[X86][Clang] Allow constexpr evaluation of F16C CVTPS2PH intrinsics (#162295)

Fixes #160312

Author: ericxu233

clang/include/clang/Basic/BuiltinsX86.td

@@ -710,11 +710,13 @@ let Features = "avx2", Attributes = [NoThrow, RequiredVectorWidth<128>] in {
   def gatherq_d : X86Builtin<"_Vector<4, int>(_Vector<4, int>, int const *, _Vector<2, long long int>, _Vector<4, int>, _Constant char)">;
 }
 
-let Features = "f16c", Attributes = [NoThrow, Const, RequiredVectorWidth<128>] in {
+let Features = "f16c",
+    Attributes = [NoThrow, Const, Constexpr, RequiredVectorWidth<128>] in {
   def vcvtps2ph : X86Builtin<"_Vector<8, short>(_Vector<4, float>, _Constant int)">;
 }
 
-let Features = "f16c", Attributes = [NoThrow, Const, RequiredVectorWidth<256>] in {
+let Features = "f16c",
+    Attributes = [NoThrow, Const, Constexpr, RequiredVectorWidth<256>] in {
   def vcvtps2ph256 : X86Builtin<"_Vector<8, short>(_Vector<8, float>, _Constant int)">;
 }
 

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -3527,7 +3527,94 @@ static bool interp__builtin_ia32_shufbitqmb_mask(InterpState &S, CodePtr OpPC,
   }
 
   pushInteger(S, RetMask, Call->getType());
+  return true;
+}
+
+static bool interp__builtin_ia32_vcvtps2ph(InterpState &S, CodePtr OpPC,
+                                           const CallExpr *Call) {
+  // Arguments are: vector of floats, rounding immediate
+  assert(Call->getNumArgs() == 2);
+
+  APSInt Imm = popToAPSInt(S, Call->getArg(1));
+  const Pointer &Src = S.Stk.pop<Pointer>();
+  const Pointer &Dst = S.Stk.peek<Pointer>();
+
+  assert(Src.getFieldDesc()->isPrimitiveArray());
+  assert(Dst.getFieldDesc()->isPrimitiveArray());
+
+  const auto *SrcVTy = Call->getArg(0)->getType()->castAs<VectorType>();
+  unsigned SrcNumElems = SrcVTy->getNumElements();
+  const auto *DstVTy = Call->getType()->castAs<VectorType>();
+  unsigned DstNumElems = DstVTy->getNumElements();
+
+  const llvm::fltSemantics &HalfSem =
+      S.getASTContext().getFloatTypeSemantics(S.getASTContext().HalfTy);
+
+  // imm[2] == 1 means use MXCSR rounding mode.
+  // In that case, we can only evaluate if the conversion is exact.
+  int ImmVal = Imm.getZExtValue();
+  bool UseMXCSR = (ImmVal & 4) != 0;
+  bool IsFPConstrained =
+      Call->getFPFeaturesInEffect(S.getASTContext().getLangOpts())
+          .isFPConstrained();
+
+  llvm::RoundingMode RM;
+  if (!UseMXCSR) {
+    switch (ImmVal & 3) {
+    case 0:
+      RM = llvm::RoundingMode::NearestTiesToEven;
+      break;
+    case 1:
+      RM = llvm::RoundingMode::TowardNegative;
+      break;
+    case 2:
+      RM = llvm::RoundingMode::TowardPositive;
+      break;
+    case 3:
+      RM = llvm::RoundingMode::TowardZero;
+      break;
+    default:
+      llvm_unreachable("Invalid immediate rounding mode");
+    }
+  } else {
+    // For MXCSR, we must check for exactness. We can use any rounding mode
+    // for the trial conversion since the result is the same if it's exact.
+    RM = llvm::RoundingMode::NearestTiesToEven;
+  }
+
+  QualType DstElemQT = Dst.getFieldDesc()->getElemQualType();
+  PrimType DstElemT = *S.getContext().classify(DstElemQT);
+
+  for (unsigned I = 0; I != SrcNumElems; ++I) {
+    Floating SrcVal = Src.elem<Floating>(I);
+    APFloat DstVal = SrcVal.getAPFloat();
+
+    bool LostInfo;
+    APFloat::opStatus St = DstVal.convert(HalfSem, RM, &LostInfo);
+
+    if (UseMXCSR && IsFPConstrained && St != APFloat::opOK) {
+      S.FFDiag(S.Current->getSource(OpPC),
+               diag::note_constexpr_dynamic_rounding);
+      return false;
+    }
 
+    INT_TYPE_SWITCH_NO_BOOL(DstElemT, {
+      // Convert the destination value's bit pattern to an unsigned integer,
+      // then reconstruct the element using the target type's 'from' method.
+      uint64_t RawBits = DstVal.bitcastToAPInt().getZExtValue();
+      Dst.elem<T>(I) = T::from(RawBits);
+    });
+  }
+
+  // Zero out remaining elements if the destination has more elements
+  // (e.g., vcvtps2ph converting 4 floats to 8 shorts).
+  if (DstNumElems > SrcNumElems) {
+    for (unsigned I = SrcNumElems; I != DstNumElems; ++I) {
+      INT_TYPE_SWITCH_NO_BOOL(DstElemT, { Dst.elem<T>(I) = T::from(0); });
+    }
+  }
+
+  Dst.initializeAllElements();
   return true;
 }
 
@@ -4956,6 +5043,10 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const CallExpr *Call,
   case X86::BI__builtin_ia32_insert128i256:
     return interp__builtin_x86_insert_subvector(S, OpPC, Call, BuiltinID);
 
+  case clang::X86::BI__builtin_ia32_vcvtps2ph:
+  case clang::X86::BI__builtin_ia32_vcvtps2ph256:
+    return interp__builtin_ia32_vcvtps2ph(S, OpPC, Call);
+
   case X86::BI__builtin_ia32_vec_ext_v4hi:
   case X86::BI__builtin_ia32_vec_ext_v16qi:
   case X86::BI__builtin_ia32_vec_ext_v8hi:

clang/lib/AST/ExprConstant.cpp

@@ -13909,6 +13909,81 @@ bool VectorExprEvaluator::VisitCallExpr(const CallExpr *E) {
       return false;
     return Success(R, E);
   }
+
+  case clang::X86::BI__builtin_ia32_vcvtps2ph:
+  case clang::X86::BI__builtin_ia32_vcvtps2ph256: {
+    APValue SrcVec;
+    if (!EvaluateAsRValue(Info, E->getArg(0), SrcVec))
+      return false;
+
+    APSInt Imm;
+    if (!EvaluateInteger(E->getArg(1), Imm, Info))
+      return false;
+
+    const auto *SrcVTy = E->getArg(0)->getType()->castAs<VectorType>();
+    unsigned SrcNumElems = SrcVTy->getNumElements();
+    const auto *DstVTy = E->getType()->castAs<VectorType>();
+    unsigned DstNumElems = DstVTy->getNumElements();
+    QualType DstElemTy = DstVTy->getElementType();
+
+    const llvm::fltSemantics &HalfSem =
+        Info.Ctx.getFloatTypeSemantics(Info.Ctx.HalfTy);
+
+    int ImmVal = Imm.getZExtValue();
+    bool UseMXCSR = (ImmVal & 4) != 0;
+    bool IsFPConstrained =
+        E->getFPFeaturesInEffect(Info.Ctx.getLangOpts()).isFPConstrained();
+
+    llvm::RoundingMode RM;
+    if (!UseMXCSR) {
+      switch (ImmVal & 3) {
+      case 0:
+        RM = llvm::RoundingMode::NearestTiesToEven;
+        break;
+      case 1:
+        RM = llvm::RoundingMode::TowardNegative;
+        break;
+      case 2:
+        RM = llvm::RoundingMode::TowardPositive;
+        break;
+      case 3:
+        RM = llvm::RoundingMode::TowardZero;
+        break;
+      default:
+        llvm_unreachable("Invalid immediate rounding mode");
+      }
+    } else {
+      RM = llvm::RoundingMode::NearestTiesToEven;
+    }
+
+    SmallVector<APValue, 8> ResultElements;
+    ResultElements.reserve(DstNumElems);
+
+    for (unsigned I = 0; I < SrcNumElems; ++I) {
+      APFloat SrcVal = SrcVec.getVectorElt(I).getFloat();
+
+      bool LostInfo;
+      APFloat::opStatus St = SrcVal.convert(HalfSem, RM, &LostInfo);
+
+      if (UseMXCSR && IsFPConstrained && St != APFloat::opOK) {
+        Info.FFDiag(E, diag::note_constexpr_dynamic_rounding);
+        return false;
+      }
+
+      APSInt DstInt(SrcVal.bitcastToAPInt(),
+                    DstElemTy->isUnsignedIntegerOrEnumerationType());
+      ResultElements.push_back(APValue(DstInt));
+    }
+
+    if (DstNumElems > SrcNumElems) {
+      APSInt Zero = Info.Ctx.MakeIntValue(0, DstElemTy);
+      for (unsigned I = SrcNumElems; I < DstNumElems; ++I) {
+        ResultElements.push_back(APValue(Zero));
+      }
+    }
+
+    return Success(ResultElements, E);
+  }
   }
 }
 

clang/test/CodeGen/X86/f16c-builtins.c

@@ -46,6 +46,31 @@ __m128 test_mm_cvtph_ps(__m128i a) {
   return _mm_cvtph_ps(a);
 }
 
+// A value exactly halfway between 1.0 and the next representable FP16 number.
+// In binary, its significand ends in ...000, followed by a tie-bit 1.
+#define POS_HALFWAY (1.0f + 0.00048828125f) // 1.0 + 2^-11, a tie-breaking case
+
+//
+// _mm_cvtps_ph (128-bit, 4 floats -> 8 shorts, 4 are zero-padded)
+//
+// Test values: -2.5f, 1.123f, POS_HALFWAY
+TEST_CONSTEXPR(match_v8hi(
+  _mm_cvtps_ph(_mm_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_NEAREST_INT),
+  0xC100, 0x3C7E, 0x3C00, 0x0000, 0, 0, 0, 0
+));
+TEST_CONSTEXPR(match_v8hi(
+  _mm_cvtps_ph(_mm_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_NEG_INF),
+  0xC100, 0x3C7D, 0x3C00, 0x0000, 0, 0, 0, 0
+));
+TEST_CONSTEXPR(match_v8hi(
+  _mm_cvtps_ph(_mm_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_POS_INF),
+  0xC100, 0x3C7E, 0x3C01, 0x0000, 0, 0, 0, 0
+));
+TEST_CONSTEXPR(match_v8hi(
+  _mm_cvtps_ph(_mm_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_ZERO),
+  0xC100, 0x3C7D, 0x3C00, 0x0000, 0, 0, 0, 0
+));
+
 __m256 test_mm256_cvtph_ps(__m128i a) {
   // CHECK-LABEL: test_mm256_cvtph_ps
   // CHECK: fpext <8 x half> %{{.*}} to <8 x float>
@@ -56,12 +81,44 @@ TEST_CONSTEXPR(match_m256(
     1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 0.5f, -2.0f, 0.0f
 ));
 
+//
+// _mm256_cvtps_ph (256-bit, 8 floats -> 8 shorts)
+//
+// Test values: -2.5f, 1.123f, POS_HALFWAY
+TEST_CONSTEXPR(match_v8hi(
+  _mm256_cvtps_ph(_mm256_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f, -2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_NEAREST_INT),
+  0xC100, 0x3C7E, 0x3C00, 0x0000, 0xC100, 0x3C7E, 0x3C00, 0x0000
+));
+TEST_CONSTEXPR(match_v8hi(
+  _mm256_cvtps_ph(_mm256_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f, -2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_NEG_INF),
+  0xC100, 0x3C7D, 0x3C00, 0x0000, 0xC100, 0x3C7D, 0x3C00, 0x0000
+));
+TEST_CONSTEXPR(match_v8hi(
+  _mm256_cvtps_ph(_mm256_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f, -2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_POS_INF),
+  0xC100, 0x3C7E, 0x3C01, 0x0000, 0xC100, 0x3C7E, 0x3C01, 0x0000
+));
+TEST_CONSTEXPR(match_v8hi(
+  _mm256_cvtps_ph(_mm256_setr_ps(-2.5f, 1.123f, POS_HALFWAY, 0.0f, -2.5f, 1.123f, POS_HALFWAY, 0.0f), _MM_FROUND_TO_ZERO),
+  0xC100, 0x3C7D, 0x3C00, 0x0000, 0xC100, 0x3C7D, 0x3C00, 0x0000
+));
+
 __m128i test_mm_cvtps_ph(__m128 a) {
   // CHECK-LABEL: test_mm_cvtps_ph
   // CHECK: call <8 x i16> @llvm.x86.vcvtps2ph.128(<4 x float> %{{.*}}, i32 0)
   return _mm_cvtps_ph(a, 0);
 }
 
+//
+// Tests for Exact Dynamic Rounding
+//
+// Test that dynamic rounding SUCCEEDS for exactly representable values.
+// We use _MM_FROUND_CUR_DIRECTION (value 4) to specify dynamic rounding.
+// Inputs: -2.5f, 0.125f, -16.0f are all exactly representable in FP16.
+TEST_CONSTEXPR(match_v8hi(
+  __builtin_ia32_vcvtps2ph256(_mm256_setr_ps(-2.5f, 0.125f, -16.0f, 0.0f, -2.5f, 0.125f, -16.0f, 0.0f), _MM_FROUND_CUR_DIRECTION),
+  0xC100, 0x3000, 0xCC00, 0x0000, 0xC100, 0x3000, 0xCC00, 0x0000
+));
+
 __m128i test_mm256_cvtps_ph(__m256 a) {
   // CHECK-LABEL: test_mm256_cvtps_ph
   // CHECK: call <8 x i16> @llvm.x86.vcvtps2ph.256(<8 x float> %{{.*}}, i32 0)