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Merge pull request #10743 from m-radomski/master

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std: correct rounding in parse_hex_float.zig
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Andrew Kelley 2022-01-31 22:59:49 -05:00 • committed by GitHub
commit d3e40b0d80
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2 changed files with 11 additions and 14 deletions
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20
lib/std/fmt/parse_hex_float.zig
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@ -202,18 +202,19 @@ pub fn parseHexFloat(comptime T: type, s: []const u8) !T {
exponent += 1;
}
// There are two cases to handle:
// - We've truncated more than 0.5ULP (R=S=1), increase the mantissa.
// - We've truncated exactly 0.5ULP (R=1 S=0), increase the mantissa if the
// result is odd (G=1).
// The two checks can be neatly folded as follows.
mantissa |= @boolToInt(mantissa & 0b100 != 0);
mantissa += 1;
// Whenever the guard bit is one (G=1) and:
// - we've truncated more than 0.5ULP (R=S=1)
// - we've truncated exactly 0.5ULP (R=1 S=0)
// Were are going to increase the mantissa (round up)
const guard_bit_and_half_or_more = (mantissa & 0b110) == 0b110;
mantissa >>= 2;
exponent += 2;
if (mantissa & (1 << (mantissa_bits + 1)) != 0) {
if (guard_bit_and_half_or_more) {
mantissa += 1;
}
if (mantissa == (1 << (mantissa_bits + 1))) {
// Renormalize, if the exponent overflows we'll catch that below.
mantissa >>= 1;
exponent += 1;
@ -338,6 +339,7 @@ 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| {

5
test/behavior/math.zig
View file

@ -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.