diff --git a/lib/std/special/compiler_rt.zig b/lib/std/special/compiler_rt.zig
index 375de6fced..0f5d6c4d23 100644
--- a/lib/std/special/compiler_rt.zig
+++ b/lib/std/special/compiler_rt.zig
@@ -726,6 +726,11 @@ comptime {
 
     if (!is_test) {
         @export(fmodl, .{ .name = "fmodl", .linkage = linkage });
+        if (long_double_is_f80) {
+            @export(fmodl, .{ .name = "fmodx", .linkage = linkage });
+        } else {
+            @export(fmodx, .{ .name = "fmodx", .linkage = linkage });
+        }
         if (long_double_is_f128) {
             @export(fmodl, .{ .name = "fmodq", .linkage = linkage });
         } else {
@@ -884,13 +889,8 @@ fn ceill(x: c_longdouble) callconv(.C) c_longdouble {
     return math.ceil(x);
 }
 
-const fmodq = @import("compiler_rt/floatfmodq.zig").fmodq;
-fn fmodl(x: c_longdouble, y: c_longdouble) callconv(.C) c_longdouble {
-    if (!long_double_is_f128) {
-        @panic("TODO implement this");
-    }
-    return @floatCast(c_longdouble, fmodq(x, y));
-}
+const fmodq = @import("compiler_rt/fmodq.zig").fmodq;
+const fmodx = @import("compiler_rt/fmodx.zig").fmodx;
 
 // Avoid dragging in the runtime safety mechanisms into this .o file,
 // unless we're trying to test this file.
diff --git a/lib/std/special/compiler_rt/divdf3.zig b/lib/std/special/compiler_rt/divdf3.zig
index 2148902de2..137f5c02f9 100644
--- a/lib/std/special/compiler_rt/divdf3.zig
+++ b/lib/std/special/compiler_rt/divdf3.zig
@@ -314,12 +314,11 @@ pub fn wideMultiply(comptime Z: type, a: Z, b: Z, hi: *Z, lo: *Z) void {
 pub fn normalize(comptime T: type, significand: *std.meta.Int(.unsigned, @typeInfo(T).Float.bits)) i32 {
     @setRuntimeSafety(builtin.is_test);
     const Z = std.meta.Int(.unsigned, @typeInfo(T).Float.bits);
-    const significandBits = std.math.floatMantissaBits(T);
-    const implicitBit = @as(Z, 1) << significandBits;
+    const integerBit = @as(Z, 1) << std.math.floatFractionalBits(T);
 
-    const shift = @clz(Z, significand.*) - @clz(Z, implicitBit);
+    const shift = @clz(Z, significand.*) - @clz(Z, integerBit);
     significand.* <<= @intCast(std.math.Log2Int(Z), shift);
-    return 1 - shift;
+    return @as(i32, 1) - shift;
 }
 
 pub fn __aeabi_ddiv(a: f64, b: f64) callconv(.AAPCS) f64 {
diff --git a/lib/std/special/compiler_rt/floatfmodq.zig b/lib/std/special/compiler_rt/fmodq.zig
similarity index 93%
rename from lib/std/special/compiler_rt/floatfmodq.zig
rename to lib/std/special/compiler_rt/fmodq.zig
index b8da727c90..3f56c49796 100644
--- a/lib/std/special/compiler_rt/floatfmodq.zig
+++ b/lib/std/special/compiler_rt/fmodq.zig
@@ -27,7 +27,7 @@ pub fn fmodq(a: f128, b: f128) callconv(.C) f128 {
 
     const signA = aPtr_u16[exp_and_sign_index] & 0x8000;
     var expA = @intCast(i32, (aPtr_u16[exp_and_sign_index] & 0x7fff));
-    var expB = bPtr_u16[exp_and_sign_index] & 0x7fff;
+    var expB = @intCast(i32, (bPtr_u16[exp_and_sign_index] & 0x7fff));
 
     // There are 3 cases where the answer is undefined, check for:
     //   - fmodq(val, 0)
@@ -55,12 +55,12 @@ pub fn fmodq(a: f128, b: f128) callconv(.C) f128 {
 
     if (expA == 0) {
         amod *= 0x1p120;
-        expA = aPtr_u16[exp_and_sign_index] -% 120;
+        expA = @as(i32, aPtr_u16[exp_and_sign_index]) - 120;
     }
 
     if (expB == 0) {
         bmod *= 0x1p120;
-        expB = bPtr_u16[exp_and_sign_index] -% 120;
+        expB = @as(i32, bPtr_u16[exp_and_sign_index]) - 120;
     }
 
     // OR in extra non-stored mantissa digit
@@ -73,7 +73,7 @@ pub fn fmodq(a: f128, b: f128) callconv(.C) f128 {
         var high = highA -% highB;
         var low = lowA -% lowB;
         if (lowA < lowB) {
-            high = highA -% 1;
+            high -%= 1;
         }
         if (high >> 63 == 0) {
             if ((high | low) == 0) {
@@ -122,5 +122,5 @@ pub fn fmodq(a: f128, b: f128) callconv(.C) f128 {
 }
 
 test {
-    _ = @import("floatfmodq_test.zig");
+    _ = @import("fmodq_test.zig");
 }
diff --git a/lib/std/special/compiler_rt/floatfmodq_test.zig b/lib/std/special/compiler_rt/fmodq_test.zig
similarity index 67%
rename from lib/std/special/compiler_rt/floatfmodq_test.zig
rename to lib/std/special/compiler_rt/fmodq_test.zig
index a272b797e3..b8baf8ae9b 100644
--- a/lib/std/special/compiler_rt/floatfmodq_test.zig
+++ b/lib/std/special/compiler_rt/fmodq_test.zig
@@ -1,5 +1,5 @@
 const std = @import("std");
-const fmodq = @import("floatfmodq.zig");
+const fmodq = @import("fmodq.zig");
 const testing = std.testing;
 
 fn test_fmodq(a: f128, b: f128, exp: f128) !void {
@@ -34,12 +34,18 @@ test "fmodq" {
     try test_fmodq(-0.0, 1.0, -0.0);
     try test_fmodq(7046119.0, 5558362.0, 1487757.0);
     try test_fmodq(9010357.0, 1957236.0, 1181413.0);
+    try test_fmodq(5192296858534827628530496329220095, 10.0, 5.0);
+    try test_fmodq(5192296858534827628530496329220095, 922337203681230954775807, 220474884073715748246157);
 
     // Denormals
-    const a: f128 = 0xedcb34a235253948765432134674p-16494;
-    const b: f128 = 0x5d2e38791cfbc0737402da5a9518p-16494;
-    const exp: f128 = 0x336ec3affb2db8618e4e7d5e1c44p-16494;
-    try test_fmodq(a, b, exp);
+    const a1: f128 = 0xedcb34a235253948765432134674p-16494;
+    const b1: f128 = 0x5d2e38791cfbc0737402da5a9518p-16494;
+    const exp1: f128 = 0x336ec3affb2db8618e4e7d5e1c44p-16494;
+    try test_fmodq(a1, b1, exp1);
+    const a2: f128 = 0x0.7654_3210_fdec_ba98_7654_3210_fdecp-16382;
+    const b2: f128 = 0x0.0012_fdac_bdef_1234_fdec_3222_1111p-16382;
+    const exp2: f128 = 0x0.0001_aecd_9d66_4a6e_67b7_d7d0_a901p-16382;
+    try test_fmodq(a2, b2, exp2);
 
     try test_fmodq_nans();
     try test_fmodq_infs();
diff --git a/lib/std/special/compiler_rt/fmodx.zig b/lib/std/special/compiler_rt/fmodx.zig
new file mode 100644
index 0000000000..efe16f9f16
--- /dev/null
+++ b/lib/std/special/compiler_rt/fmodx.zig
@@ -0,0 +1,108 @@
+const builtin = @import("builtin");
+const std = @import("std");
+const math = std.math;
+const normalize = @import("divdf3.zig").normalize;
+
+// fmodx - floating modulo large, returns the remainder of division for f80 types
+// Logic and flow heavily inspired by MUSL fmodl for 113 mantissa digits
+pub fn fmodx(a: f80, b: f80) callconv(.C) f80 {
+    @setRuntimeSafety(builtin.is_test);
+
+    const T = f80;
+    const Z = std.meta.Int(.unsigned, @bitSizeOf(T));
+
+    const significandBits = math.floatMantissaBits(T);
+    const fractionalBits = math.floatFractionalBits(T);
+    const exponentBits = math.floatExponentBits(T);
+
+    const signBit = (@as(Z, 1) << (significandBits + exponentBits));
+    const maxExponent = ((1 << exponentBits) - 1);
+
+    var aRep = @bitCast(Z, a);
+    var bRep = @bitCast(Z, b);
+
+    const signA = aRep & signBit;
+    var expA = @intCast(i32, (@bitCast(Z, a) >> significandBits) & maxExponent);
+    var expB = @intCast(i32, (@bitCast(Z, b) >> significandBits) & maxExponent);
+
+    // There are 3 cases where the answer is undefined, check for:
+    //   - fmodx(val, 0)
+    //   - fmodx(val, NaN)
+    //   - fmodx(inf, val)
+    // The sign on checked values does not matter.
+    // Doing (a * b) / (a * b) procudes undefined results
+    // because the three cases always produce undefined calculations:
+    //   - 0 / 0
+    //   - val * NaN
+    //   - inf / inf
+    if (b == 0 or math.isNan(b) or expA == maxExponent) {
+        return (a * b) / (a * b);
+    }
+
+    // Remove the sign from both
+    aRep &= ~signBit;
+    bRep &= ~signBit;
+    if (aRep <= bRep) {
+        if (aRep == bRep) {
+            return 0 * a;
+        }
+        return a;
+    }
+
+    if (expA == 0) expA = normalize(f80, &aRep);
+    if (expB == 0) expB = normalize(f80, &bRep);
+
+    var highA: u64 = 0;
+    var highB: u64 = 0;
+    var lowA: u64 = @truncate(u64, aRep);
+    var lowB: u64 = @truncate(u64, bRep);
+
+    while (expA > expB) : (expA -= 1) {
+        var high = highA -% highB;
+        var low = lowA -% lowB;
+        if (lowA < lowB) {
+            high -%= 1;
+        }
+        if (high >> 63 == 0) {
+            if ((high | low) == 0) {
+                return 0 * a;
+            }
+            highA = 2 *% high + (low >> 63);
+            lowA = 2 *% low;
+        } else {
+            highA = 2 *% highA + (lowA >> 63);
+            lowA = 2 *% lowA;
+        }
+    }
+
+    var high = highA -% highB;
+    var low = lowA -% lowB;
+    if (lowA < lowB) {
+        high -%= 1;
+    }
+    if (high >> 63 == 0) {
+        if ((high | low) == 0) {
+            return 0 * a;
+        }
+        highA = high;
+        lowA = low;
+    }
+
+    while ((lowA >> fractionalBits) == 0) {
+        lowA = 2 *% lowA;
+        expA = expA - 1;
+    }
+
+    // Combine the exponent with the sign and significand, normalize if happened to be denormalized
+    if (expA < -fractionalBits) {
+        return @bitCast(T, signA);
+    } else if (expA <= 0) {
+        return @bitCast(T, (lowA >> @intCast(math.Log2Int(u64), 1 - expA)) | signA);
+    } else {
+        return @bitCast(T, lowA | (@as(Z, @intCast(u16, expA)) << significandBits) | signA);
+    }
+}
+
+test {
+    _ = @import("fmodx_test.zig");
+}
diff --git a/lib/std/special/compiler_rt/fmodx_test.zig b/lib/std/special/compiler_rt/fmodx_test.zig
new file mode 100644
index 0000000000..a5d0887ea4
--- /dev/null
+++ b/lib/std/special/compiler_rt/fmodx_test.zig
@@ -0,0 +1,51 @@
+const std = @import("std");
+const fmodx = @import("fmodx.zig");
+const testing = std.testing;
+
+fn test_fmodx(a: f80, b: f80, exp: f80) !void {
+    const res = fmodx.fmodx(a, b);
+    try testing.expect(exp == res);
+}
+
+fn test_fmodx_nans() !void {
+    try testing.expect(std.math.isNan(fmodx.fmodx(1.0, std.math.nan(f80))));
+    try testing.expect(std.math.isNan(fmodx.fmodx(1.0, -std.math.nan(f80))));
+    try testing.expect(std.math.isNan(fmodx.fmodx(std.math.nan(f80), 1.0)));
+    try testing.expect(std.math.isNan(fmodx.fmodx(-std.math.nan(f80), 1.0)));
+}
+
+fn test_fmodx_infs() !void {
+    try testing.expect(fmodx.fmodx(1.0, std.math.inf(f80)) == 1.0);
+    try testing.expect(fmodx.fmodx(1.0, -std.math.inf(f80)) == 1.0);
+    try testing.expect(std.math.isNan(fmodx.fmodx(std.math.inf(f80), 1.0)));
+    try testing.expect(std.math.isNan(fmodx.fmodx(-std.math.inf(f80), 1.0)));
+}
+
+test "fmodx" {
+    try test_fmodx(6.4, 4.0, 2.4);
+    try test_fmodx(6.4, -4.0, 2.4);
+    try test_fmodx(-6.4, 4.0, -2.4);
+    try test_fmodx(-6.4, -4.0, -2.4);
+    try test_fmodx(3.0, 2.0, 1.0);
+    try test_fmodx(-5.0, 3.0, -2.0);
+    try test_fmodx(3.0, 2.0, 1.0);
+    try test_fmodx(1.0, 2.0, 1.0);
+    try test_fmodx(0.0, 1.0, 0.0);
+    try test_fmodx(-0.0, 1.0, -0.0);
+    try test_fmodx(7046119.0, 5558362.0, 1487757.0);
+    try test_fmodx(9010357.0, 1957236.0, 1181413.0);
+    try test_fmodx(9223372036854775807, 10.0, 7.0);
+
+    // Denormals
+    const a1: f80 = 0x0.76e5_9a51_1a92_9ca4p-16381;
+    const b1: f80 = 0x0.2e97_1c3c_8e7d_e03ap-16381;
+    const exp1: f80 = 0x0.19b7_61d7_fd96_dc30p-16381;
+    try test_fmodx(a1, b1, exp1);
+    const a2: f80 = 0x0.76e5_9a51_1a92_9ca4p-16381;
+    const b2: f80 = 0x0.0e97_1c3c_8e7d_e03ap-16381;
+    const exp2: f80 = 0x0.022c_b86c_a6a3_9ad4p-16381;
+    try test_fmodx(a2, b2, exp2);
+
+    try test_fmodx_nans();
+    try test_fmodx_infs();
+}