diff --git a/src/stage1/softfloat.hpp b/src/stage1/softfloat.hpp
index ec0315d684..a0d270d55f 100644
--- a/src/stage1/softfloat.hpp
+++ b/src/stage1/softfloat.hpp
@@ -57,7 +57,7 @@ static inline bool zig_f128_isNaN(float128_t *aPtr) {
 }
 
 static inline bool zig_extF80_isNaN(extFloat80_t *aPtr) {
-    return aPtr->signExp & 0x7FFF && aPtr->signif;
+    return (aPtr->signExp & 0x7FFF) == 0x7FFF && aPtr->signif & UINT64_C(0x7FFFFFFFFFFFFFFF);
 }
 
 #endif
diff --git a/test/behavior/floatop_stage1.zig b/test/behavior/floatop_stage1.zig
index b1b7eb2b92..303288a118 100644
--- a/test/behavior/floatop_stage1.zig
+++ b/test/behavior/floatop_stage1.zig
@@ -4,6 +4,7 @@ const math = std.math;
 const pi = std.math.pi;
 const e = std.math.e;
 const Vector = std.meta.Vector;
+const has_f80_rt = @import("builtin").cpu.arch == .x86_64;
 
 const epsilon = 0.000001;
 
@@ -27,6 +28,10 @@ fn testSqrt() !void {
         var a: f64 = 25;
         try expect(@sqrt(a) == 5);
     }
+    if (has_f80_rt) {
+        var a: f80 = 25;
+        try expect(@sqrt(a) == 5);
+    }
     {
         const a: comptime_float = 25.0;
         try expect(@sqrt(a) == 5.0);
@@ -86,6 +91,10 @@ fn testSin() !void {
         var a: f64 = 0;
         try expect(@sin(a) == 0);
     }
+    // {
+    //     var a: f80 = 0;
+    //     try expect(@sin(a) == 0);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, 2.2, 3.3, 4.4 };
         var result = @sin(v);
@@ -116,6 +125,10 @@ fn testCos() !void {
         var a: f64 = 0;
         try expect(@cos(a) == 1);
     }
+    // {
+    //     var a: f80 = 0;
+    //     try expect(@cos(a) == 1);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, 2.2, 3.3, 4.4 };
         var result = @cos(v);
@@ -146,6 +159,10 @@ fn testExp() !void {
         var a: f64 = 0;
         try expect(@exp(a) == 1);
     }
+    // {
+    //     var a: f80 = 0;
+    //     try expect(@exp(a) == 1);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, 2.2, 0.3, 0.4 };
         var result = @exp(v);
@@ -176,6 +193,10 @@ fn testExp2() !void {
         var a: f64 = 2;
         try expect(@exp2(a) == 4);
     }
+    // {
+    //     var a: f80 = 2;
+    //     try expect(@exp2(a) == 4);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, 2.2, 0.3, 0.4 };
         var result = @exp2(v);
@@ -208,6 +229,10 @@ fn testLog() !void {
         var a: f64 = e;
         try expect(@log(a) == 1 or @log(a) == @bitCast(f64, @as(u64, 0x3ff0000000000000)));
     }
+    // {
+    //     var a: f80 = e;
+    //     try expect(@log(a) == 1);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, 2.2, 0.3, 0.4 };
         var result = @log(v);
@@ -238,6 +263,10 @@ fn testLog2() !void {
         var a: f64 = 4;
         try expect(@log2(a) == 2);
     }
+    // {
+    //     var a: f80 = 4;
+    //     try expect(@log2(a) == 2);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, 2.2, 0.3, 0.4 };
         var result = @log2(v);
@@ -268,6 +297,10 @@ fn testLog10() !void {
         var a: f64 = 1000;
         try expect(@log10(a) == 3);
     }
+    // {
+    //     var a: f80 = 1000;
+    //     try expect(@log10(a) == 3);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, 2.2, 0.3, 0.4 };
         var result = @log10(v);
@@ -304,6 +337,12 @@ fn testFabs() !void {
         try expect(@fabs(a) == 2.5);
         try expect(@fabs(b) == 2.5);
     }
+    // {
+    //     var a: f80 = -2.5;
+    //     var b: f80 = 2.5;
+    //     try expect(@fabs(a) == 2.5);
+    //     try expect(@fabs(b) == 2.5);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, -2.2, 0.3, -0.4 };
         var result = @fabs(v);
@@ -334,6 +373,10 @@ fn testFloor() !void {
         var a: f64 = 3.5;
         try expect(@floor(a) == 3);
     }
+    // {
+    //     var a: f80 = 3.5;
+    //     try expect(@floor(a) == 3);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, -2.2, 0.3, -0.4 };
         var result = @floor(v);
@@ -364,6 +407,10 @@ fn testCeil() !void {
         var a: f64 = 3.5;
         try expect(@ceil(a) == 4);
     }
+    // {
+    //     var a: f80 = 3.5;
+    //     try expect(@ceil(a) == 4);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, -2.2, 0.3, -0.4 };
         var result = @ceil(v);
@@ -394,6 +441,10 @@ fn testTrunc() !void {
         var a: f64 = -3.5;
         try expect(@trunc(a) == -3);
     }
+    // {
+    //     var a: f80 = -3.5;
+    //     try expect(@trunc(a) == -3);
+    // }
     {
         var v: Vector(4, f32) = [_]f32{ 1.1, -2.2, 0.3, -0.4 };
         var result = @trunc(v);
@@ -403,3 +454,33 @@ fn testTrunc() !void {
         try expect(math.approxEqAbs(f32, @trunc(@as(f32, -0.4)), result[3], epsilon));
     }
 }
+
+test "floating point comparisons" {
+    if (has_f80_rt) try testFloatComparisons();
+    comptime try testFloatComparisons();
+}
+
+fn testFloatComparisons() !void {
+    inline for ([_]type{ f16, f32, f64, f80, f128 }) |ty| {
+        // No decimal part
+        {
+            const x: ty = 1.0;
+            try expect(x == 1);
+            try expect(x != 0);
+            try expect(x > 0);
+            try expect(x < 2);
+            try expect(x >= 1);
+            try expect(x <= 1);
+        }
+        // Non-zero decimal part
+        {
+            const x: ty = 1.5;
+            try expect(x != 1);
+            try expect(x != 2);
+            try expect(x > 1);
+            try expect(x < 2);
+            try expect(x >= 1);
+            try expect(x <= 2);
+        }
+    }
+}
diff --git a/test/behavior/math_stage1.zig b/test/behavior/math_stage1.zig
index 72824348dd..298f2ffc85 100644
--- a/test/behavior/math_stage1.zig
+++ b/test/behavior/math_stage1.zig
@@ -5,6 +5,7 @@ const expectEqualSlices = std.testing.expectEqualSlices;
 const maxInt = std.math.maxInt;
 const minInt = std.math.minInt;
 const mem = std.mem;
+const has_f80_rt = @import("builtin").cpu.arch == .x86_64;
 
 test "allow signed integer division/remainder when values are comptime known and positive or exact" {
     try expect(5 / 3 == 1);
@@ -194,6 +195,8 @@ fn testSqrt(comptime T: type, x: T) !void {
 test "@fabs" {
     try testFabs(f128, 12.0);
     comptime try testFabs(f128, 12.0);
+    if (has_f80_rt) try testFabs(f80, 12.0);
+    // comptime try testFabs(f80, 12.0);
     try testFabs(f64, 12.0);
     comptime try testFabs(f64, 12.0);
     try testFabs(f32, 12.0);
@@ -217,6 +220,8 @@ test "@floor" {
     // FIXME: Generates a floorl function call
     // testFloor(f128, 12.0);
     comptime try testFloor(f128, 12.0);
+    // try testFloor(f80, 12.0);
+    comptime try testFloor(f80, 12.0);
     try testFloor(f64, 12.0);
     comptime try testFloor(f64, 12.0);
     try testFloor(f32, 12.0);
@@ -240,6 +245,8 @@ test "@ceil" {
     // FIXME: Generates a ceill function call
     //testCeil(f128, 12.0);
     comptime try testCeil(f128, 12.0);
+    // try testCeil(f80, 12.0);
+    comptime try testCeil(f80, 12.0);
     try testCeil(f64, 12.0);
     comptime try testCeil(f64, 12.0);
     try testCeil(f32, 12.0);
@@ -263,6 +270,14 @@ test "@trunc" {
     // FIXME: Generates a truncl function call
     //testTrunc(f128, 12.0);
     comptime try testTrunc(f128, 12.0);
+    // try testTrunc(f80, 12.0);
+    // comptime try testTrunc(f80, 12.0);
+    comptime {
+        const x: f80 = 12.0;
+        const y = x + 0.8;
+        const z = @trunc(y);
+        try expectEqual(x, z);
+    }
     try testTrunc(f64, 12.0);
     comptime try testTrunc(f64, 12.0);
     try testTrunc(f32, 12.0);
@@ -294,6 +309,8 @@ test "@round" {
     // FIXME: Generates a roundl function call
     //testRound(f128, 12.0);
     comptime try testRound(f128, 12.0);
+    // try testRound(f80, 12.0);
+    comptime try testRound(f80, 12.0);
     try testRound(f64, 12.0);
     comptime try testRound(f64, 12.0);
     try testRound(f32, 12.0);
@@ -333,10 +350,12 @@ test "NaN comparison" {
     try testNanEqNan(f32);
     try testNanEqNan(f64);
     try testNanEqNan(f128);
+    if (has_f80_rt) try testNanEqNan(f80);
     comptime try testNanEqNan(f16);
     comptime try testNanEqNan(f32);
     comptime try testNanEqNan(f64);
     comptime try testNanEqNan(f128);
+    // comptime try testNanEqNan(f80);
 }
 
 fn testNanEqNan(comptime F: type) !void {
diff --git a/test/behavior/muladd.zig b/test/behavior/muladd.zig
index cff7b5e2ad..46e938e1a9 100644
--- a/test/behavior/muladd.zig
+++ b/test/behavior/muladd.zig
@@ -25,6 +25,12 @@ fn testMulAdd() !void {
         var c: f64 = 6.25;
         try expect(@mulAdd(f64, a, b, c) == 20);
     }
+    // {
+    //     var a: f16 = 5.5;
+    //     var b: f80 = 2.5;
+    //     var c: f80 = 6.25;
+    //     try expect(@mulAdd(f80, a, b, c) == 20);
+    // }
     if (builtin.os.tag == .macos and builtin.cpu.arch == .aarch64) {
         // https://github.com/ziglang/zig/issues/9900
         return error.SkipZigTest;
diff --git a/test/behavior/type_stage1.zig b/test/behavior/type_stage1.zig
index a87cba08d4..911e6963c9 100644
--- a/test/behavior/type_stage1.zig
+++ b/test/behavior/type_stage1.zig
@@ -13,8 +13,9 @@ test "Type.Float" {
     try testing.expect(f16 == @Type(TypeInfo{ .Float = TypeInfo.Float{ .bits = 16 } }));
     try testing.expect(f32 == @Type(TypeInfo{ .Float = TypeInfo.Float{ .bits = 32 } }));
     try testing.expect(f64 == @Type(TypeInfo{ .Float = TypeInfo.Float{ .bits = 64 } }));
+    try testing.expect(f80 == @Type(TypeInfo{ .Float = TypeInfo.Float{ .bits = 80 } }));
     try testing.expect(f128 == @Type(TypeInfo{ .Float = TypeInfo.Float{ .bits = 128 } }));
-    try testTypes(&[_]type{ f16, f32, f64, f128 });
+    try testTypes(&[_]type{ f16, f32, f64, f80, f128 });
 }
 
 test "Type.Array" {
diff --git a/test/behavior/widening.zig b/test/behavior/widening.zig
index afca1ae143..035d4fa4f9 100644
--- a/test/behavior/widening.zig
+++ b/test/behavior/widening.zig
@@ -2,6 +2,7 @@ const std = @import("std");
 const expect = std.testing.expect;
 const mem = std.mem;
 const builtin = @import("builtin");
+const has_f80_rt = @import("builtin").cpu.arch == .x86_64;
 
 test "integer widening" {
     var a: u8 = 250;
@@ -27,6 +28,10 @@ test "float widening" {
     try expect(a == b);
     try expect(b == c);
     try expect(c == d);
+    if (has_f80_rt) {
+        var e: f80 = c;
+        try expect(c == e);
+    }
 }
 
 test "float widening f16 to f128" {