Commit 7f024d6786b690e7980d6e47d7ae70fa8528a0df | zig

author	Mateusz Radomski <radomski.main@protonmail.com> 2022-01-31 09:54:19+01:00
committer	Andrew Kelley <andrew@ziglang.org> 2022-01-31 20:59:32-07:00
log	7f024d6786b690e7980d6e47d7ae70fa8528a0df
tree	6b7ffeda08e97cfe833359aa1c3769f80c3bb2d5
parent	627cf6ce482349c172150d058660f7a1646c2aac

std: correct rounding in parse_hex_float.zig

2 files changed, 13 insertions(+), 14 deletions(-)

lib
- std
  - fmt
    parse_hex_float.zig
test
- behavior
  - math.zig

lib/std/fmt/parse_hex_float.zig+13-9

 @@ -202,18 +202,21 @@ pub fn parseHexFloat(comptime T: type, s: []const u8) !T {
         exponent += 1;
     }
-    // There are two cases to handle:
+    // Whenever the guard bit is one (G=1) and:
-    // - We've truncated more than 0.5ULP (R=S=1), increase the mantissa.
+    //   - we've truncated more than 0.5ULP (R=S=1)
-    // - We've truncated exactly 0.5ULP (R=1 S=0), increase the mantissa if the
+    //   - we've truncated exactly 0.5ULP (R=1 S=0)
-    //   result is odd (G=1).
+    // Were are going to increase the mantissa (round up)
-    // The two checks can be neatly folded as follows.
+    var exactly_half = (mantissa & 0b11) == 0b10;
-    mantissa |= @boolToInt(mantissa & 0b100 != 0);
+    var more_than_half = (mantissa & 0b11) == 0b11;
-    mantissa += 1;
     mantissa >>= 2;
+    var guardBit = mantissa & 1 == 1;
     exponent += 2;
-    if (mantissa & (1 << (mantissa_bits + 1)) != 0) {
+    if (guardBit and (exactly_half or more_than_half)) {
+        mantissa += 1;
+    }
+    if (mantissa == (1 << (mantissa_bits + 1))) {
         // Renormalize, if the exponent overflows we'll catch that below.
         mantissa >>= 1;
         exponent += 1;
 @@ -338,6 +341,7 @@ test "f128" {
         // // Min denormalized value.
         .{ .s = "0x1p-16494", .v = math.f128_true_min },
         .{ .s = "-0x1p-16494", .v = -math.f128_true_min },
+        .{ .s = "0x1.edcb34a235253948765432134674fp-1", .v = 0x1.edcb34a235253948765432134674fp-1 },
     };
     for (cases) |case| {

test/behavior/math.zig-5

 @@ -632,11 +632,6 @@ test "allow signed integer division/remainder when values are comptime known and
 }
 test "quad hex float literal parsing accurate" {
-    if (builtin.zig_backend != .stage1) {
-        // TODO https://github.com/ziglang/zig/issues/10737
-        return error.SkipZigTest;
-    }
     const a: f128 = 0x1.1111222233334444555566667777p+0;
     // implied 1 is dropped, with an exponent of 0 (0x3fff) after biasing.