Does FixedBufferAllocator work on architectures below 32 bits? | Zig Programming Language | Page 1

uncut briar Oct 23, 2023, 2:27 PM

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I'm writing a small program for a 328P based microcontroller, which has 8-bit registers, and i think it uses a 16-bit pointer size, but when i tried to create a FBA, the compiler complains about this pointer size, as seen in fig.1.

var alloc_data: [100]u8 = undefined;

pub fn main() !void {
    var fba = std.heap.FixedBufferAllocator.init(&alloc_data);
    const alloc = fba.allocator();
    _ = alloc;
}

The allocator is used to temporarily store formatted strings to display via uart and an LCD right after, which has a limited character length, so FBA is a good choice for this.

Is there a built-in allocator that would work for this use case? Otherwise, is there a recommended 3rd party allocator that would work?
0.10 is used instead of 0.11 or master because of issue #16417.

tidal wren Oct 23, 2023, 2:30 PM

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this appears to be a problem with how the allocator interface itself assumes the pointer size to be 32 or 64 bits (or greater)

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there'd need to be a fix in the interface itself to allow for this use case to make it possible to use literally any allocator

uncut briar Oct 23, 2023, 2:34 PM

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so, there's no way to use any allocator? That means i'll likely have to implement my own Writer for both use cases; a little annoying

tidal wren Oct 23, 2023, 2:36 PM

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just to note, there is std.fmt.bufPrint

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also, I've written a couple utility functions for bounded formatting:

pub fn fixedLenFmt(
    comptime fmt_str: []const u8,
    args: anytype,
    comptime reference_args: @TypeOf(args),
) error{ Overflow, Underflow }![std.fmt.count(fmt_str, reference_args)]u8 {
    const bounded = try boundedFmt(fmt_str, args, reference_args);
    if (bounded.len != bounded.buffer.len) return error.Underflow;
    return bounded.constSlice()[0..bounded.buffer.len].*;
}

pub fn boundedFmt(
    comptime fmt_str: []const u8,
    args: anytype,
    comptime reference_args: @TypeOf(args),
) error{Overflow}!std.BoundedArray(u8, std.fmt.count(fmt_str, reference_args)) {
    const len = comptime std.fmt.count(fmt_str, reference_args);
    var result: std.BoundedArray(u8, len) = .{};
    try result.writer().print(fmt_str, args);
    return result;
}

usage being:

const int_str = try boundedFmt("{d}", .{@as(u32, runtime_int)}, .{std.math.maxInt(u32)});

trail flower Oct 23, 2023, 2:37 PM

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You could just edit the standard library. At least the issue with alignBackwardAnyAlign could be solved with a cast.

tidal wren Oct 23, 2023, 2:41 PM

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At any rate, I would suggest filing an issue on github - that way this may be amended

uncut briar Oct 23, 2023, 2:44 PM

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tidal wren just to note, there is `std.fmt.bufPrint`

    var buffer: [50]u8 = undefined;
    const string = try std.fmt.bufPrint(&buffer, "{}", .{regs.TC1.TCNT1});
    _ = string;

this crashes the compiler

uncut briar Oct 23, 2023, 2:45 PM

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tidal wren At any rate, I would suggest filing an issue on github - that way this may be am...

will do

tidal wren Oct 23, 2023, 2:45 PM

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uncut briar ```rs var buffer: [50]u8 = undefined; const string = try std.fmt.bufPrin...

unfortunate, seems like that target perhaps isn't handled very well by llvm yet

uncut briar Oct 23, 2023, 2:47 PM

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yeah, i think llvm 17 has better support, but this is using llvm 15

tidal wren Oct 23, 2023, 2:47 PM

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? zig is on llvm 16 currently

uncut briar Oct 23, 2023, 2:49 PM

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this is 0.10, not 0.11

tidal wren Oct 23, 2023, 2:49 PM

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ahhh I see

trail flower Oct 23, 2023, 2:56 PM

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I think 0.12 is using llvm 17. I tried the example in issue #16417 that should reproduce the error and it seems to compile fine on the latest 0.12 dev release.

uncut briar Oct 23, 2023, 3:38 PM

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updated to 0.12, but i'm stuck with this issue

tidal wren Oct 23, 2023, 3:39 PM

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I believe you have to call functions using inline asm inside a naked function

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see _start in the std.start file for an example

uncut briar Oct 23, 2023, 3:46 PM

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yeah, _start should be called by asm (".section .vectors\njmp _start\n")

tidal wren Oct 23, 2023, 3:47 PM

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well then does it even need to be .Naked?

uncut briar Oct 23, 2023, 3:48 PM

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good question

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it appears that it doesn't need to

tidal wren Oct 23, 2023, 3:50 PM

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well then there ygo

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though I wonder whether that assembly needs to do any setup then

uncut briar Oct 23, 2023, 3:56 PM

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it does do setup

tidal wren Oct 23, 2023, 3:56 PM

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fair enough, guess you're set then

uncut briar Oct 23, 2023, 4:05 PM

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not exactly
using the provided boundedFmt

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and this happens when using either FBA or fmt.bufPrint

final raptor Oct 23, 2023, 4:11 PM

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zig's compiler-rt isn't fully feature complete, so its likely missing some of the functions for smaller bit-width architectures

uncut briar Oct 23, 2023, 4:37 PM

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i think this is bugged?

wise meteor Oct 23, 2023, 4:41 PM

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if x is comptime known then the type of the second argument is something like struct { comptime u32 = x } so when you tell the third argument to use the same type, it breaks because you can't change the comptime value

#Does FixedBufferAllocator work on architectures below 32 bits?