format

    fn format(data: FormatData, w: *Writer) Writer.Error!void {
        if (data.flags.comma) {
            if (data.constant == .no_init) return;
            try w.writeByte(',');
        }
        if (data.flags.space) {
            if (data.constant == .no_init) return;
            try w.writeByte(' ');
        }
        if (data.flags.percent)
            try w.print("{f} ", .{data.constant.typeOf(data.builder).fmt(data.builder, .percent)});
        assert(data.constant != .no_init);
        if (std.enums.tagName(Constant, data.constant)) |name| return w.writeAll(name);
        switch (data.constant.unwrap()) {
            .constant => |constant| {
                const item = data.builder.constant_items.get(constant);
                switch (item.tag) {
                    .positive_integer,
                    .negative_integer,
                    => |tag| {
                        const extra: *align(@alignOf(std.math.big.Limb)) const Integer =
                            @ptrCast(data.builder.constant_limbs.items[item.data..][0..Integer.limbs]);
                        const limbs = data.builder.constant_limbs
                            .items[item.data + Integer.limbs ..][0..extra.limbs_len];
                        const bigint: std.math.big.int.Const = .{
                            .limbs = limbs,
                            .positive = switch (tag) {
                                .positive_integer => true,
                                .negative_integer => false,
                                else => unreachable,
                            },
                        };
                        const ExpectedContents = extern struct {
                            const expected_limbs = @divExact(512, @bitSizeOf(std.math.big.Limb));
                            string: [
                                (std.math.big.int.Const{
                                    .limbs = &@as([expected_limbs]std.math.big.Limb, @splat(maxInt(std.math.big.Limb))),
                                    .positive = false,
                                }).sizeInBaseUpperBound(10)
                            ]u8,
                            limbs: [
                                std.math.big.int.calcToStringLimbsBufferLen(expected_limbs, 10)
                            ]std.math.big.Limb,
                        };
                        var bfa_buf: ExpectedContents = undefined;
                        var bfa: std.heap.BufferFirstAllocator = .init(@ptrCast(&bfa_buf), data.builder.gpa);
                        const allocator = bfa.allocator();
                        const str = bigint.toStringAlloc(allocator, 10, undefined) catch return error.WriteFailed;
                        defer allocator.free(str);
                        try w.writeAll(str);
                    },
                    .half,
                    .bfloat,
                    => |tag| try w.print("0x{c}{X:0>4}", .{ @as(u8, switch (tag) {
                        .half => 'H',
                        .bfloat => 'R',
                        else => unreachable,
                    }), item.data >> switch (tag) {
                        .half => 0,
                        .bfloat => 16,
                        else => unreachable,
                    } }),
                    .float => {
                        const Float = struct {
                            fn Repr(comptime T: type) type {
                                return packed struct(@Int(.unsigned, @bitSizeOf(T))) {
                                    mantissa: @Int(.unsigned, std.math.floatMantissaBits(T)),
                                    exponent: @Int(.unsigned, std.math.floatExponentBits(T)),
                                    sign: u1,
                                };
                            }
                        };
                        const Mantissa64 = @FieldType(Float.Repr(f64), "mantissa");
                        const Exponent32 = @FieldType(Float.Repr(f32), "exponent");
                        const Exponent64 = @FieldType(Float.Repr(f64), "exponent");

                        const repr: Float.Repr(f32) = @bitCast(item.data);
                        const denormal_shift = switch (repr.exponent) {
                            std.math.minInt(Exponent32) => @as(
                                std.math.Log2Int(Mantissa64),
                                @clz(repr.mantissa),
                            ) + 1,
                            else => 0,
                        };
                        try w.print("0x{X:0>16}", .{@as(u64, @bitCast(Float.Repr(f64){
                            .mantissa = std.math.shl(
                                Mantissa64,
                                repr.mantissa,
                                std.math.floatMantissaBits(f64) - std.math.floatMantissaBits(f32) +
                                    denormal_shift,
                            ),
                            .exponent = switch (repr.exponent) {
                                std.math.minInt(Exponent32) => if (repr.mantissa > 0)
                                    @as(Exponent64, std.math.floatExponentMin(f32) +
                                        std.math.floatExponentMax(f64)) - denormal_shift
                                else
                                    std.math.minInt(Exponent64),
                                else => @as(Exponent64, repr.exponent) +
                                    (std.math.floatExponentMax(f64) - std.math.floatExponentMax(f32)),
                                maxInt(Exponent32) => maxInt(Exponent64),
                            },
                            .sign = repr.sign,
                        }))});
                    },
                    .double => {
                        const extra = data.builder.constantExtraData(Double, item.data);
                        try w.print("0x{X:0>8}{X:0>8}", .{ extra.hi, extra.lo });
                    },
                    .fp128,
                    .ppc_fp128,
                    => |tag| {
                        const extra = data.builder.constantExtraData(Fp128, item.data);
                        try w.print("0x{c}{X:0>8}{X:0>8}{X:0>8}{X:0>8}", .{
                            @as(u8, switch (tag) {
                                .fp128 => 'L',
                                .ppc_fp128 => 'M',
                                else => unreachable,
                            }),
                            extra.lo_hi,
                            extra.lo_lo,
                            extra.hi_hi,
                            extra.hi_lo,
                        });
                    },
                    .x86_fp80 => {
                        const extra = data.builder.constantExtraData(Fp80, item.data);
                        try w.print("0xK{X:0>4}{X:0>8}{X:0>8}", .{
                            extra.hi, extra.lo_hi, extra.lo_lo,
                        });
                    },
                    .null,
                    .none,
                    .zeroinitializer,
                    .undef,
                    .poison,
                    => |tag| try w.writeAll(@tagName(tag)),
                    .structure,
                    .packed_structure,
                    .array,
                    .vector,
                    => |tag| {
                        var extra = data.builder.constantExtraDataTrail(Aggregate, item.data);
                        const len: u32 = @intCast(extra.data.type.aggregateLen(data.builder));
                        const vals = extra.trail.next(len, Constant, data.builder);
                        try w.writeAll(switch (tag) {
                            .structure => "{ ",
                            .packed_structure => "<{ ",
                            .array => "[",
                            .vector => "<",
                            else => unreachable,
                        });
                        for (vals, 0..) |val, index| {
                            if (index > 0) try w.writeAll(", ");
                            try w.print("{f}", .{val.fmt(data.builder, .{ .percent = true })});
                        }
                        try w.writeAll(switch (tag) {
                            .structure => " }",
                            .packed_structure => " }>",
                            .array => "]",
                            .vector => ">",
                            else => unreachable,
                        });
                    },
                    .splat => {
                        const extra = data.builder.constantExtraData(Splat, item.data);
                        const len = extra.type.vectorLen(data.builder);
                        try w.writeByte('<');
                        for (0..len) |index| {
                            if (index > 0) try w.writeAll(", ");
                            try w.print("{f}", .{extra.value.fmt(data.builder, .{ .percent = true })});
                        }
                        try w.writeByte('>');
                    },
                    .string => try w.print("c{f}", .{
                        @as(String, @fromBackingInt(item.data)).fmtQ(data.builder),
                    }),
                    .blockaddress => |tag| {
                        const extra = data.builder.constantExtraData(BlockAddress, item.data);
                        const function = extra.function.ptrConst(data.builder);
                        try w.print("{s}({f}, {f})", .{
                            @tagName(tag),
                            function.global.fmt(data.builder),
                            extra.block.toInst(function).fmt(extra.function, data.builder, .{}),
                        });
                    },
                    .dso_local_equivalent,
                    .no_cfi,
                    => |tag| {
                        const function: Function.Index = @fromBackingInt(item.data);
                        try w.print("{s} {f}", .{
                            @tagName(tag),
                            function.ptrConst(data.builder).global.fmt(data.builder),
                        });
                    },
                    .trunc,
                    .ptrtoint,
                    .inttoptr,
                    .bitcast,
                    .addrspacecast,
                    => |tag| {
                        const extra = data.builder.constantExtraData(Cast, item.data);
                        try w.print("{s} ({f} to {f})", .{
                            @tagName(tag),
                            extra.val.fmt(data.builder, .{ .percent = true }),
                            extra.type.fmt(data.builder, .percent),
                        });
                    },
                    .getelementptr,
                    .@"getelementptr inbounds",
                    => |tag| {
                        var extra = data.builder.constantExtraDataTrail(GetElementPtr, item.data);
                        const indices =
                            extra.trail.next(extra.data.info.indices_len, Constant, data.builder);
                        try w.print("{s} ({f}, {f}", .{
                            @tagName(tag),
                            extra.data.type.fmt(data.builder, .percent),
                            extra.data.base.fmt(data.builder, .{ .percent = true }),
                        });
                        for (indices) |index| try w.print(", {f}", .{index.fmt(data.builder, .{ .percent = true })});
                        try w.writeByte(')');
                    },
                    .add,
                    .@"add nsw",
                    .@"add nuw",
                    .sub,
                    .@"sub nsw",
                    .@"sub nuw",
                    .shl,
                    .xor,
                    => |tag| {
                        const extra = data.builder.constantExtraData(Binary, item.data);
                        try w.print("{s} ({f}, {f})", .{
                            @tagName(tag),
                            extra.lhs.fmt(data.builder, .{ .percent = true }),
                            extra.rhs.fmt(data.builder, .{ .percent = true }),
                        });
                    },
                    .@"asm",
                    .@"asm sideeffect",
                    .@"asm alignstack",
                    .@"asm sideeffect alignstack",
                    .@"asm inteldialect",
                    .@"asm sideeffect inteldialect",
                    .@"asm alignstack inteldialect",
                    .@"asm sideeffect alignstack inteldialect",
                    .@"asm unwind",
                    .@"asm sideeffect unwind",
                    .@"asm alignstack unwind",
                    .@"asm sideeffect alignstack unwind",
                    .@"asm inteldialect unwind",
                    .@"asm sideeffect inteldialect unwind",
                    .@"asm alignstack inteldialect unwind",
                    .@"asm sideeffect alignstack inteldialect unwind",
                    => |tag| {
                        const extra = data.builder.constantExtraData(Assembly, item.data);
                        try w.print("{s} {f}, {f}", .{
                            @tagName(tag),
                            extra.assembly.fmtQ(data.builder),
                            extra.constraints.fmtQ(data.builder),
                        });
                    },
                }
            },
            .global => |global| try w.print("{f}", .{global.fmt(data.builder)}),
        }
    }