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[APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc. #143959

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[APFloat] Add exp function for APFloat::IEEESsingle using expf implementation from LLVM libc. #143959

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aeaac93
[APFloat] Add exp function for APFloat::IEEESsingle using expf implem…
lntue Jun 12, 2025
32f6609
Fix typo and add more tests.
lntue Jun 13, 2025
d43491b
Add directional rounding version to LLVM libc and use that in APFloat.
lntue Jun 13, 2025
f134cb7
Only change rounding modes if they are different.
lntue Jun 13, 2025
d18947d
Omit declarations when LLVM_INTEGRATE_LIBC is off and fix default rou…
lntue Jun 13, 2025
79ee1b5
Remove trailing ).
lntue Jun 13, 2025
b1ea332
Wrap static rounding expf function with #pragma STDC FENV_ACCESS ON
lntue Jun 15, 2025
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16 changes: 16 additions & 0 deletions libc/src/__support/math/expf.h
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Original file line number Diff line number Diff line change
Expand Up @@ -109,6 +109,22 @@ static constexpr float expf(float x) {
return static_cast<float>(exp_hi * exp_mid * exp_lo);
}

#pragma STDC FENV_ACCESS ON

// Directional rounding version of expf.
LIBC_INLINE static float expf(float x, int rounding_mode) {
int current_rounding_mode = fputil::get_round();
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Use the pragma to enable fenv_access?

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Done.

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I would still be more comfortable if you just add an exp implementation with a template argument for the rounding mode, and didn't use fenv access anywhere in the stack. This has just merely shuffled the problem into the libc code directly

if (rounding_mode == current_rounding_mode)
return expf(x);

fputil::set_round(rounding_mode);
float result = expf(x);
fputil::set_round(current_rounding_mode);
return result;
}

#pragma STDC FENV_ACCESS DEFAULT

} // namespace math

} // namespace LIBC_NAMESPACE_DECL
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11 changes: 11 additions & 0 deletions llvm/CMakeLists.txt
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Expand Up @@ -644,6 +644,17 @@ endif()

set(LLVM_ENABLE_Z3_SOLVER_DEFAULT "${Z3_FOUND}")

set(LLVM_INTEGRATE_LIBC "OFF" CACHE STRING "Use LLVM libc code 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} )
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10 changes: 10 additions & 0 deletions llvm/include/llvm/ADT/APFloat.h
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Expand Up @@ -1524,6 +1524,10 @@ class APFloat : public APFloatBase {
friend APFloat frexp(const APFloat &X, int &Exp, roundingMode RM);
friend IEEEFloat;
friend DoubleAPFloat;

#ifdef LLVM_INTEGRATE_LIBC
friend APFloat exp(const APFloat &X, roundingMode RM);
#endif // LLVM_INTEGRATE_LIBC
};

static_assert(sizeof(APFloat) == sizeof(detail::IEEEFloat),
Expand Down Expand Up @@ -1657,6 +1661,12 @@ inline APFloat maximumnum(const APFloat &A, const APFloat &B) {
return A < B ? B : A;
}

#ifdef LLVM_INTEGRATE_LIBC
/// Implement IEEE 754-2019 exp functions.
LLVM_READONLY
APFloat exp(const APFloat &X, RoundingMode RM = APFloat::rmNearestTiesToEven);
#endif // LLVM_INTEGRATE_LIBC

inline raw_ostream &operator<<(raw_ostream &OS, const APFloat &V) {
V.print(OS);
return OS;
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3 changes: 3 additions & 0 deletions llvm/include/llvm/Config/llvm-config.h.cmake
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Expand Up @@ -137,4 +137,7 @@
coverage bugs, and to 0 otherwise. */
#cmakedefine01 LLVM_ENABLE_DEBUGLOC_ORIGIN_TRACKING

/* Define if LLVM and clang uses LLVM libc for math computations. */
#cmakedefine LLVM_INTEGRATE_LIBC

#endif
32 changes: 32 additions & 0 deletions llvm/lib/Support/APFloat.cpp
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Expand Up @@ -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())) \
Expand Down Expand Up @@ -5601,6 +5606,33 @@ float APFloat::convertToFloat() const {
return Temp.getIEEE().convertToFloat();
}

#ifdef LLVM_INTEGRATE_LIBC
static constexpr int getFEnvRoundingMode(llvm::RoundingMode rm) {
switch (rm) {
case APFloat::rmTowardPositive:
return FE_UPWARD;
case APFloat::rmTowardNegative:
return FE_DOWNWARD;
case APFloat::rmTowardZero:
return FE_TOWARDZERO;
default:
// TODO: fix rmNearestTiesToAway for platform without FE_TONEARESTFROMZERO.
return FE_TONEAREST;
};
}

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) {
float result = LIBC_NAMESPACE::shared::expf(
X.convertToFloat(), getFEnvRoundingMode(rounding_mode));
return APFloat(result);
}
llvm_unreachable("Unexpected semantics");
}
#endif // LLVM_INTEGRATE_LIBC

} // namespace llvm

#undef APFLOAT_DISPATCH_ON_SEMANTICS
6 changes: 6 additions & 0 deletions llvm/lib/Support/CMakeLists.txt
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Expand Up @@ -379,3 +379,9 @@ if(LLVM_WITH_Z3)
${Z3_INCLUDE_DIR}
)
endif()

if(LLVM_INTEGRATE_LIBC)
set_property(TARGET LLVMSupport PROPERTY CXX_STANDARD 17)
target_include_directories(LLVMSupport PRIVATE "${LLVM_INCLUDE_DIR}/../../libc")
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I tested this build locally, and somehow the way this was declared caused a fatal include error trying to build APFloat.cpp, complaining that it couldn't find shared/math.h

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It's ugly but maybe we can have these return std::optional and just fail out if the support wasn't built?

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I'd expect that this would become a required dependency before we can actually start using it somewhere.

target_compile_options(LLVMSupport PRIVATE "-Wno-c99-extensions") # _Complex warnings.
endif()
2 changes: 1 addition & 1 deletion llvm/lib/Target/AMDGPU/AMDGPULibCalls.cpp
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Expand Up @@ -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:
Expand Down
77 changes: 77 additions & 0 deletions llvm/unittests/ADT/APFloatTest.cpp
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Expand Up @@ -16,6 +16,7 @@
#include "llvm/Support/FormatVariadic.h"
#include "gtest/gtest.h"
#include <cmath>
#include <limits>
#include <ostream>
#include <string>
#include <tuple>
Expand Down Expand Up @@ -8356,4 +8357,80 @@ TEST(APFloatTest, hasSignBitInMSB) {
EXPECT_FALSE(APFloat::hasSignBitInMSB(APFloat::Float8E8M0FNU()));
}

#ifdef LLVM_INTEGRATE_LIBC
TEST(APFloatTest, expf) {
std::array<llvm::RoundingMode, 4> allRoundingModes = {
APFloat::rmNearestTiesToEven, APFloat::rmTowardPositive,
APFloat::rmTowardNegative, APFloat::rmTowardZero};
for (auto rm : allRoundingModes) {
// exp(+-0) = 1 for all rounding modes.
EXPECT_EQ(1.0f, llvm::exp(APFloat(0.0f), rm).convertToFloat());
EXPECT_EQ(1.0f, llvm::exp(APFloat(-0.0f), rm).convertToFloat());
// exp(+Inf) = +Inf for all rounding modes.
EXPECT_EQ(std::numeric_limits<float>::infinity(),
llvm::exp(APFloat::getInf(APFloat::IEEEsingle(), false), rm)
.convertToFloat());
// exp(-Inf) = 0 for all rounding modes.
EXPECT_EQ(0.0f, llvm::exp(APFloat::getInf(APFloat::IEEEsingle(), true), rm)
.convertToFloat());
// exp(NaN) = NaN for all rounding modes.
EXPECT_TRUE(llvm::exp(APFloat::getNaN(APFloat::IEEEsingle()), rm).isNaN());
}
// exp(1)
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());
// exp(float max)
EXPECT_EQ(std::numeric_limits<float>::infinity(),
llvm::exp(APFloat::getLargest(APFloat::IEEEsingle(), false),
APFloat::rmNearestTiesToEven)
.convertToFloat());
EXPECT_EQ(std::numeric_limits<float>::infinity(),
llvm::exp(APFloat::getLargest(APFloat::IEEEsingle(), false),
APFloat::rmTowardPositive)
.convertToFloat());
EXPECT_EQ(std::numeric_limits<float>::max(),
llvm::exp(APFloat::getLargest(APFloat::IEEEsingle(), false),
APFloat::rmTowardNegative)
.convertToFloat());
EXPECT_EQ(std::numeric_limits<float>::max(),
llvm::exp(APFloat::getLargest(APFloat::IEEEsingle(), false),
APFloat::rmTowardZero)
.convertToFloat());
// exp(min_denormal)
EXPECT_EQ(1.0f, llvm::exp(APFloat::getSmallest(APFloat::IEEEsingle(), false),
APFloat::rmNearestTiesToEven)
.convertToFloat());
EXPECT_EQ(0x1.000002p0f,
llvm::exp(APFloat::getSmallest(APFloat::IEEEsingle(), false),
APFloat::rmTowardPositive)
.convertToFloat());
EXPECT_EQ(1.0f, llvm::exp(APFloat::getSmallest(APFloat::IEEEsingle(), false),
APFloat::rmTowardNegative)
.convertToFloat());
EXPECT_EQ(1.0f, llvm::exp(APFloat::getSmallest(APFloat::IEEEsingle(), false),
APFloat::rmTowardZero)
.convertToFloat());
// Default rounding mode.
// exp(-1)
EXPECT_EQ(0x1.78b564p-2f, llvm::exp(APFloat(-1.0f)).convertToFloat());
EXPECT_EQ(
0x1.78b564p-2f,
llvm::exp(APFloat(-1.0f), APFloat::rmTowardPositive).convertToFloat());
EXPECT_EQ(
0x1.78b562p-2f,
llvm::exp(APFloat(-1.0f), APFloat::rmTowardNegative).convertToFloat());
EXPECT_EQ(0x1.78b562p-2f,
llvm::exp(APFloat(-1.0f), APFloat::rmTowardZero).convertToFloat());
}
#endif // LLVM_INTEGRATE_LIBC

} // namespace
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