Metal : writing to a field with 3 or less ti.i8 elements causes overwrites

[Mirebeau]

Describe the bug

Writing to a field with 3 or less ti.i8 elements leads to overwrites on other fields.

To Reproduce

Log/Screenshots

$ python BugFieldMetal.py
[Taichi] version 1.7.4, llvm 15.0.7, commit b4b956fd, osx, python 3.13.7
[Taichi] Starting on arch=metal
[W 11/03/25 18:59:07.458 17818148] TaichiWarning
While compiling `ext_arr_to_tensor_c20_0`, File "/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/lib/python3.13/site-packages/taichi/_kernels.py", line 146, in ext_arr_to_tensor:
        tensor[I] = arr[I - offset]
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Assign may lose precision: i32 <- i64
a = [1 2 3] , b =  [1 0 0]
[W 11/03/25 18:59:07.493 17818148] TaichiWarning
While compiling `set_b_c80_0`, File "/Users/jean-mariemirebeau/Dropbox/Programmes/GithubM1/AGDT/AGDT_NotebooksDev/Notebooks_Algo/InPreparation_Scripts/BugFieldMetal.py", line 11, in set_b:
def set_b(): b[1] = -1
             ^^^^^^^^^
Assign may lose precision: i8 <- i32
a = [65281     2     3] , b =  [ 1 -1  0]

Additional comments

ti diagnose
[Taichi] version 1.7.4, llvm 15.0.7, commit b4b956f, osx, python 3.13.7

---

** Taichi Programming Language **

---

Docs: https://docs.taichi-lang.org/
GitHub: https://github.com/taichi-dev/taichi/
Forum: https://forum.taichi.graphics/

Taichi system diagnose:

python: 3.13.7 | packaged by conda-forge | (main, Sep 3 2025, 14:24:46) [Clang 19.1.7 ]
system: darwin
executable: /Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/bin/python3.13
platform: macOS-15.6.1-arm64-arm-64bit-Mach-O
architecture: 64bit Mach-O
uname: uname_result(system='Darwin', node='MacBook-Pro-de-Jean-Marie.local', release='24.6.0', version='Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:29 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6000', machine='arm64')
/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/lib/python3.13/site-packages/taichi/tools/diagnose.py:20: DeprecationWarning: 'locale.getdefaultlocale' is deprecated and slated for removal in Python 3.15. Use setlocale(), getencoding() and getlocale() instead.
print(f'locale: {".".join(locale.getdefaultlocale())}')
locale: en_US.UTF-8
PATH: /Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/bin:/Users/jean-mariemirebeau/opt/miniconda3/condabin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Library/Apple/usr/bin:/usr/local/MacGPG2/bin:/Library/TeX/texbin:/Users/jean-mariemirebeau/.vscode/extensions/ms-python.debugpy-2025.14.1-darwin-arm64/bundled/scripts/noConfigScripts
PYTHONPATH: ['/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/bin', '/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/lib/python313.zip', '/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/lib/python3.13', '/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/lib/python3.13/lib-dynload', '/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/lib/python3.13/site-packages']

lsb_release not available: [Errno 2] No such file or directory: 'lsb_release'

import: <module 'taichi' from '/Users/jean-mariemirebeau/opt/miniconda3/envs/agdt_313/lib/python3.13/site-packages/taichi/init.py'>

cpu: True
metal: True
opengl: False
cuda: False
vulkan: True

glewinfo not available: [Errno 2] No such file or directory: 'glewinfo'

nvidia-smi not available: [Errno 2] No such file or directory: 'nvidia-smi'
[Taichi] version 1.7.4, llvm 15.0.7, commit b4b956f, osx, python 3.13.7

[Taichi] version 1.7.4, llvm 15.0.7, commit b4b956f, osx, python 3.13.7
[Taichi] Starting on arch=arm64

[W 11/03/25 19:04:15.128 17823841] [misc.py:adaptive_arch_select@758] Arch=[<Arch.opengl: 5>] is not supported, falling back to CPU
[Taichi] version 1.7.4, llvm 15.0.7, commit b4b956f, osx, python 3.13.7
[Taichi] Starting on arch=arm64

[W 11/03/25 19:04:15.408 17823880] [misc.py:adaptive_arch_select@758] Arch=[<Arch.cuda: 3>] is not supported, falling back to CPU
[Taichi] version 1.7.4, llvm 15.0.7, commit b4b956f, osx, python 3.13.7
[Taichi] Starting on arch=arm64

[Taichi] version 1.7.4, llvm 15.0.7, commit b4b956f, osx, python 3.13.7

---

** Taichi Programming Language **

---

Docs: https://docs.taichi-lang.org/
GitHub: https://github.com/taichi-dev/taichi/
Forum: https://forum.taichi.graphics/

                               TAICHI EXAMPLES                                    

────────────────────────────────────────────────────────────────────────────────────
0: ad_gravity 25: karman_vortex_street 50: patterns
1: circle_packing_image 26: keyboard 51: pbf2d
2: comet 27: laplace 52: physarum
3: cornell_box 28: laplace_equation 53: poisson_disk_sampling
4: diff_sph 29: mandelbrot_zoom 54: print_offset
5: differential_evolution 30: marching_squares 55: rasterizer
6: euler 31: mass_spring_3d_ggui 56: regression
7: eulerfluid2d 32: mass_spring_game 57: sdf_renderer
8: explicit_activation 33: mass_spring_game_ggui 58: simple_derivative
9: export_mesh 34: mciso_advanced 59: simple_texture
10: export_ply 35: mgpcg 60: simple_uv
11: export_videos 36: mgpcg_advanced 61: snow_phaseField
12: fem128 37: minimal 62: stable_fluid
13: fem128_ggui 38: minimization 63: stable_fluid_ggui
14: fem99 39: mpm128 64: stable_fluid_graph
15: fractal 40: mpm128_ggui 65: taichi_bitmasked
16: fractal3d_ggui 41: mpm3d 66: taichi_dynamic
17: fullscreen 42: mpm3d_ggui 67: taichi_logo
18: game_of_life 43: mpm88 68: taichi_ngp
19: gui_image_io 44: mpm88_graph 69: taichi_sparse
20: gui_widgets 45: mpm99 70: texture_graph
21: implicit_fem 46: mpm_lagrangian_forces 71: tutorial
22: implicit_mass_spring 47: nbody 72: two_stream_instability
23: initial_value_problem 48: odop_solar 73: vortex_rings
24: jacobian 49: oit_renderer 74: waterwave
────────────────────────────────────────────────────────────────────────────────────
42
Running example minimal ...
[Taichi] Starting on arch=arm64
42.0

Running time: 0.12s

Consider attaching this log when maintainers ask about system information.

Running time: 2.99s

[Mirebeau]

The same problem arises if the size of the ti.i8 field is not a multiple of 4.
In addition, the "field.fill" method does not work.

import numpy as np
import taichi as ti
ti.init(arch=ti.gpu,debug=False) # arch = metal on MacOs 15.6.1. Bug both in debug and release mode

b = ti.field(ti.i8,7) # Bug disappears if the size of b is >=4
a = ti.field(ti.i32,3)
b.fill(2)
a.from_numpy(np.array([1,2,3]))

@ti.kernel
def set_b(): b[5] = -1

# Note that b is incompletely filled with "2"
print("a =",a.to_numpy(),", b = ",b.to_numpy()) # Output : a = [1 2 3] , b =  [2 2 2 2 1 0 0]
set_b()
# Note the side effect on a
print("a =",a.to_numpy(),", b = ",b.to_numpy()) #Output : a = [65281     2     3] , b =  [ 2  2  2  2  1 -1  0]