Motiejus Jakštys
4aa90373a3
Replace GCC/glibc extensions (__float128, strtof128, __uint128_t) with portable C11 code so astgen.c compiles with TCC and other C11 compilers. The f64 vs f128 decision uses an exact algebraic round-trip test: a decimal value m*10^e round-trips through f64 iff the odd part of m*5^e fits in 53 binary bits. This requires only integer arithmetic, no floating-point. For f128 encoding, strtold parses the value at 80-bit extended precision, then bit manipulation converts to IEEE 754 binary128 layout (both formats share 15-bit exponent; bottom 49 of 112 fraction bits are zero-padded). The big integer multiply-add path uses a portable mul64() helper instead of __uint128_t. 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 build fmt-zig0 test-zig0
Full test and static analysis with all supported compilers and valgrind (run before commit, takes a while):
zig build all-zig0 -Dvalgrind
Debugging tips
Test runs infinitely? Build the test program executable:
$ 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.