From f5540778b56636cffd89f794c6b1a9266d7d36f8 Mon Sep 17 00:00:00 2001
From: Cody Tapscott <topolarity@tapscott.me>
Date: Mon, 25 Apr 2022 16:32:42 -0700
Subject: [PATCH] stdlib: Fix hex-float printing for f80

---
 lib/std/fmt.zig          | 7 ++++---
 lib/std/math/signbit.zig | 6 ++++++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/lib/std/fmt.zig b/lib/std/fmt.zig
index 80a2e87c5c..994350ffa7 100644
--- a/lib/std/fmt.zig
+++ b/lib/std/fmt.zig
@@ -1124,6 +1124,7 @@ pub fn formatFloatHexadecimal(
     const TU = std.meta.Int(.unsigned, std.meta.bitCount(T));
 
     const mantissa_bits = math.floatMantissaBits(T);
+    const fractional_bits = math.floatFractionalBits(T);
     const exponent_bits = math.floatExponentBits(T);
     const mantissa_mask = (1 << mantissa_bits) - 1;
     const exponent_mask = (1 << exponent_bits) - 1;
@@ -1155,14 +1156,14 @@ pub fn formatFloatHexadecimal(
         // Adjust the exponent for printing.
         exponent += 1;
     } else {
-        // Add the implicit 1.
-        mantissa |= 1 << mantissa_bits;
+        if (fractional_bits == mantissa_bits)
+            mantissa |= 1 << fractional_bits; // Add the implicit integer bit.
     }
 
     // Fill in zeroes to round the mantissa width to a multiple of 4.
     if (T == f16) mantissa <<= 2 else if (T == f32) mantissa <<= 1;
 
-    const mantissa_digits = (mantissa_bits + 3) / 4;
+    const mantissa_digits = (fractional_bits + 3) / 4;
 
     if (options.precision) |precision| {
         // Round if needed.
diff --git a/lib/std/math/signbit.zig b/lib/std/math/signbit.zig
index 056add5ae6..a72d6ab119 100644
--- a/lib/std/math/signbit.zig
+++ b/lib/std/math/signbit.zig
@@ -9,6 +9,7 @@ pub fn signbit(x: anytype) bool {
         f16 => signbit16(x),
         f32 => signbit32(x),
         f64 => signbit64(x),
+        f80 => signbit80(x),
         f128 => signbit128(x),
         else => @compileError("signbit not implemented for " ++ @typeName(T)),
     };
@@ -29,6 +30,11 @@ fn signbit64(x: f64) bool {
     return bits >> 63 != 0;
 }
 
+fn signbit80(x: f80) bool {
+    const bits = @bitCast(u80, x);
+    return bits >> 79 != 0;
+}
+
 fn signbit128(x: f128) bool {
     const bits = @bitCast(u128, x);
     return bits >> 127 != 0;