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[libc][math] Fix signed zeros for erff. #97742

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merged 1 commit into from
Jul 5, 2024
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[libc][math] Fix signed zeros for erff. #97742

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24 changes: 15 additions & 9 deletions libc/src/math/generic/erff.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -127,15 +127,6 @@ LLVM_LIBC_FUNCTION(float, erff, (float x)) {
uint32_t x_u = xbits.uintval();
uint32_t x_abs = x_u & 0x7fff'ffffU;

// Exceptional values
if (LIBC_UNLIKELY(x_abs == 0x3f65'9229U)) // |x| = 0x1.cb2452p-1f
return x < 0.0f ? fputil::round_result_slightly_down(-0x1.972ea8p-1f)
: fputil::round_result_slightly_up(0x1.972ea8p-1f);
if (LIBC_UNLIKELY(x_abs == 0x4004'1e6aU)) // |x| = 0x1.083cd4p+1f
return x < 0.0f ? fputil::round_result_slightly_down(-0x1.fe3462p-1f)
: fputil::round_result_slightly_up(0x1.fe3462p-1f);

// if (LIBC_UNLIKELY(x_abs > 0x407a'd444U)) {
if (LIBC_UNLIKELY(x_abs >= 0x4080'0000U)) {
const float ONE[2] = {1.0f, -1.0f};
const float SMALL[2] = {-0x1.0p-25f, 0x1.0p-25f};
Expand All @@ -149,6 +140,21 @@ LLVM_LIBC_FUNCTION(float, erff, (float x)) {
return ONE[sign] + SMALL[sign];
}

// Exceptional mask = common 0 bits of 2 exceptional values.
constexpr uint32_t EXCEPT_MASK = 0x809a'6184U;

if (LIBC_UNLIKELY((x_abs & EXCEPT_MASK) == 0)) {
// Exceptional values
if (LIBC_UNLIKELY(x_abs == 0x3f65'9229U)) // |x| = 0x1.cb2452p-1f
return x < 0.0f ? fputil::round_result_slightly_down(-0x1.972ea8p-1f)
: fputil::round_result_slightly_up(0x1.972ea8p-1f);
if (LIBC_UNLIKELY(x_abs == 0x4004'1e6aU)) // |x| = 0x1.083cd4p+1f
return x < 0.0f ? fputil::round_result_slightly_down(-0x1.fe3462p-1f)
: fputil::round_result_slightly_up(0x1.fe3462p-1f);
if (x_abs == 0U)
return x;
}

// Polynomial approximation:
// erf(x) ~ x * (c0 + c1 * x^2 + c2 * x^4 + ... + c7 * x^14)
double xd = static_cast<double>(x);
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