[LLVM] Add __builtin_readsteadycounter intrinsic and builtin for realtime clocks (#81331)

Summary:
This patch adds a new intrinsic and builtin function mirroring the
existing `__builtin_readcyclecounter`. The difference is that this
implementation targets a separate counter that some targets have which
returns a fixed frequency clock that can be used to determine elapsed
time, this is different compared to the cycle counter which often has
variable frequency.

This patch only adds support for the NVPTX and AMDGPU targets.

This is done as a new and separate builtin rather than an argument to
`readcyclecounter` to avoid needing to change existing code and to make
the separation more explicit.

Author: jhuber6

clang/docs/LanguageExtensions.rst

@@ -2764,6 +2764,39 @@ Query for this feature with ``__has_builtin(__builtin_readcyclecounter)``. Note
 that even if present, its use may depend on run-time privilege or other OS
 controlled state.
 
+``__builtin_readsteadycounter``
+------------------------------
+
+``__builtin_readsteadycounter`` is used to access the fixed frequency counter
+register (or a similar steady-rate clock) on those targets that support it.
+The function is similar to ``__builtin_readcyclecounter`` above except that the
+frequency is fixed, making it suitable for measuring elapsed time.
+
+**Syntax**:
+
+.. code-block:: c++
+
+  __builtin_readsteadycounter()
+
+**Example of Use**:
+
+.. code-block:: c++
+
+  unsigned long long t0 = __builtin_readsteadycounter();
+  do_something();
+  unsigned long long t1 = __builtin_readsteadycounter();
+  unsigned long long secs_to_do_something = (t1 - t0) / tick_rate;
+
+**Description**:
+
+The ``__builtin_readsteadycounter()`` builtin returns the frequency counter value.
+When not supported by the target, the return value is always zero. This builtin
+takes no arguments and produces an unsigned long long result. The builtin does 
+not guarantee any particular frequency, only that it is stable. Knowledge of the 
+counter's true frequency will need to be provided by the user.
+
+Query for this feature with ``__has_builtin(__builtin_readsteadycounter)``.
+
 ``__builtin_dump_struct``
 -------------------------
 

clang/docs/ReleaseNotes.rst

@@ -117,6 +117,9 @@ C23 Feature Support
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
+- Added ``__builtin_readsteadycounter`` for reading fixed frequency hardware
+  counters.
+
 New Compiler Flags
 ------------------
 

clang/include/clang/Basic/Builtins.td

@@ -1110,6 +1110,12 @@ def ReadCycleCounter : Builtin {
   let Prototype = "unsigned long long int()";
 }
 
+def ReadSteadyCounter : Builtin {
+  let Spellings = ["__builtin_readsteadycounter"];
+  let Attributes = [NoThrow];
+  let Prototype = "unsigned long long int()";
+}
+
 def Trap : Builtin {
   let Spellings = ["__builtin_trap"];
   let Attributes = [NoThrow, NoReturn];

clang/lib/CodeGen/CGBuiltin.cpp

@@ -3443,6 +3443,10 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     Function *F = CGM.getIntrinsic(Intrinsic::readcyclecounter);
     return RValue::get(Builder.CreateCall(F));
   }
+  case Builtin::BI__builtin_readsteadycounter: {
+    Function *F = CGM.getIntrinsic(Intrinsic::readsteadycounter);
+    return RValue::get(Builder.CreateCall(F));
+  }
   case Builtin::BI__builtin___clear_cache: {
     Value *Begin = EmitScalarExpr(E->getArg(0));
     Value *End = EmitScalarExpr(E->getArg(1));

clang/test/CodeGen/builtins.c

@@ -496,6 +496,12 @@ long long test_builtin_readcyclecounter(void) {
   return __builtin_readcyclecounter();
 }
 
+// CHECK-LABEL: define{{.*}} i64 @test_builtin_readsteadycounter
+long long test_builtin_readsteadycounter(void) {
+  // CHECK: call i64 @llvm.readsteadycounter()
+  return __builtin_readsteadycounter();
+}
+
 /// __builtin_launder should be a NOP in C since there are no vtables.
 // CHECK-LABEL: define{{.*}} void @test_builtin_launder
 void test_builtin_launder(int *p) {

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -1179,6 +1179,12 @@ enum NodeType {
   /// counter-like register (or other high accuracy low latency clock source).
   READCYCLECOUNTER,
 
+  /// READSTEADYCOUNTER - This corresponds to the readfixedcounter intrinsic.
+  /// It has the same semantics as the READCYCLECOUNTER implementation except
+  /// that the result is the content of the architecture-specific fixed
+  /// frequency counter suitable for measuring elapsed time.
+  READSTEADYCOUNTER,
+
   /// HANDLENODE node - Used as a handle for various purposes.
   HANDLENODE,
 

llvm/include/llvm/IR/Intrinsics.td

@@ -870,6 +870,8 @@ def int_pcmarker      : DefaultAttrsIntrinsic<[], [llvm_i32_ty]>;
 
 def int_readcyclecounter : DefaultAttrsIntrinsic<[llvm_i64_ty]>;
 
+def int_readsteadycounter : DefaultAttrsIntrinsic<[llvm_i64_ty]>;
+
 // The assume intrinsic is marked InaccessibleMemOnly so that proper control
 // dependencies will be maintained.
 def int_assume : DefaultAttrsIntrinsic<

llvm/include/llvm/Support/TargetOpcodes.def

@@ -352,6 +352,9 @@ HANDLE_TARGET_OPCODE(G_INTRINSIC_ROUNDEVEN)
 /// INTRINSIC readcyclecounter
 HANDLE_TARGET_OPCODE(G_READCYCLECOUNTER)
 
+/// INTRINSIC readsteadycounter
+HANDLE_TARGET_OPCODE(G_READSTEADYCOUNTER)
+
 /// Generic load (including anyext load)
 HANDLE_TARGET_OPCODE(G_LOAD)
 

llvm/include/llvm/Target/GenericOpcodes.td

@@ -1101,6 +1101,12 @@ def G_READCYCLECOUNTER : GenericInstruction {
   let hasSideEffects = true;
 }
 
+def G_READSTEADYCOUNTER : GenericInstruction {
+  let OutOperandList = (outs type0:$dst);
+  let InOperandList = (ins);
+  let hasSideEffects = true;
+}
+
 //------------------------------------------------------------------------------
 // Memory ops
 //------------------------------------------------------------------------------

llvm/include/llvm/Target/GlobalISel/SelectionDAGCompat.td

@@ -168,6 +168,7 @@ def : GINodeEquiv<G_FMAXNUM_IEEE, fmaxnum_ieee>;
 def : GINodeEquiv<G_FMAXIMUM, fmaximum>;
 def : GINodeEquiv<G_FMINIMUM, fminimum>;
 def : GINodeEquiv<G_READCYCLECOUNTER, readcyclecounter>;
+def : GINodeEquiv<G_READSTEADYCOUNTER, readsteadycounter>;
 def : GINodeEquiv<G_ROTR, rotr>;
 def : GINodeEquiv<G_ROTL, rotl>;
 def : GINodeEquiv<G_LROUND, lround>;