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diff --git a/lib/std/special/compiler_rt/mulXf3.zig b/lib/std/special/compiler_rt/mulXf3.zig
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+// Ported from:
+//
+// https://github.com/llvm/llvm-project/blob/2ffb1b0413efa9a24eb3c49e710e36f92e2cb50b/compiler-rt/lib/builtins/fp_mul_impl.inc
+
+const std = @import("std");
+const builtin = @import("builtin");
+const compiler_rt = @import("../compiler_rt.zig");
+
+pub extern fn __multf3(a: f128, b: f128) f128 {
+ return mulXf3(f128, a, b);
+}
+pub extern fn __muldf3(a: f64, b: f64) f64 {
+ return mulXf3(f64, a, b);
+}
+pub extern fn __mulsf3(a: f32, b: f32) f32 {
+ return mulXf3(f32, a, b);
+}
+
+fn mulXf3(comptime T: type, a: T, b: T) T {
+ @setRuntimeSafety(builtin.is_test);
+ const Z = @IntType(false, T.bit_count);
+
+ const typeWidth = T.bit_count;
+ const significandBits = std.math.floatMantissaBits(T);
+ const exponentBits = std.math.floatExponentBits(T);
+
+ const signBit = (Z(1) << (significandBits + exponentBits));
+ const maxExponent = ((1 << exponentBits) - 1);
+ const exponentBias = (maxExponent >> 1);
+
+ const implicitBit = (Z(1) << significandBits);
+ const quietBit = implicitBit >> 1;
+ const significandMask = implicitBit - 1;
+
+ const absMask = signBit - 1;
+ const exponentMask = absMask ^ significandMask;
+ const qnanRep = exponentMask | quietBit;
+ const infRep = @bitCast(Z, std.math.inf(T));
+
+ const aExponent = @truncate(u32, (@bitCast(Z, a) >> significandBits) & maxExponent);
+ const bExponent = @truncate(u32, (@bitCast(Z, b) >> significandBits) & maxExponent);
+ const productSign: Z = (@bitCast(Z, a) ^ @bitCast(Z, b)) & signBit;
+
+ var aSignificand: Z = @bitCast(Z, a) & significandMask;
+ var bSignificand: Z = @bitCast(Z, b) & significandMask;
+ var scale: i32 = 0;
+
+ // Detect if a or b is zero, denormal, infinity, or NaN.
+ if (aExponent -% 1 >= maxExponent -% 1 or bExponent -% 1 >= maxExponent -% 1) {
+ const aAbs: Z = @bitCast(Z, a) & absMask;
+ const bAbs: Z = @bitCast(Z, b) & absMask;
+
+ // NaN * anything = qNaN
+ if (aAbs > infRep) return @bitCast(T, @bitCast(Z, a) | quietBit);
+ // anything * NaN = qNaN
+ if (bAbs > infRep) return @bitCast(T, @bitCast(Z, b) | quietBit);
+
+ if (aAbs == infRep) {
+ // infinity * non-zero = +/- infinity
+ if (bAbs != 0) {
+ return @bitCast(T, aAbs | productSign);
+ } else {
+ // infinity * zero = NaN
+ return @bitCast(T, qnanRep);
+ }
+ }
+
+ if (bAbs == infRep) {
+ //? non-zero * infinity = +/- infinity
+ if (aAbs != 0) {
+ return @bitCast(T, bAbs | productSign);
+ } else {
+ // zero * infinity = NaN
+ return @bitCast(T, qnanRep);
+ }
+ }
+
+ // zero * anything = +/- zero
+ if (aAbs == 0) return @bitCast(T, productSign);
+ // anything * zero = +/- zero
+ if (bAbs == 0) return @bitCast(T, productSign);
+
+ // one or both of a or b is denormal, the other (if applicable) is a
+ // normal number. Renormalize one or both of a and b, and set scale to
+ // include the necessary exponent adjustment.
+ if (aAbs < implicitBit) scale +%= normalize(T, &aSignificand);
+ if (bAbs < implicitBit) scale +%= normalize(T, &bSignificand);
+ }
+
+ // Or in the implicit significand bit. (If we fell through from the
+ // denormal path it was already set by normalize( ), but setting it twice
+ // won't hurt anything.)
+ aSignificand |= implicitBit;
+ bSignificand |= implicitBit;
+
+ // Get the significand of a*b. Before multiplying the significands, shift
+ // one of them left to left-align it in the field. Thus, the product will
+ // have (exponentBits + 2) integral digits, all but two of which must be
+ // zero. Normalizing this result is just a conditional left-shift by one
+ // and bumping the exponent accordingly.
+ var productHi: Z = undefined;
+ var productLo: Z = undefined;
+ wideMultiply(Z, aSignificand, bSignificand << exponentBits, &productHi, &productLo);
+
+ var productExponent: i32 = @bitCast(i32, aExponent +% bExponent) -% exponentBias +% scale;
+
+ // Normalize the significand, adjust exponent if needed.
+ if ((productHi & implicitBit) != 0) {
+ productExponent +%= 1;
+ } else {
+ productHi = (productHi << 1) | (productLo >> (typeWidth - 1));
+ productLo = productLo << 1;
+ }
+
+ // If we have overflowed the type, return +/- infinity.
+ if (productExponent >= maxExponent) return @bitCast(T, infRep | productSign);
+
+ if (productExponent <= 0) {
+ // Result is denormal before rounding
+ //
+ // If the result is so small that it just underflows to zero, return
+ // a zero of the appropriate sign. Mathematically there is no need to
+ // handle this case separately, but we make it a special case to
+ // simplify the shift logic.
+ const shift: u32 = @truncate(u32, Z(1) -% @bitCast(u32, productExponent));
+ if (shift >= typeWidth) return @bitCast(T, productSign);
+
+ // Otherwise, shift the significand of the result so that the round
+ // bit is the high bit of productLo.
+ wideRightShiftWithSticky(Z, &productHi, &productLo, shift);
+ } else {
+ // Result is normal before rounding; insert the exponent.
+ productHi &= significandMask;
+ productHi |= Z(@bitCast(u32, productExponent)) << significandBits;
+ }
+
+ // Insert the sign of the result:
+ productHi |= productSign;
+
+ // Final rounding. The final result may overflow to infinity, or underflow
+ // to zero, but those are the correct results in those cases. We use the
+ // default IEEE-754 round-to-nearest, ties-to-even rounding mode.
+ if (productLo > signBit) productHi +%= 1;
+ if (productLo == signBit) productHi +%= productHi & 1;
+ return @bitCast(T, productHi);
+}
+
+fn wideMultiply(comptime Z: type, a: Z, b: Z, hi: *Z, lo: *Z) void {
+ @setRuntimeSafety(builtin.is_test);
+ switch (Z) {
+ u32 => {
+ // 32x32 --> 64 bit multiply
+ const product = u64(a) * u64(b);
+ hi.* = @truncate(u32, product >> 32);
+ lo.* = @truncate(u32, product);
+ },
+ u64 => {
+ const S = struct {
+ fn loWord(x: u64) u64 {
+ return @truncate(u32, x);
+ }
+ fn hiWord(x: u64) u64 {
+ return @truncate(u32, x >> 32);
+ }
+ };
+ // 64x64 -> 128 wide multiply for platforms that don't have such an operation;
+ // many 64-bit platforms have this operation, but they tend to have hardware
+ // floating-point, so we don't bother with a special case for them here.
+ // Each of the component 32x32 -> 64 products
+ const plolo: u64 = S.loWord(a) * S.loWord(b);
+ const plohi: u64 = S.loWord(a) * S.hiWord(b);
+ const philo: u64 = S.hiWord(a) * S.loWord(b);
+ const phihi: u64 = S.hiWord(a) * S.hiWord(b);
+ // Sum terms that contribute to lo in a way that allows us to get the carry
+ const r0: u64 = S.loWord(plolo);
+ const r1: u64 = S.hiWord(plolo) +% S.loWord(plohi) +% S.loWord(philo);
+ lo.* = r0 +% (r1 << 32);
+ // Sum terms contributing to hi with the carry from lo
+ hi.* = S.hiWord(plohi) +% S.hiWord(philo) +% S.hiWord(r1) +% phihi;
+ },
+ u128 => {
+ const Word_LoMask = u64(0x00000000ffffffff);
+ const Word_HiMask = u64(0xffffffff00000000);
+ const Word_FullMask = u64(0xffffffffffffffff);
+ const S = struct {
+ fn Word_1(x: u128) u64 {
+ return @truncate(u32, x >> 96);
+ }
+ fn Word_2(x: u128) u64 {
+ return @truncate(u32, x >> 64);
+ }
+ fn Word_3(x: u128) u64 {
+ return @truncate(u32, x >> 32);
+ }
+ fn Word_4(x: u128) u64 {
+ return @truncate(u32, x);
+ }
+ };
+ // 128x128 -> 256 wide multiply for platforms that don't have such an operation;
+ // many 64-bit platforms have this operation, but they tend to have hardware
+ // floating-point, so we don't bother with a special case for them here.
+
+ const product11: u64 = S.Word_1(a) * S.Word_1(b);
+ const product12: u64 = S.Word_1(a) * S.Word_2(b);
+ const product13: u64 = S.Word_1(a) * S.Word_3(b);
+ const product14: u64 = S.Word_1(a) * S.Word_4(b);
+ const product21: u64 = S.Word_2(a) * S.Word_1(b);
+ const product22: u64 = S.Word_2(a) * S.Word_2(b);
+ const product23: u64 = S.Word_2(a) * S.Word_3(b);
+ const product24: u64 = S.Word_2(a) * S.Word_4(b);
+ const product31: u64 = S.Word_3(a) * S.Word_1(b);
+ const product32: u64 = S.Word_3(a) * S.Word_2(b);
+ const product33: u64 = S.Word_3(a) * S.Word_3(b);
+ const product34: u64 = S.Word_3(a) * S.Word_4(b);
+ const product41: u64 = S.Word_4(a) * S.Word_1(b);
+ const product42: u64 = S.Word_4(a) * S.Word_2(b);
+ const product43: u64 = S.Word_4(a) * S.Word_3(b);
+ const product44: u64 = S.Word_4(a) * S.Word_4(b);
+
+ const sum0: u128 = u128(product44);
+ const sum1: u128 = u128(product34) +%
+ u128(product43);
+ const sum2: u128 = u128(product24) +%
+ u128(product33) +%
+ u128(product42);
+ const sum3: u128 = u128(product14) +%
+ u128(product23) +%
+ u128(product32) +%
+ u128(product41);
+ const sum4: u128 = u128(product13) +%
+ u128(product22) +%
+ u128(product31);
+ const sum5: u128 = u128(product12) +%
+ u128(product21);
+ const sum6: u128 = u128(product11);
+
+ const r0: u128 = (sum0 & Word_FullMask) +%
+ ((sum1 & Word_LoMask) << 32);
+ const r1: u128 = (sum0 >> 64) +%
+ ((sum1 >> 32) & Word_FullMask) +%
+ (sum2 & Word_FullMask) +%
+ ((sum3 << 32) & Word_HiMask);
+
+ lo.* = r0 +% (r1 << 64);
+ hi.* = (r1 >> 64) +%
+ (sum1 >> 96) +%
+ (sum2 >> 64) +%
+ (sum3 >> 32) +%
+ sum4 +%
+ (sum5 << 32) +%
+ (sum6 << 64);
+ },
+ else => @compileError("unsupported"),
+ }
+}
+
+fn normalize(comptime T: type, significand: *@IntType(false, T.bit_count)) i32 {
+ @setRuntimeSafety(builtin.is_test);
+ const Z = @IntType(false, T.bit_count);
+ const significandBits = std.math.floatMantissaBits(T);
+ const implicitBit = Z(1) << significandBits;
+
+ const shift = @clz(Z, significand.*) - @clz(Z, implicitBit);
+ significand.* <<= @intCast(std.math.Log2Int(Z), shift);
+ return 1 - shift;
+}
+
+fn wideRightShiftWithSticky(comptime Z: type, hi: *Z, lo: *Z, count: u32) void {
+ @setRuntimeSafety(builtin.is_test);
+ const typeWidth = Z.bit_count;
+ const S = std.math.Log2Int(Z);
+ if (count < typeWidth) {
+ const sticky = @truncate(u8, lo.* << @intCast(S, typeWidth -% count));
+ lo.* = (hi.* << @intCast(S, typeWidth -% count)) | (lo.* >> @intCast(S, count)) | sticky;
+ hi.* = hi.* >> @intCast(S, count);
+ } else if (count < 2 * typeWidth) {
+ const sticky = @truncate(u8, hi.* << @intCast(S, 2 * typeWidth -% count) | lo.*);
+ lo.* = hi.* >> @intCast(S, count -% typeWidth) | sticky;
+ hi.* = 0;
+ } else {
+ const sticky = @truncate(u8, hi.* | lo.*);
+ lo.* = sticky;
+ hi.* = 0;
+ }
+}
+
+test "import mulXf3" {
+ _ = @import("mulXf3_test.zig");
+}
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