1 files changed, 71 insertions, 0 deletions
diff --git a/lib/std/special/compiler_rt/floattidf.zig b/lib/std/special/compiler_rt/floattidf.zig
new file mode 100644
index 0000000000..42ef6df7e4
--- /dev/null
+++ b/lib/std/special/compiler_rt/floattidf.zig
@@ -0,0 +1,71 @@
+const builtin = @import("builtin");
+const is_test = builtin.is_test;
+const std = @import("std");
+const maxInt = std.math.maxInt;
+
+const DBL_MANT_DIG = 53;
+
+pub extern fn __floattidf(arg: i128) f64 {
+    @setRuntimeSafety(is_test);
+
+    if (arg == 0)
+        return 0.0;
+
+    var ai = arg;
+    const N: u32 = 128;
+    const si = ai >> @intCast(u7, (N - 1));
+    ai = ((ai ^ si) -% si);
+    var a = @bitCast(u128, ai);
+
+    const sd = @bitCast(i32, N - @clz(u128, a)); // number of significant digits
+    var e: i32 = sd - 1; // exponent
+    if (sd > DBL_MANT_DIG) {
+        //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
+        //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
+        //                                                12345678901234567890123456
+        //  1 = msb 1 bit
+        //  P = bit DBL_MANT_DIG-1 bits to the right of 1
+        //  Q = bit DBL_MANT_DIG bits to the right of 1
+        //  R = "or" of all bits to the right of Q
+        switch (sd) {
+            DBL_MANT_DIG + 1 => {
+                a <<= 1;
+            },
+            DBL_MANT_DIG + 2 => {},
+            else => {
+                const shift1_amt = @intCast(i32, sd - (DBL_MANT_DIG + 2));
+                const shift1_amt_u7 = @intCast(u7, shift1_amt);
+
+                const shift2_amt = @intCast(i32, N + (DBL_MANT_DIG + 2)) - sd;
+                const shift2_amt_u7 = @intCast(u7, shift2_amt);
+
+                a = (a >> shift1_amt_u7) | @boolToInt((a & (@intCast(u128, maxInt(u128)) >> shift2_amt_u7)) != 0);
+            },
+        }
+        // finish
+        a |= @boolToInt((a & 4) != 0); // Or P into R
+        a += 1; // round - this step may add a significant bit
+        a >>= 2; // dump Q and R
+        // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
+        if ((a & (u128(1) << DBL_MANT_DIG)) != 0) {
+            a >>= 1;
+            e += 1;
+        }
+        // a is now rounded to DBL_MANT_DIG bits
+    } else {
+        a <<= @intCast(u7, DBL_MANT_DIG - sd);
+        // a is now rounded to DBL_MANT_DIG bits
+    }
+
+    const s = @bitCast(u128, arg) >> (128 - 32);
+    const high: u64 = (@intCast(u64, s) & 0x80000000) | // sign
+        (@intCast(u32, (e + 1023)) << 20) | // exponent
+        (@truncate(u32, a >> 32) & 0x000fffff); // mantissa-high
+    const low: u64 = @truncate(u32, a); // mantissa-low
+
+    return @bitCast(f64, low | (high << 32));
+}
+
+test "import floattidf" {
+    _ = @import("floattidf_test.zig");
+}