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Migrate the ARM entry point to global_asm!. #383

Merged
merged 2 commits into from
Feb 16, 2022
Merged

Migrate the ARM entry point to global_asm!. #383

merged 2 commits into from
Feb 16, 2022

Commits on Feb 15, 2022

  1. Migrate the ARM entry point to global_asm!. 

    Building `libtock_runtime` no longer requires an external toolchain.

Help Needed
-----------

This generates significantly different code for the entry point than the old
implementation. I don't have an ARM board to test with, or enough knowledge to
understand why the difference exists, so I need help testing/debugging this.
    @jrvanwhy
    jrvanwhy committed Feb 15, 2022
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  2. Tweak the ARM assembly to generate smaller instructions. 

    LLVM doesn't automatically pick the smallest instructions for the add/mov/sub mnemonics like GCC did. This means that moving to global_asm! made `start` larger. This change explicitly names the smaller instructions, and generates code of identical size to the GCC toolchain.

There is one remaining difference between the external assembly and this `global_asm!` implementation: LLVM translates `mov r5, r0` into `mov r5, r0`, whereas GCC translated it into `adds r5, r0, #0`.
    @jrvanwhy
    jrvanwhy committed Feb 15, 2022
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