[APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc.

Author: lntue

llvm/CMakeLists.txt

@@ -659,6 +659,17 @@ endif()
 
 set(LLVM_ENABLE_Z3_SOLVER_DEFAULT "${Z3_FOUND}")
 
+set(LLVM_INTEGRATE_LIBC "OFF" CACHE STRING "Use LLVM libc codes directly if available.")
+
+if(LLVM_INTEGRATE_LIBC)
+  message(STATUS "LLVM_INTEGRATE_LIBC is ${LLVM_INTEGRATE_LIBC}")
+  include(FindLibcCommonUtils)
+  if(NOT TARGET llvm-libc-common-utilities)
+    message(STATUS "LLVM_INTEGRATE_LIBC is set but cannot find LLVM libc at ${libc_path}.")
+    set(LLVM_INTEGRATE_LIBC OFF)
+  endif()
+endif()
+
 
 if( LLVM_TARGETS_TO_BUILD STREQUAL "all" )
   set( LLVM_TARGETS_TO_BUILD ${LLVM_ALL_TARGETS} )

llvm/include/llvm/ADT/APFloat.h

@@ -1523,6 +1523,7 @@ class APFloat : public APFloatBase {
   friend int ilogb(const APFloat &Arg) { return ilogb(Arg.getIEEE()); }
   friend APFloat scalbn(APFloat X, int Exp, roundingMode RM);
   friend APFloat frexp(const APFloat &X, int &Exp, roundingMode RM);
+  friend APFloat exp(const APFloat &X, roundingMode RM);
   friend IEEEFloat;
   friend DoubleAPFloat;
 };
@@ -1658,6 +1659,10 @@ inline APFloat maximumnum(const APFloat &A, const APFloat &B) {
   return A < B ? B : A;
 }
 
+/// Implement IEEE 754-2019 exp functions.
+LLVM_READONLY
+APFloat exp(const APFloat &X, RoundingMode RM);
+
 inline raw_ostream &operator<<(raw_ostream &OS, const APFloat &V) {
   V.print(OS);
   return OS;

llvm/include/llvm/Config/llvm-config.h.cmake

@@ -146,4 +146,7 @@
    coverage bugs, and to 0 otherwise. */
 #cmakedefine01 LLVM_ENABLE_DEBUGLOC_TRACKING_ORIGIN
 
+/* Define if LLVM and clang uses LLVM libc for math computations. */
+#cmakedefine LLVM_INTEGRATE_LIBC
+
 #endif

llvm/lib/Support/APFloat.cpp

@@ -28,6 +28,11 @@
 #include <cstring>
 #include <limits.h>
 
+#ifdef LLVM_INTEGRATE_LIBC
+// Shared headers from LLVM libc
+#include "shared/math.h"
+#endif // LLVM_INTEGRATE_LIBC
+
 #define APFLOAT_DISPATCH_ON_SEMANTICS(METHOD_CALL)                             \
   do {                                                                         \
     if (usesLayout<IEEEFloat>(getSemantics()))                                 \
@@ -5601,6 +5606,35 @@ float APFloat::convertToFloat() const {
   return Temp.getIEEE().convertToFloat();
 }
 
+#ifdef LLVM_INTEGRATE_LIBC
+APFloat exp(const APFloat &X, RoundingMode rounding_mode) {
+  assert((&X.getSemantics() == (const llvm::fltSemantics *)&semIEEEsingle) &&
+         "Float semantics is not IEEEsingle");
+  if (&X.getSemantics() == (const llvm::fltSemantics *)&semIEEEsingle) {
+    int current_rounding_mode = fegetround();
+    switch (rounding_mode) {
+    case APFloat::rmNearestTiesToEven:
+      fesetround(FE_TONEAREST);
+      break;
+    case APFloat::rmTowardPositive:
+      fesetround(FE_UPWARD);
+      break;
+    case APFloat::rmTowardNegative:
+      fesetround(FE_DOWNWARD);
+      break;
+    case APFloat::rmTowardZero:
+      fesetround(FE_TOWARDZERO);
+      break;
+    default:
+    }
+    float result = LIBC_NAMESPACE::shared::expf(X.convertToFloat());
+    fesetround(current_rounding_mode);
+    return APFloat(result);
+  }
+  llvm_unreachable("Unexpected semantics");
+}
+#endif // LLVM_INTEGRATE_LIBC
+
 } // namespace llvm
 
 #undef APFLOAT_DISPATCH_ON_SEMANTICS

llvm/lib/Support/CMakeLists.txt

@@ -384,3 +384,9 @@ target_include_directories(LLVMSupport
   PRIVATE
   ${LLVM_THIRD_PARTY_DIR}/siphash/include
 )
+
+if(LLVM_INTEGRATE_LIBC)
+  set_property(TARGET LLVMSupport PROPERTY CXX_STANDARD 17)
+  target_include_directories(LLVMSupport PRIVATE "${LLVM_INCLUDE_DIR}/../../libc")
+  target_compile_options(LLVMSupport PRIVATE "-Wno-c99-extensions") # _Complex warnings.
+endif()

llvm/lib/Target/AMDGPU/AMDGPULibCalls.cpp

@@ -1540,7 +1540,7 @@ bool AMDGPULibCalls::evaluateScalarMathFunc(const FuncInfo &FInfo, double &Res0,
     return true;
 
   case AMDGPULibFunc::EI_EXP:
-    Res0 = exp(opr0);
+    Res0 = std::exp(opr0);
     return true;
 
   case AMDGPULibFunc::EI_EXP2:

llvm/unittests/ADT/APFloatTest.cpp

@@ -8356,4 +8356,23 @@ TEST(APFloatTest, hasSignBitInMSB) {
   EXPECT_FALSE(APFloat::hasSignBitInMSB(APFloat::Float8E8M0FNU()));
 }
 
+#ifdef LLVM_INTEGRATE_LIBC
+TEST(APFloatTest, expf) {
+  EXPECT_EQ(
+      1.0f,
+      llvm::exp(APFloat(0.0f), APFloat::rmNearestTiesToEven).convertToFloat());
+  EXPECT_EQ(
+      0x1.5bf0a8p1f,
+      llvm::exp(APFloat(1.0f), APFloat::rmNearestTiesToEven).convertToFloat());
+  EXPECT_EQ(
+      0x1.5bf0aap1f,
+      llvm::exp(APFloat(1.0f), APFloat::rmTowardPositive).convertToFloat());
+  EXPECT_EQ(
+      0x1.5bf0a8p1f,
+      llvm::exp(APFloat(1.0f), APFloat::rmTowardNegative).convertToFloat());
+  EXPECT_EQ(0x1.5bf0a8p1f,
+            llvm::exp(APFloat(1.0f), APFloat::rmTowardZero).convertToFloat());
+}
+#endif // LLVM_INTEGRATE_LIBC
+
 } // namespace