Project: modernc.org/libc (the emission side is cznic/ccgo#61 and cznic/ccgo#62) ## Environment - modernc.org/libc v1.75.7 on linux/amd64 (probes below) and as pinned by the originating project for darwin, linux and windows on amd64 and arm64. - ccgo v4.35.0 built with `go install modernc.org/ccgo/v4@v4.35.0`, Go 1.27.0, gcc 14.2.0 as the host compiler. ## Origin This report records findings made by the third-party project github.com/Tryanks/cpython-go, a ccgo transpilation of CPython 3.14 (commit ab9a153, 2026-09-05). The project is not mine; I am filing the findings so they are not lost. The lists of libm functions and the volatile-access case were checked here; the darwin and windows items are taken from the project's sources. ## 1. libm functions libpython/ccgo_shims.go of the project: // Compiler builtins the transpiled code calls that modernc.org/libc does not // provide. generator.go rewrites `iqlibc.X__builtin_<name>(` to // `_ccgo_builtin_<name>(`. func _ccgo_builtin_nextafter(tls *libc.TLS, x, y float64) float64 { return math.Nextafter(x, y) } func _ccgo_builtin_asinh(tls *libc.TLS, x float64) float64 { return math.Asinh(x) } func _ccgo_builtin_log1p(tls *libc.TLS, x float64) float64 { return math.Log1p(x) } func _ccgo_builtin_fma(tls *libc.TLS, x, y, z float64) float64 { return math.FMA(x, y, z) } Transpiling a file that calls most of <math.h> (u_libm_many.c in the reproducer directory) shows which functions ccgo rewrites to __builtin_ names that the libc package does not define (the generated Go then does not compile, see cznic/ccgo#62): acosh asinh asinhf atanh cbrt erf erfc exp2 expm1 fdim fmaf lgamma log1p log1pf logb nextafter nextafterf nexttoward remainder remquo rint scalbn tgamma For comparison, these are defined as X__builtin_<name> in libc v1.75.7 on linux/amd64: copysign fma fmax fmin hypot ilogb log2 round; and these are reached as plain X<name>: acos asin atan atan2 ceil cos cosh exp fabs floor fmod frexp ldexp log log10 modf pow sin sinh sqrt tan tanh. In the project's generated code the affected CPython call sites include _math_nextafter_impl (Modules/mathmodule.c), _cmath_acos_impl (Modules/cmathmodule.c, asinh) and, for windows, _dl_mul (Python/dtoa.c, fma). ## 2. Fortify and other builtins - __builtin___strncat_chk: defined by the project in libpython/ccgo_shims.go ("the _FORTIFY_SOURCE variant of strncat"); reached from expat (generated function _expat_default_handler) on darwin. - __builtin___snprintf_chk, __builtin___sprintf_chk, __builtin___vsnprintf_chk (darwin): routed by the project to its own formatter in libpython/printf.go together with printf, fprintf and vfprintf; listed in shimmedLibc["darwin"] of generator.go, whose comment says the darwin implementations "panic with TODO for arguments CPython uses". - __builtin_log2 (darwin): routed, same list. - __builtin_flt_rounds: generator.go passes -D__builtin_flt_rounds()=1 with the comment "FLT_ROUNDS expands to builtins libc does not provide." - __builtin_clzl (windows): "Windows C long is 32-bit even on amd64 and arm64. modernc's generic non-Linux implementation uses LeadingZeros64 for its uint32 ulong alias, making every nonzero result 32 too large." (libpython/libc_windows.go) - __builtin_snprintf and __mingw_vsnprintf (windows): routed to the project's formatter (libpython/printf_windows.go). ## 3. 8/16-bit atomic helpers for volatile accesses libpython/ccgo_shims.go: "Atomic loads missing from modernc.org/libc for 8/16 bit operands." The project defines _ccgo_AtomicLoadPUint8, _ccgo_AtomicLoadPUint16, _ccgo_AtomicStorePUint8 and _ccgo_AtomicStorePUint16 and rewrites the generated calls with sed: shell(gsed, "-i", `s/libc\.Atomic\(Load\|Store\)PUint\(8\|16\)(/_ccgo_Atomic\1PUint\2(/g`, result) The 32/64-bit variants (AtomicLoadPInt32, AtomicLoadPUint32, AtomicLoadPUint64, AtomicStorePInt32, AtomicStorePUint32, AtomicStorePUint64) are used by the same generated code and resolve. In the project the 8-bit calls come from the generated functions __tscmp (Modules/_operator.c, constant-time compare over volatile unsigned char) and _stack_overflow. Probe (v_volatile_subword.c): ```c #include <stdio.h> #include <stdint.h> static int tscmp(const volatile unsigned char *a, const volatile unsigned char *b, int n) { int r = 0; for (int i = 0; i < n; i++) r |= a[i] ^ b[i]; return r; } static volatile uint16_t flag16; static volatile uint8_t flag8; int main(void) { unsigned char x[3] = {1, 2, 3}, y[3] = {1, 2, 4}; flag16 = 300; flag8 = 7; printf("%d %d %d\n", tscmp(x, y, 3), flag16, flag8); return 0; } ``` gcc v_volatile_subword.c && ./a.out # prints: 7 300 7 ccgo -o v.go v_volatile_subword.c && go run v.go ccgo emits libc.AtomicLoadPUint8 (3 times), libc.AtomicLoadPUint16, libc.AtomicStorePUint8 and libc.AtomicStorePUint16; the Go compiler stops at the first missing one: out/v_volatile_subword.go:59:7: undefined: libc.AtomicStorePUint16 ## 4. Related - #52 (fixed 2026-07) concerned the failure path of the __atomic_compare_exchange* helpers; the missing 8/16-bit AtomicLoadP/AtomicStoreP functions above are a different family. - cznic/ccgo#61: __atomic_exchange_n, __atomic_compare_exchange_n and __atomic_thread_fence are undefined in ccgo; the project supplies all 62 _Py_atomic_* functions from CPython's pyatomic_gcc.h in Go (libpython/atomics.go). - cznic/ccgo#62: the missing libm functions are emitted with the internal qualifier iqlibc.