cc/arch: describe functions and their variables in DWARF

[jgarzik]

The abbreviation table held exactly one shape -- a compile unit with no children -- so a c17 binary described its files and nothing inside them. gdb fell back to the ELF symbol table for function names, which is why backtraces named functions while info args, info locals, print x and ptype all came back empty.

There are now DIEs for functions, for the parameters and locals inside them, and for the types they need: base types, pointers, arrays, and aggregates by size.

A variable's location is built from the same address computation the loads and stores go through -- stack_mem on x86-64, loc_addr_parts on aarch64 -- and emitted as DW_OP_breg<n> <offset>, naming the base register directly rather than going through DW_AT_frame_base. That keeps it true for an over-aligned frame, whose locals are not addressed from the frame pointer at all, and it means a location cannot disagree with the code that reads the variable.

(gdb) info locals
arr = {5, 0, 0, 0}
c = 5

A variable with no memory to point at gets no location rather than a made-up one, and gdb says "optimized out" on its own.

Two things bit while building it, both worth the comments they now carry. aarch64's register enum omits x18 -- platform-reserved, never allocated -- so every variant after X17 sits one below its register number, and a DWARF number taken from the discriminant made the frame pointer x29 come out as 28. And a DW_FORM_block1 states its own length, which cannot be measured between labels here: a label keyed by register and offset collides the moment two functions put a variable in the same place, which it did, in about three hundred tests.

Known gaps, both stated rather than papered over:

• A local that SSA promoted into registers has no stack home and so no location. At -O0 that is most of them, because c17 promotes at every optimization level. Describing those needs either location lists or a decision not to promote under -g; the second is a codegen change and wants its own commit.
• Aggregates carry DW_AT_byte_size and no members, so gdb prints a struct as {<No data fields>}. A member's name is a StringId and no string table reaches the backend, so member DIEs need the CodeGenerator trait widened first.

[Copilot]

Pull request overview

This PR enhances the cc backend’s DWARF emission so debuggers can see subprogram DIEs, variable DIEs (params + locals), and the minimal set of type DIEs needed to interpret them, with variable locations derived from the backend’s real addressing computations (x86-64 stack_mem, aarch64 loc_addr_parts).

Changes:

• Collect per-function debug metadata during codegen (function extent + locals/params + locations) and emit it into .debug_info.
• Expand DWARF abbreviation/info generation to include subprograms, variables (with/without locations), and basic/pointer/array/aggregate types.
• Add target-specific DWARF register numbering helpers and .sleb128 directive support; add tests covering subprogram DIE presence and location base register correctness.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cc/tests/codegen/debug_info.rs	Adds regression tests for subprogram DIE emission and correct base register usage in variable locations.
cc/arch/x86_64/regalloc.rs	Defines System V AMD64 DWARF register numbering for debug location expressions.
cc/arch/x86_64/frame.rs	Collects per-function DIE info (including variable locations) and emits a function-end label for DW_AT_high_pc.
cc/arch/x86_64/codegen.rs	Switches debug-info emission to pass collected function DIEs + unit info into DWARF generator.
cc/arch/lir.rs	Adds .sleb128 directive emission needed for DWARF location expressions.
cc/arch/dwarf.rs	Major DWARF expansion: new abbrev shapes, type DIE collection/emission, function/variable DIE emission, and SLEB128 length logic.
cc/arch/codegen.rs	Stores collected per-function debug DIEs in CodeGenBase.
cc/arch/aarch64/regalloc.rs	Defines AAPCS64 DWARF register numbering with explicit handling for the missing x18 enum variant.
cc/arch/aarch64/frame.rs	Collects per-function DIE info (including variable locations) and emits a function-end label for DW_AT_high_pc.
cc/arch/aarch64/codegen.rs	Switches debug-info emission to pass collected function DIEs + unit info into DWARF generator.

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