Refine generated instructions for staticly linked programs of Arm32

[DrXiao]

Previously, for the Arm architecture, the compiler generated the following sequence at the program entry point for staticly linked executables:

• a 'bl' instruction to call a global initialization function, and then execute the main function.
• a 'b' instruction.
• a sequence of instructions to invoke the '_exit' system all.

After further investigation, since the main function inherently invokes '_exit' upon completion, the entry sequence can be simplified. We now use a single 'b' instruction to perform an unconditional jump to the global initialization and the main routines.

Additionally, the implementation of the '__syscall' function has been corrected. Previously, it modified registers r4-r7 without preserving their original values for the caller. This commit ensures that these callee-saved registers are properly saved and restored.

Finally, This refinement improves the efficiency and correctness of the generated instructions for statically linked programs.

---

Summary by cubic

Simplifies ARM32 static entry by branching directly to global init + main, and fixes __syscall to preserve callee-saved registers. Fewer startup instructions and correct ABI behavior.

• Bug Fixes

  • __syscall now saves/restores r4–r7 with stmdb/ldm.
  • Extra args are loaded from the correct stack offsets after the push.

• Refactors

  • Replaced the entry sequence with a single b; removed explicit _exit code.
  • Recomputed ELF offsets to match the shorter static entry path.

^{Written for commit fc28ca9. Summary will update on new commits.}

[jserv]

Consolidate the comments, illustrating with Arm instructions.

[cubic-dev-ai]

1 issue found across 1 file

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/arm-codegen.c">

<violation number="1" location="src/arm-codegen.c:695">
P2: Dead instruction: `r7` is loaded from `[sp+28]` but immediately overwritten by `mov r7, r0` on the next line. This wastes one instruction (4 bytes in the binary) and reads from a stack slot that has no corresponding argument. Remove this load to match the actual 7-parameter signature of `__syscall`.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[jserv]

Thank @DrXiao for contributing!