth arguments specify the rounding mode and exception
+behavior as described above.
+
+Semantics:
+""""""""""
+
+The value produced is the floating point quotient of the two value operands and
+has the same type as the operands.
+
+
+'``llvm.experimental.constrained.frem``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+::
+
+      declare <type> 
+      @llvm.experimental.constrained.frem(<type> <op1>, <type> <op2>,
+                                          metadata <rounding mode>,
+                                          metadata  <exception behavior>)
+
+Overview:
+"""""""""
+
+The '``llvm.experimental.constrained.frem``' intrinsic returns the remainder
+from the division of its two operands.
+
+
+Arguments:
+""""""""""
+
+The first two arguments to the '``llvm.experimental.constrained.frem``'
+intrinsic must be :ref:`floating point <t_floating>` or :ref:`vector <t_vector>`
+of floating point values. Both arguments must have identical types.
+
+The third and fourth arguments specify the rounding mode and exception
+behavior as described above.  The rounding mode argument has no effect, since
+the result of frem is never rounded, but the argument is included for
+consistency with the other constrained floating point intrinsics.
+
+Semantics:
+""""""""""
+
+The value produced is the floating point remainder from the division of the two
+value operands and has the same type as the operands.  The remainder has the
+same sign as the dividend. 
+
+
 General Intrinsics
 ------------------
 

include/llvm/CodeGen/ISDOpcodes.h

@@ -245,6 +245,12 @@ namespace ISD {
     /// Simple binary floating point operators.
     FADD, FSUB, FMUL, FDIV, FREM,
 
+    /// Constrained versions of the binary floating point operators.
+    /// These will be lowered to the simple operators before final selection.
+    /// They are used to limit optimizations while the DAG is being
+    /// optimized.
+    STRICT_FADD, STRICT_FSUB, STRICT_FMUL, STRICT_FDIV, STRICT_FREM,
+
     /// FMA - Perform a * b + c with no intermediate rounding step.
     FMA,
 

include/llvm/CodeGen/SelectionDAGISel.h

@@ -270,6 +270,8 @@ class SelectionDAGISel : public MachineFunctionPass {
   SDNode *MorphNode(SDNode *Node, unsigned TargetOpc, SDVTList VTs,
                     ArrayRef<SDValue> Ops, unsigned EmitNodeInfo);
 
+  SDNode *MutateStrictFPToFP(SDNode *Node, unsigned NewOpc);
+
   /// Prepares the landing pad to take incoming values or do other EH
   /// personality specific tasks. Returns true if the block should be
   /// instruction selected, false if no code should be emitted for it.

include/llvm/IR/IntrinsicInst.h

@@ -152,6 +152,45 @@ namespace llvm {
     }
   };
 
+  /// This is the common base class for constrained floating point intrinsics.
+  class ConstrainedFPIntrinsic : public IntrinsicInst {
+  public:
+    enum RoundingMode {
+      rmInvalid,
+      rmDynamic,
+      rmToNearest,
+      rmDownward,
+      rmUpward,
+      rmTowardZero
+    };
+
+    enum ExceptionBehavior {
+      ebInvalid,
+      ebIgnore,
+      ebMayTrap,
+      ebStrict
+    };
+
+    RoundingMode getRoundingMode() const;
+    ExceptionBehavior getExceptionBehavior() const;
+
+    // Methods for support type inquiry through isa, cast, and dyn_cast:
+    static inline bool classof(const IntrinsicInst *I) {
+      switch (I->getIntrinsicID()) {
+      case Intrinsic::experimental_constrained_fadd:
+      case Intrinsic::experimental_constrained_fsub:
+      case Intrinsic::experimental_constrained_fmul:
+      case Intrinsic::experimental_constrained_fdiv:
+      case Intrinsic::experimental_constrained_frem:
+        return true;
+      default: return false;
+      }
+    }
+    static inline bool classof(const Value *V) {
+      return isa<IntrinsicInst>(V) && classof(cast<IntrinsicInst>(V));
+    }
+  };
+
   /// This is the common base class for memset/memcpy/memmove.
   class MemIntrinsic : public IntrinsicInst {
   public:

include/llvm/IR/Intrinsics.td

@@ -389,6 +389,9 @@ def int_memset  : Intrinsic<[],
                              llvm_i32_ty, llvm_i1_ty],
                             [IntrArgMemOnly, NoCapture<0>, WriteOnly<0>]>;
 
+// FIXME: Add version of these floating point intrinsics which allow non-default
+// rounding modes and FP exception handling.
+
 let IntrProperties = [IntrNoMem] in {
   def int_fma  : Intrinsic<[llvm_anyfloat_ty],
                            [LLVMMatchType<0>, LLVMMatchType<0>,
@@ -442,6 +445,39 @@ def int_objectsize : Intrinsic<[llvm_anyint_ty], [llvm_anyptr_ty, llvm_i1_ty],
                                [IntrNoMem]>,
                                GCCBuiltin<"__builtin_object_size">;
 
+//===--------------- Constrained Floating Point Intrinsics ----------------===//
+//
+
+let IntrProperties = [IntrInaccessibleMemOnly] in {
+  def int_experimental_constrained_fadd : Intrinsic<[ llvm_anyfloat_ty ],
+                                                    [ LLVMMatchType<0>,
+                                                      LLVMMatchType<0>,
+                                                      llvm_metadata_ty,
+                                                      llvm_metadata_ty ]>;
+  def int_experimental_constrained_fsub : Intrinsic<[ llvm_anyfloat_ty ],
+                                                    [ LLVMMatchType<0>,
+                                                      LLVMMatchType<0>,
+                                                      llvm_metadata_ty,
+                                                      llvm_metadata_ty ]>;
+  def int_experimental_constrained_fmul : Intrinsic<[ llvm_anyfloat_ty ],
+                                                    [ LLVMMatchType<0>,
+                                                      LLVMMatchType<0>,
+                                                      llvm_metadata_ty,
+                                                      llvm_metadata_ty ]>;
+  def int_experimental_constrained_fdiv : Intrinsic<[ llvm_anyfloat_ty ],
+                                                    [ LLVMMatchType<0>,
+                                                      LLVMMatchType<0>,
+                                                      llvm_metadata_ty,
+                                                      llvm_metadata_ty ]>;
+  def int_experimental_constrained_frem : Intrinsic<[ llvm_anyfloat_ty ],
+                                                    [ LLVMMatchType<0>,
+                                                      LLVMMatchType<0>,
+                                                      llvm_metadata_ty,
+                                                      llvm_metadata_ty ]>;
+}
+// FIXME: Add intrinsic for fcmp, fptrunc, fpext, fptoui and fptosi.
+
+
 //===------------------------- Expect Intrinsics --------------------------===//
 //
 def int_expect : Intrinsic<[llvm_anyint_ty], [LLVMMatchType<0>,