codegen.c: chunk array literals at 64 elements to reduce register pressure

[matz]

Summary

• Array literals (e.g. 100 elements) were consuming up to 99 registers by loading all values before constructing the array
• Restore 3.4-era chunking at GEN_LIT_ARY_MAX (64 elements), capping nregs at 64 for any-size array literal
• This fixes compatibility with mruby/c which has a smaller register limit

fixes #6731

Co-authored-by: Claude noreply@anthropic.com

[gemini-code-assist]

Summary of Changes

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This pull request optimizes the code generation process for array literals by re-implementing a chunking strategy. This modification ensures that large array literals do not consume an excessive number of registers, thereby improving performance and compatibility, particularly for systems with constrained register resources. The core change involves adjusting the limits and logic used during the compilation of array elements.

Highlights

• Register Pressure Reduction: Array literals are now chunked at 64 elements (GEN_LIT_ARY_MAX) during code generation, which caps the number of registers used for any-sized array literal at 64. This prevents excessive register consumption that previously occurred when loading all values before array construction.
• Compatibility Fix: This change restores a 3.4-era chunking mechanism, resolving compatibility issues with environments like mruby/c which have smaller register limits, specifically addressing issue Revert array construction behavior to 3.4 style (chunked via OP_ARYPUSH) #6731.

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Changelog

• mrbgems/mruby-compiler/core/codegen.c
  • Updated the initial stack limit variable slimit from GEN_VAL_STACK_MAX to GEN_LIT_ARY_MAX within the codegen_array function.
  • Modified the conditional logic that adjusts the slimit value, changing the comparison from cursp() >= GEN_LIT_ARY_MAX to cursp() >= slimit and the assigned value from INT16_MAX to GEN_VAL_STACK_MAX.

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[gemini-code-assist]

Code Review

This pull request addresses an issue with excessive register usage for large array literals by reintroducing chunking. The change restores behavior from an older version, capping the number of registers used and fixing compatibility with platforms like mruby/c. The logic appears sound and correctly implements the intended fix. I have one minor suggestion to improve code clarity.

[gemini-code-assist]

For better readability and to make the logic more self-contained, it's clearer to use GEN_LIT_ARY_MAX directly in this condition instead of slimit. This avoids having to look back to line 3824 to understand the condition's threshold.

  if (cursp() >= GEN_LIT_ARY_MAX) slimit = GEN_VAL_STACK_MAX;