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Fix JS number rounding on x87 #1244

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Merged
merged 2 commits into from
Nov 21, 2025
Merged

Fix JS number rounding on x87 #1244

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7511dbc
Fix JS number rounding on x87
bnoordhuis Nov 20, 2025
6b58370
squash! more save/restore points
bnoordhuis Nov 21, 2025
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4 changes: 2 additions & 2 deletions .github/workflows/ci.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -58,7 +58,7 @@ jobs:
- { os: ubuntu-latest, configType: asan+ubsan, runTest262: true }
- { os: ubuntu-latest, configType: msan }
- { os: ubuntu-latest, configType: tcc }
- { os: ubuntu-latest, arch: x86 }
- { os: ubuntu-latest, arch: x86, runTest262: true }
- { os: ubuntu-latest, arch: riscv64 }
- { os: ubuntu-latest, arch: s390x }

Expand Down Expand Up @@ -97,7 +97,7 @@ jobs:
uses: jirutka/setup-alpine@v1
with:
arch: ${{ matrix.config.arch }}
packages: "build-base make cmake"
packages: "build-base make cmake git"

- name: uname
run: uname -a
Expand Down
19 changes: 19 additions & 0 deletions cutils.h
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Expand Up @@ -634,6 +634,25 @@ int js_thread_join(js_thread_t thrd);

#endif /* !defined(EMSCRIPTEN) && !defined(__wasi__) */

// JS requires strict rounding behavior. Turn on 64-bits double precision
// and disable x87 80-bits extended precision for intermediate floating-point
// results. 0x300 is extended precision, 0x200 is double precision.
// Note that `*&cw` in the asm constraints looks redundant but isn't.
#if defined(__i386__) && !defined(_MSC_VER)
#define JS_X87_FPCW_SAVE_AND_ADJUST(cw) \
unsigned short cw; \
__asm__ __volatile__("fnstcw %0" : "=m"(*&cw)); \
do { \
unsigned short t = 0x200 | (cw & ~0x300); \
__asm__ __volatile__("fldcw %0" : /*empty*/ : "m"(*&t)); \
} while (0)
#define JS_X87_FPCW_RESTORE(cw) \
__asm__ __volatile__("fldcw %0" : /*empty*/ : "m"(*&cw))
#else
#define JS_X87_FPCW_SAVE_AND_ADJUST(cw)
#define JS_X87_FPCW_RESTORE(cw)
#endif

#ifdef __cplusplus
} /* extern "C" { */
#endif
Expand Down
34 changes: 29 additions & 5 deletions quickjs.c
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Expand Up @@ -13395,12 +13395,17 @@ static int js_is_array(JSContext *ctx, JSValueConst val)

static double js_math_pow(double a, double b)
{
double d;

if (unlikely(!isfinite(b)) && fabs(a) == 1) {
/* not compatible with IEEE 754 */
return NAN;
d = NAN;
} else {
return pow(a, b);
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
d = pow(a, b);
JS_X87_FPCW_RESTORE(fpcw);
}
return d;
}

JSValue JS_NewBigInt64(JSContext *ctx, int64_t v)
Expand Down Expand Up @@ -13820,11 +13825,15 @@ static no_inline __exception int js_binary_arith_slow(JSContext *ctx, JSValue *s
}
break;
case OP_div:
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
sp[-2] = js_number((double)v1 / (double)v2);
JS_X87_FPCW_RESTORE(fpcw);
return 0;
case OP_mod:
if (v1 < 0 || v2 <= 0) {
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
sp[-2] = js_number(fmod(v1, v2));
JS_X87_FPCW_RESTORE(fpcw);
return 0;
} else {
v = (int64_t)v1 % (int64_t)v2;
Expand Down Expand Up @@ -13888,6 +13897,7 @@ static no_inline __exception int js_binary_arith_slow(JSContext *ctx, JSValue *s
if (JS_ToFloat64Free(ctx, &d2, op2))
goto exception;
handle_float64:
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
switch(op) {
case OP_sub:
dr = d1 - d2;
Expand All @@ -13907,6 +13917,7 @@ static no_inline __exception int js_binary_arith_slow(JSContext *ctx, JSValue *s
default:
abort();
}
JS_X87_FPCW_RESTORE(fpcw);
sp[-2] = js_float64(dr);
}
return 0;
Expand Down Expand Up @@ -14023,7 +14034,9 @@ static no_inline __exception int js_add_slow(JSContext *ctx, JSValue *sp)
}
if (JS_ToFloat64Free(ctx, &d2, op2))
goto exception;
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
sp[-2] = js_float64(d1 + d2);
JS_X87_FPCW_RESTORE(fpcw);
}
return 0;
exception:
Expand Down Expand Up @@ -17998,8 +18011,10 @@ static JSValue JS_CallInternal(JSContext *caller_ctx, JSValueConst func_obj,
sp[-2] = js_int32(r);
sp--;
} else if (JS_VALUE_IS_BOTH_FLOAT(op1, op2)) {
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
sp[-2] = js_float64(JS_VALUE_GET_FLOAT64(op1) +
JS_VALUE_GET_FLOAT64(op2));
JS_X87_FPCW_RESTORE(fpcw);
sp--;
} else {
add_slow:
Expand Down Expand Up @@ -18066,8 +18081,10 @@ static JSValue JS_CallInternal(JSContext *caller_ctx, JSValueConst func_obj,
sp[-2] = js_int32(r);
sp--;
} else if (JS_VALUE_IS_BOTH_FLOAT(op1, op2)) {
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
sp[-2] = js_float64(JS_VALUE_GET_FLOAT64(op1) -
JS_VALUE_GET_FLOAT64(op2));
JS_X87_FPCW_RESTORE(fpcw);
sp--;
} else {
goto binary_arith_slow;
Expand Down Expand Up @@ -18098,7 +18115,9 @@ static JSValue JS_CallInternal(JSContext *caller_ctx, JSValueConst func_obj,
sp[-2] = js_int32(r);
sp--;
} else if (JS_VALUE_IS_BOTH_FLOAT(op1, op2)) {
JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
d = JS_VALUE_GET_FLOAT64(op1) * JS_VALUE_GET_FLOAT64(op2);
JS_X87_FPCW_RESTORE(fpcw);
mul_fp_res:
sp[-2] = js_float64(d);
sp--;
Expand Down Expand Up @@ -52277,7 +52296,9 @@ static double set_date_fields(double fields[minimum_length(7)], int is_local) {
int yi, mi, i;
int64_t days;
volatile double temp; /* enforce evaluation order */
double d = NAN;

JS_X87_FPCW_SAVE_AND_ADJUST(fpcw);
/* emulate 21.4.1.15 MakeDay ( year, month, date ) */
y = fields[0];
m = fields[1];
Expand All @@ -52287,7 +52308,7 @@ static double set_date_fields(double fields[minimum_length(7)], int is_local) {
if (mn < 0)
mn += 12;
if (ym < -271821 || ym > 275760)
return NAN;
goto out;

yi = ym;
mi = mn;
Expand Down Expand Up @@ -52319,14 +52340,17 @@ static double set_date_fields(double fields[minimum_length(7)], int is_local) {
/* emulate 21.4.1.16 MakeDate ( day, time ) */
tv = (temp = day * 86400000) + time; /* prevent generation of FMA */
if (!isfinite(tv))
return NAN;
goto out;

/* adjust for local time and clip */
if (is_local) {
int64_t ti = tv < INT64_MIN ? INT64_MIN : tv >= 0x1p63 ? INT64_MAX : (int64_t)tv;
tv += getTimezoneOffset(ti) * 60000;
}
return time_clip(tv);
d = time_clip(tv);
out:
JS_X87_FPCW_RESTORE(fpcw);
return d;
}

static JSValue get_date_field(JSContext *ctx, JSValueConst this_val,
Expand Down