Updates

[jgarzik]

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[Copilot]

Pull request overview

This PR implements two-register struct returns for structs between 9-16 bytes in size, following the System V AMD64 ABI and AAPCS64 specifications. Previously, all structs larger than 8 bytes used the sret convention (hidden pointer parameter); now structs 9-16 bytes are returned efficiently in two general-purpose registers (RAX+RDX on x86-64, X0+X1 on AArch64).

Key Changes:

• Increased max_aggregate_register_bits from 64 to 128 to enable two-register struct returns
• Added is_two_reg_return flag to track instructions that use two-register returns
• Implemented IR linearization logic to load 9-16 byte structs into two registers and emit return instructions with the new flag
• Added architecture-specific codegen for both x86-64 and AArch64 to handle storing/loading two-register returns

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
cc/target.rs	Updated max_aggregate_register_bits from 64 to 128 bits and revised comments to reflect two-register return support
cc/ir/mod.rs	Added is_two_reg_return field to Instruction struct to flag two-register struct returns
cc/ir/linearize.rs	Implemented logic for detecting 9-16 byte struct returns, loading struct data into two temporaries, and emitting return/call instructions with the two-register flag
cc/arch/x86_64/codegen.rs	Added x86-64 codegen for moving two-register returns (RAX+RDX) and storing them to stack locations
cc/arch/aarch64/codegen.rs	Added AArch64 codegen for moving two-register returns (X0+X1) and storing them to stack locations
cc/tests/datatypes/struct_type.rs	Added test cases for 16-byte structs (boundary case) and 24-byte structs (sret boundary)
cc/doc/C99.md	Removed "Multi-Register Struct Returns" and "Array Parameter Qualifiers" sections since multi-register returns are now implemented
cc/README.md	Removed "multi-register returns" from the list of unimplemented features

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