[clang] Add fixed point precision macros (#81207)

This defines the builtin macros specified in `7.18a.3 Precision macros` of ISO/IEC TR 18037:2008. These are the `__*__` versions of them and the formal definitions in stdfix.h can use them.
llvm · Feb 14, 2024 · ad49657 · ad49657
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diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -305,6 +305,12 @@ DWARF Support in Clang
 Floating Point Support in Clang
 -------------------------------
 
+Fixed Point Support in Clang
+----------------------------
+
+- Support fixed point precision macros according to ``7.18a.3`` of
+  `ISO/IEC TR 18037:2008 <https://standards.iso.org/ittf/PubliclyAvailableStandards/c051126_ISO_IEC_TR_18037_2008.zip>`_.
+
 AST Matchers
 ------------
 

diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -768,6 +768,60 @@ void InitializeOpenCLFeatureTestMacros(const TargetInfo &TI,
   Builder.defineMacro("__opencl_c_int64");
 }
 
+llvm::SmallString<32> ConstructFixedPointLiteral(llvm::APFixedPoint Val,
+                                                 llvm::StringRef Suffix) {
+  if (Val.isSigned() && Val == llvm::APFixedPoint::getMin(Val.getSemantics())) {
+    // When representing the min value of a signed fixed point type in source
+    // code, we cannot simply write `-<lowest value>`. For example, the min
+    // value of a `short _Fract` cannot be written as `-1.0hr`. This is because
+    // the parser will read this (and really any negative numerical literal) as
+    // a UnaryOperator that owns a FixedPointLiteral with a positive value
+    // rather than just a FixedPointLiteral with a negative value. Compiling
+    // `-1.0hr` results in an overflow to the maximal value of that fixed point
+    // type. The correct way to represent a signed min value is to instead split
+    // it into two halves, like `(-0.5hr-0.5hr)` which is what the standard
+    // defines SFRACT_MIN as.
+    llvm::SmallString<32> Literal;
+    Literal.push_back('(');
+    llvm::SmallString<32> HalfStr =
+        ConstructFixedPointLiteral(Val.shr(1), Suffix);
+    Literal += HalfStr;
+    Literal += HalfStr;
+    Literal.push_back(')');
+    return Literal;
+  }
+
+  llvm::SmallString<32> Str(Val.toString());
+  Str += Suffix;
+  return Str;
+}
+
+void DefineFixedPointMacros(const TargetInfo &TI, MacroBuilder &Builder,
+                            llvm::StringRef TypeName, llvm::StringRef Suffix,
+                            unsigned Width, unsigned Scale, bool Signed) {
+  // Saturation doesn't affect the size or scale of a fixed point type, so we
+  // don't need it here.
+  llvm::FixedPointSemantics FXSema(
+      Width, Scale, Signed, /*IsSaturated=*/false,
+      !Signed && TI.doUnsignedFixedPointTypesHavePadding());
+  llvm::SmallString<32> MacroPrefix("__");
+  MacroPrefix += TypeName;
+  Builder.defineMacro(MacroPrefix + "_EPSILON__",
+                      ConstructFixedPointLiteral(
+                          llvm::APFixedPoint::getEpsilon(FXSema), Suffix));
+  Builder.defineMacro(MacroPrefix + "_FBIT__", Twine(Scale));
+  Builder.defineMacro(
+      MacroPrefix + "_MAX__",
+      ConstructFixedPointLiteral(llvm::APFixedPoint::getMax(FXSema), Suffix));
+
+  // ISO/IEC TR 18037:2008 doesn't specify MIN macros for unsigned types since
+  // they're all just zero.
+  if (Signed)
+    Builder.defineMacro(
+        MacroPrefix + "_MIN__",
+        ConstructFixedPointLiteral(llvm::APFixedPoint::getMin(FXSema), Suffix));
+}
+
 static void InitializePredefinedMacros(const TargetInfo &TI,
                                        const LangOptions &LangOpts,
                                        const FrontendOptions &FEOpts,
@@ -1097,6 +1151,47 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
              TI.getTypeWidth(TI.getIntMaxType()) &&
          "uintmax_t and intmax_t have different widths?");
 
+  if (LangOpts.FixedPoint) {
+    // Each unsigned type has the same width as their signed type.
+    DefineFixedPointMacros(TI, Builder, "SFRACT", "HR", TI.getShortFractWidth(),
+                           TI.getShortFractScale(), /*Signed=*/true);
+    DefineFixedPointMacros(TI, Builder, "USFRACT", "UHR",
+                           TI.getShortFractWidth(),
+                           TI.getUnsignedShortFractScale(), /*Signed=*/false);
+    DefineFixedPointMacros(TI, Builder, "FRACT", "R", TI.getFractWidth(),
+                           TI.getFractScale(), /*Signed=*/true);
+    DefineFixedPointMacros(TI, Builder, "UFRACT", "UR", TI.getFractWidth(),
+                           TI.getUnsignedFractScale(), /*Signed=*/false);
+    DefineFixedPointMacros(TI, Builder, "LFRACT", "LR", TI.getLongFractWidth(),
+                           TI.getLongFractScale(), /*Signed=*/true);
+    DefineFixedPointMacros(TI, Builder, "ULFRACT", "ULR",
+                           TI.getLongFractWidth(),
+                           TI.getUnsignedLongFractScale(), /*Signed=*/false);
+    DefineFixedPointMacros(TI, Builder, "SACCUM", "HK", TI.getShortAccumWidth(),
+                           TI.getShortAccumScale(), /*Signed=*/true);
+    DefineFixedPointMacros(TI, Builder, "USACCUM", "UHK",
+                           TI.getShortAccumWidth(),
+                           TI.getUnsignedShortAccumScale(), /*Signed=*/false);
+    DefineFixedPointMacros(TI, Builder, "ACCUM", "K", TI.getAccumWidth(),
+                           TI.getAccumScale(), /*Signed=*/true);
+    DefineFixedPointMacros(TI, Builder, "UACCUM", "UK", TI.getAccumWidth(),
+                           TI.getUnsignedAccumScale(), /*Signed=*/false);
+    DefineFixedPointMacros(TI, Builder, "LACCUM", "LK", TI.getLongAccumWidth(),
+                           TI.getLongAccumScale(), /*Signed=*/true);
+    DefineFixedPointMacros(TI, Builder, "ULACCUM", "ULK",
+                           TI.getLongAccumWidth(),
+                           TI.getUnsignedLongAccumScale(), /*Signed=*/false);
+
+    Builder.defineMacro("__SACCUM_IBIT__", Twine(TI.getShortAccumIBits()));
+    Builder.defineMacro("__USACCUM_IBIT__",
+                        Twine(TI.getUnsignedShortAccumIBits()));
+    Builder.defineMacro("__ACCUM_IBIT__", Twine(TI.getAccumIBits()));
+    Builder.defineMacro("__UACCUM_IBIT__", Twine(TI.getUnsignedAccumIBits()));
+    Builder.defineMacro("__LACCUM_IBIT__", Twine(TI.getLongAccumIBits()));
+    Builder.defineMacro("__ULACCUM_IBIT__",
+                        Twine(TI.getUnsignedLongAccumIBits()));
+  }
+
   if (TI.hasFloat16Type())
     DefineFloatMacros(Builder, "FLT16", &TI.getHalfFormat(), "F16");
   DefineFloatMacros(Builder, "FLT", &TI.getFloatFormat(), "F");

diff --git a/clang/test/Preprocessor/fixed-point.c b/clang/test/Preprocessor/fixed-point.c
@@ -0,0 +1,67 @@
+/// Assert the fixed point precision macros according to ISO/IEC TR 18037:2008 7.18a.3 are
+/// defined when -ffixed-point is provided.
+
+// RUN: %clang_cc1 -triple=x86_64 -E -dM -ffixed-point -x c < /dev/null | FileCheck -match-full-lines %s
+// RUN: %clang_cc1 -triple=x86_64 -E -dM -ffixed-point -x c++ < /dev/null | FileCheck -match-full-lines %s
+
+/// These are the implementation-defined values for x86_64.
+// CHECK-DAG:#define __SFRACT_EPSILON__ 0.0078125HR
+// CHECK-DAG:#define __SFRACT_FBIT__ 7
+// CHECK-DAG:#define __SFRACT_MAX__ 0.9921875HR
+// CHECK-DAG:#define __SFRACT_MIN__ (-0.5HR-0.5HR)
+
+// CHECK-DAG:#define __USFRACT_EPSILON__ 0.00390625UHR
+// CHECK-DAG:#define __USFRACT_FBIT__ 8
+// CHECK-DAG:#define __USFRACT_MAX__ 0.99609375UHR
+
+// CHECK-DAG:#define __FRACT_EPSILON__ 0.000030517578125R
+// CHECK-DAG:#define __FRACT_FBIT__ 15
+// CHECK-DAG:#define __FRACT_MAX__ 0.999969482421875R
+// CHECK-DAG:#define __FRACT_MIN__ (-0.5R-0.5R)
+
+// CHECK-DAG:#define __UFRACT_EPSILON__ 0.0000152587890625UR
+// CHECK-DAG:#define __UFRACT_FBIT__ 16
+// CHECK-DAG:#define __UFRACT_MAX__ 0.9999847412109375UR
+
+// CHECK-DAG:#define __LFRACT_EPSILON__ 0.0000000004656612873077392578125LR
+// CHECK-DAG:#define __LFRACT_FBIT__ 31
+// CHECK-DAG:#define __LFRACT_MAX__ 0.9999999995343387126922607421875LR
+// CHECK-DAG:#define __LFRACT_MIN__ (-0.5LR-0.5LR)
+
+// CHECK-DAG:#define __ULFRACT_EPSILON__ 0.00000000023283064365386962890625ULR
+// CHECK-DAG:#define __ULFRACT_FBIT__ 32
+// CHECK-DAG:#define __ULFRACT_MAX__ 0.99999999976716935634613037109375ULR
+
+// CHECK-DAG:#define __SACCUM_EPSILON__ 0.0078125HK
+// CHECK-DAG:#define __SACCUM_FBIT__ 7
+// CHECK-DAG:#define __SACCUM_MAX__ 255.9921875HK
+// CHECK-DAG:#define __SACCUM_MIN__ (-128.0HK-128.0HK)
+
+// CHECK-DAG:#define __USACCUM_EPSILON__ 0.00390625UHK
+// CHECK-DAG:#define __USACCUM_FBIT__ 8
+// CHECK-DAG:#define __USACCUM_MAX__ 255.99609375UHK
+
+// CHECK-DAG:#define __ACCUM_EPSILON__ 0.000030517578125K
+// CHECK-DAG:#define __ACCUM_FBIT__ 15
+// CHECK-DAG:#define __ACCUM_MAX__ 65535.999969482421875K
+// CHECK-DAG:#define __ACCUM_MIN__ (-32768.0K-32768.0K)
+
+// CHECK-DAG:#define __UACCUM_EPSILON__ 0.0000152587890625UK
+// CHECK-DAG:#define __UACCUM_FBIT__ 16
+// CHECK-DAG:#define __UACCUM_MAX__ 65535.9999847412109375UK
+
+// CHECK-DAG:#define __LACCUM_EPSILON__ 0.0000000004656612873077392578125LK
+// CHECK-DAG:#define __LACCUM_FBIT__ 31
+// CHECK-DAG:#define __LACCUM_MAX__ 4294967295.9999999995343387126922607421875LK
+// CHECK-DAG:#define __LACCUM_MIN__ (-2147483648.0LK-2147483648.0LK)
+
+// CHECK-DAG:#define __ULACCUM_EPSILON__ 0.00000000023283064365386962890625ULK
+// CHECK-DAG:#define __ULACCUM_FBIT__ 32
+// CHECK-DAG:#define __ULACCUM_MAX__ 4294967295.99999999976716935634613037109375ULK
+
+// CHECK-DAG:#define __SACCUM_IBIT__ 8
+// CHECK-DAG:#define __USACCUM_IBIT__ 8
+// CHECK-DAG:#define __ACCUM_IBIT__ 16
+// CHECK-DAG:#define __UACCUM_IBIT__ 16
+// CHECK-DAG:#define __LACCUM_IBIT__ 32
+// CHECK-DAG:#define __ULACCUM_IBIT__ 32
diff --git a/clang/test/Preprocessor/no-fixed-point.c b/clang/test/Preprocessor/no-fixed-point.c
@@ -0,0 +1,7 @@
+/// Assert the fixed point precision macros according to ISO/IEC TR 18037:2008 7.18a.3 are not
+/// defined when -ffixed-point is not provided.
+
+// RUN: %clang_cc1 -triple=x86_64 -E -dM -x c < /dev/null | FileCheck -match-full-lines %s
+// RUN: %clang_cc1 -triple=x86_64 -E -dM -x c++ < /dev/null | FileCheck -match-full-lines %s
+
+// CHECK-NOT:#define __SFRACT_FBIT__ 7
diff --git a/llvm/include/llvm/ADT/APFixedPoint.h b/llvm/include/llvm/ADT/APFixedPoint.h
@@ -260,6 +260,7 @@ class APFixedPoint {
 
   static APFixedPoint getMax(const FixedPointSemantics &Sema);
   static APFixedPoint getMin(const FixedPointSemantics &Sema);
+  static APFixedPoint getEpsilon(const FixedPointSemantics &Sema);
 
   /// Given a floating point semantic, return the next floating point semantic
   /// with a larger exponent and larger or equal mantissa.

diff --git a/llvm/lib/Support/APFixedPoint.cpp b/llvm/lib/Support/APFixedPoint.cpp
@@ -129,6 +129,12 @@ APFixedPoint APFixedPoint::getMin(const FixedPointSemantics &Sema) {
   return APFixedPoint(Val, Sema);
 }
 
+APFixedPoint APFixedPoint::getEpsilon(const FixedPointSemantics &Sema) {
+  APSInt Val(Sema.getWidth(), !Sema.isSigned());
+  Val.setBit(/*BitPosition=*/0);
+  return APFixedPoint(Val, Sema);
+}
+
 bool FixedPointSemantics::fitsInFloatSemantics(
     const fltSemantics &FloatSema) const {
   // A fixed point semantic fits in a floating point semantic if the maximum