exp2_64() broken for negative inputs

[LewisGaul]

Remember to search before filing a new report

• I searched for this bug and did not find it in the issue tracker, and furthermore, the title I used above will make this new bug report turn up in the search results for my query.

Zig Version

0.9.0-dev.1417+bea447a63

Steps to Reproduce

const std = @import("std");
const print = std.debug.print;
const exp2 = std.math.exp2;

test {
    const val1: f64 = -8;
    print("in: {}, out: {}\n", .{val1, exp2(val1)});

    const val2: f64 = -0x1.a05cc754481d1p-2;
    print("in: {}, out: {}\n", .{val2, exp2(val2)});
}

Expected Behavior

Should successfully compute 2^x where x is negative.

Actual Behavior

2^(-8) gives inf, and the other negative power leads to a runtime crash:

$zig0.9 test tmp.zig
Test [1/1] test ""... in: -8.0e+00, out: inf
thread 10835 panic: integer overflow
/mnt/c/Users/legaul/Documents/personal/zig/build/lib/zig/std/math/exp2.zig:407:9: 0x217a7c in std.math.exp2.exp2_64 (test)
    i_0 += tblsiz / 2;
        ^
/mnt/c/Users/legaul/Documents/personal/zig/build/lib/zig/std/math/exp2.zig:20:23: 0x2168e6 in std.math.exp2.exp2 (test)
        f64 => exp2_64(x),
                      ^
/home/legaul/tmp/tmp.zig:10:44: 0x21512f in test "" (test)
    print("in: {}, out: {}\n", .{val2, exp2(val2)});
                                           ^
/mnt/c/Users/legaul/Documents/personal/zig/build/lib/zig/std/special/test_runner.zig:77:28: 0x2400a2 in std.special.main (test)
        } else test_fn.func();
                           ^
/mnt/c/Users/legaul/Documents/personal/zig/build/lib/zig/std/start.zig:517:22: 0x234cac in std.start.callMain (test)
            root.main();
                     ^
/mnt/c/Users/legaul/Documents/personal/zig/build/lib/zig/std/start.zig:469:12: 0x2182de in std.start.callMainWithArgs (test)
    return @call(.{ .modifier = .always_inline }, callMain, .{});
           ^
/mnt/c/Users/legaul/Documents/personal/zig/build/lib/zig/std/start.zig:383:17: 0x216e26 in std.start.posixCallMainAndExit (test)
    std.os.exit(@call(.{ .modifier = .always_inline }, callMainWithArgs, .{ argc, argv, envp }));
                ^
/mnt/c/Users/legaul/Documents/personal/zig/build/lib/zig/std/start.zig:296:5: 0x216c32 in std.start._start (test)
    @call(.{ .modifier = .never_inline }, posixCallMainAndExit, .{});
    ^
error: the following test command crashed:
zig-cache/o/47b60d05dca7afaac0e6df86d296a7b9/test /mnt/c/Users/legaul/Documents/personal/zig/build/zig

Note that the same problem is not observed for exp() or exp2_32().

[LewisGaul]

I came across this while working on #4026 (currently adding tests). I can work on a fix for this today/tomorrow.