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turbo: set the Thumb bit on code pointers only when compiling for Thumb - #11361

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turbo: set the Thumb bit on code pointers only when compiling for Thumb#11361
mikeysklar wants to merge 1 commit into
adafruit:mainfrom
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@mikeysklar mikeysklar commented Sep 13, 2026 •
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What

Turbo needs this to support XTENSA and RISC-V.

Bit 0 is set only when __thumb__ is defined. Other targets get the pointer unchanged.

Why

Every native call uses this macro. On Xtensa and RISC-V, bit 0 lands one byte past the code and hard-faults.

MICROPY_MAKE_POINTER_CALLABLE
MicroPython py/mpconfig.h (p); each port overrides
MicroPython stm32, rp2, samd, nrf, mimxrt, renesas-ra | 1
MicroPython esp32, esp8266 plain cast
CircuitPython py/circuitpy_mpconfig.h:202, all ports | 1
Guard is __thumb__, not MICROPY_EMIT_THUMB RP2 loads native .mpy with the emitter off
Effect on main none: only used under MICROPY_ENABLE_NATIVE_CODE or MICROPY_EMIT_INLINE_ASM, which no non-Thumb board enables

Hardware tested

Build only. Main at 42af95d52e, GCC 15.2.1, Ubuntu 24.04.

Board main this PR bytes differing
Metro M4 AirLift Lite 488,912 B 488,912 B 35, version strings and one merged-string pointer ("e")
Metro RP2350 924,332 B 924,332 B 44, version strings only

How I tested it

Built each board at main and at this PR, then compared firmware.bin byte by byte.

build-pr3cmp-main-metro_m4_airlift_lite 488912 B vs build-pr3cmp-pr3-metro_m4_airlift_lite 488912 B, bytes differing: 35
< 3.4.0; CircuitPython 10.4.0-alpha.1-3-g42af95d52e on 2026-09-12
> 3.4.0; CircuitPython 10.4.0-alpha.1-4-g0b83ba43dc on 2026-09-12
build-pr3cmp-main-adafruit_metro_rp2350 924332 B vs build-pr3cmp-pr3-adafruit_metro_rp2350 924332 B, bytes differing: 44
< 10.4.0-alpha.1-3-g42af95d52e
> 10.4.0-alpha.1-4-g0b83ba43dc

Scope

Needed before native .mpy can run on Espressif. No functional change on any board in main.

AI assistance

Claude Code was used.

@mikeysklar @claude
py: set the Thumb bit on code pointers only when compiling for Thumb
0b83ba4 
On Xtensa and RISC-V, bit 0 makes native calls jump to an odd address.
ARM needs it for Thumb.

Set it only when __thumb__ is defined. RP2 loads native .mpy with the
emitter off, so not MICROPY_EMIT_THUMB.

No change on main: no non-ARM port runs native code yet.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
@mikeysklar mikeysklar changed the title py: set the Thumb bit on code pointers only when compiling for Thumb turbo: set the Thumb bit on code pointers only when compiling for Thumb Sep 13, 2026
dhalbert
dhalbert approved these changes Sep 13, 2026
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@dhalbert dhalbert left a comment

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Thanks - makes sense. A little surprised we didn't hit this before.

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@dhalbert dhalbert dhalbert approved these changes

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@mikeysklar @dhalbert