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[FFI] Fix JSON parser/writer for the fast-math flag #18221

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Merged
tqchen merged 1 commit into from
Aug 22, 2025
Merged

[FFI] Fix JSON parser/writer for the fast-math flag #18221

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24 changes: 18 additions & 6 deletions ffi/src/ffi/extra/json_parser.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -298,26 +298,38 @@ class JSONParserContext {
private:
static double FastMathSafePosInf() {
#ifdef __FAST_MATH__
const uint64_t inf_bits = 0x7FF0000000000000ULL;
return *reinterpret_cast<const double*>(&inf_bits);
union {
uint64_t from;
double to;
} u;
u.from = 0x7FF0000000000000ULL; // write "from", read "to"
return u.to;
#else
return std::numeric_limits<double>::infinity();
#endif
}

static double FastMathSafeNegInf() {
#ifdef __FAST_MATH__
const uint64_t inf_bits = 0xFFF0000000000000ULL;
return *reinterpret_cast<const double*>(&inf_bits);
union {
uint64_t from;
double to;
} u;
u.from = 0xFFF0000000000000ULL; // write "from", read "to"
return u.to;
#else
return -std::numeric_limits<double>::infinity();
#endif
}

static double FastMathSafeNaN() {
#ifdef __FAST_MATH__
const uint64_t nan_bits = 0x7FF8000000000000ULL;
return *reinterpret_cast<const double*>(&nan_bits);
union {
uint64_t from;
double to;
} u;
u.from = 0x7FF8000000000000ULL; // write "from", read "to"
return u.to;
#else
return std::numeric_limits<double>::quiet_NaN();
#endif
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14 changes: 12 additions & 2 deletions ffi/src/ffi/extra/json_writer.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -66,7 +66,12 @@ class JSONWriter {
// IEEE 754 standard: https://en.wikipedia.org/wiki/IEEE_754
// NaN is encoded as all 1s in the exponent and non-zero in the mantissa
static_assert(sizeof(double) == sizeof(uint64_t), "Unexpected double size");
uint64_t bits = *reinterpret_cast<const uint64_t*>(&x);
union {
double from;
uint64_t to;
} u;
u.from = x; // write "from", read "to"
uint64_t bits = u.to;
uint64_t exponent = (bits >> 52) & 0x7FF;
uint64_t mantissa = bits & 0xFFFFFFFFFFFFFull;
return (exponent == 0x7FF) && (mantissa != 0);
Expand All @@ -81,7 +86,12 @@ class JSONWriter {
// IEEE 754 standard: https://en.wikipedia.org/wiki/IEEE_754
// Inf is encoded as all 1s in the exponent and zero in the mantissa
static_assert(sizeof(double) == sizeof(uint64_t), "Unexpected double size");
uint64_t bits = *reinterpret_cast<const uint64_t*>(&x);
union {
double from;
uint64_t to;
} u;
u.from = x; // write "from", read "to"
uint64_t bits = u.to;
uint64_t exponent = (bits >> 52) & 0x7FF;
uint64_t mantissa = bits & 0xFFFFFFFFFFFFFull;
// inf is encoded as all 1s in the exponent and zero in the mantissa
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