Motiejus Jakštys
d14607caef
Three issues fixed to enable the "return integer" test
(export fn f() u32 { return 42; }):
1. AIR tag storage: AirInstTag was a C enum (4 bytes) but the Zig-side
CAir expects u8 (matching Air.Inst.Tag = enum(u8)). The byte-for-byte
comparison read padding bytes instead of subsequent tags. Fixed by
storing tags as uint8_t in Air and Sema structs.
2. New ZIR_INST_RET_NODE handler: resolves the operand, coerces from
comptime_int to the function return type via coerceIntRef, and emits
AIR_INST_RET.
3. Return type resolution in zirFunc: reads the actual return type from
ZIR extra data (ret_ty_body_len == 1) instead of hardcoding void.
Also rewrites the AIR datas comparison to be tag-aware with canonical
ref renumbering, so structurally equivalent AIR compares equal even
when InternPool indices differ between C and Zig.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
About
zig0 aspires to be an interpreter of zig 0.15.2 written in C.
This is written with help from LLM:
- Lexer:
- Datastructures 100% human.
- Helper functions 100% human.
- Lexing functions 50/50 human/bot.
- Parser:
- Datastructures 100% human.
- Helper functions 50/50.
- Parser functions 5/95 human/bot.
- AstGen: TBD.
Testing
Quick test:
./zig-out/bin/zig build fmt-zig0 test-zig0
Full test and static analysis with all supported compilers and valgrind (run before commit, takes a while):
./zig-out/bin/zig build all-zig0 -Dvalgrind
Debugging tips
Test runs infinitely? Build the test program executable:
$ ./zig-out/bin/zig build test-zig0 -Dzig0-no-exec
And then run it, capturing the stack trace:
gdb -batch \
-ex "python import threading; threading.Timer(1.0, lambda: gdb.post_event(lambda: gdb.execute('interrupt'))).start()" \
-ex run \
-ex "bt full" \
-ex quit \
zig-out/bin/test
You are welcome to replace -ex "bt full" with anything other of interest.
Float handling
Float literals are parsed with strtold() (C11 standard, portable). On
x86-64 Linux, long double is 80-bit extended precision (63 fraction bits).
When a float doesn't round-trip through f64, it's emitted as f128 (ZIR
float128 instruction). The 80-bit extended value is converted to IEEE 754
binary128 encoding by bit manipulation — both formats share the same 15-bit
exponent with bias 16383. The top 63 of binary128's 112 fraction bits come
from the 80-bit value; the bottom 49 are zero-padded.
This means float128 literals lose ~49 bits of precision compared to the
upstream Zig implementation (which uses native f128). This is acceptable
because stage0 is a bootstrap tool — the real Zig compiler re-parses all
source with full f128 precision in later stages. The test comparison mask
in astgen_test.zig skips float128 payloads to account for this.
Previous approach used __float128/strtof128 (GCC/glibc extensions) for
full precision, but these are not portable to TCC and other C11 compilers.
Rebuilding zig
If you need to rebuild zig-out/bin/zig for some reason, here's how:
~/code/zig-bootstrap/out-0.15.2/zig-x86_64-linux-musl-x86_64_v3/zig build \
--zig-lib-dir lib/ -Dtarget=x86_64-linux-musl -Dcpu=x86_64_v3 \
-Dstatic-llvm --search-prefix \
$HOME/code/zig-bootstrap/out-0.15.2/x86_64-linux-musl-x86_64_v3/ \
-Ddebug-extensions=true -Dlog=false -Doptimize=ReleaseSafe