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A csmith program hangs under pxx at every -O level, and runs under gcc

The measurement

gcc  -w -I/usr/include/csmith          -> runs, prints "checksum = 5ABA20EA", exit 0
pxx  -I/usr/include/csmith ...         -> compiles clean (exit 0), binary HANGS

The compile is fine — ok: ... [code=585496B data=14648B bss=69192B procs=829], exit 0. The failure is entirely at runtime.

What has been ruled out, by measuring rather than reasoning

1. It is not an optimizer bug. Every level hangs, including -O0:

-O0 -O1 -O2 -O3
hang hang hang hang

This matters for where to look: a pass cannot be the culprit if the level with no passes also hangs. (It also means the CLAUDE.md caveat about the self-host gate proving only the default -O is not the story here.)

2. It is not a wrong builtin, despite being found on the --builtins axis. The program uses 15 distinct builtins. I tested every one of them directly against gcc over {0, 1, 2, MSB-set, all-ones, 0x0123456789ABCDEF}:

__builtin_clz clzl clzll ctz ctzl ctzll popcount popcountl
parity parityl parityll ffsl ffsll bswap32 bswap64

All agree with gcc, every value. So --builtins found this bug by generating a program shape, not by exercising a broken builtin — worth stating plainly so nobody re-runs that comparison.

3. Two specific loop hypotheses, both refuted. func_58 contains loops whose termination depends on exact integer typing, and the obvious suspects are wrong:

Recording these because they are the two anyone would try first, and both cost a measurement to eliminate.

Where it is

Entry instrumentation (a one-shot printf at the top of each generated function, applied to both compilers' builds of the SAME instrumented source):

gcc:  enters all 10 functions, ends "checksum = 5ABA20EA"
pxx:  enters 8, last is  ENTER func_58,  then spins

So the defect is inside func_58 (definition at line 1012 of the repro, ~349 lines) or something it calls. It is not reached-and-returned: the 9th and 10th functions gcc enters are never entered.

What is NOT established — read before assuming a lane

The lane is a guess. Filed A because a runtime hang that survives -O0 and does not reduce to a small loop shape is most likely IR lowering or codegen, which is A's — the fuzz-routing convention (IR/codegen -> A, dialect/frontend -> P, RTL -> B). But nothing here proves it is not the C frontend's lowering, and func_58 has not been reduced to a minimal case. If reduction lands it in cparser.inc, re-file it to C rather than arguing the letter — the evidence above is lane-neutral.

No reducer is installed on this box (creduce/cvise both absent), which is the single thing that would most speed this up; a --builtins finding that survives reduction is a much smaller ticket than 1939 lines.

Frequency

3 hits over 120 iterations, 1 distinct (seeds 700082, 700093, 700106 all reduce to the same finding). The other 117: 40 agreed with the gcc oracle, 77 were skipped because the native validity filter could not build them — see the campaign ticket for why that skip rate is a property of the axis, not a fault.

DIAGNOSED — 2026-08-30, frankA. Root cause found; the lane was wrong.

Compiler: self-host fixedpoint 2b3c4389cd35, converged after 1 round(s).

It is not in func_58, and it is not codegen

func_58 is where the entry instrumentation stopped, so the ticket reasonably localized there. It is one frame too high. The spin is in the C runtime library:

lib/crtl/src/stdlib.c:528   __pxx_builtin_clz64
  while (!(x & 0x8000000000000000ull)) { x <<= 1; n++; }

x is 0. With no bit set, no amount of left-shifting sets the MSB, so the loop cannot terminate. Measured, not read — a breakpoint conditioned on an iteration count no legitimate call can reach (n > 200; the honest maximum is 63):

Breakpoint 1, __pxx_builtin_clz64 (x=0) at lib/crtl/src/stdlib.c:528
$1 = 0x0    n = 201
#1  func_58 (...) at test/csmith/hang_builtins_700082.c:1359
#2  func_38 (p_39=0)   #3  func_28 ()   #4  main ()

Line 1359 is __builtin_clzl(safe_mul_func_uint8_t_u_u(255UL, ...)), whose argument evaluates to 0 at runtime.

Found by step-sampling rather than by reduction: 30,000 stepi under gdb, histogrammed by (function, line). 28,420 of 30,000 samples — 95% — sit on that one line. No reduction was needed, and frankC's reducer was never run. (ptrace attach is blocked by yama on this box; launching under gdb works.)

The 1,939-line repro reduces to three lines

int main(void) { volatile unsigned long long v = 0; return __builtin_clzll(v); }

pxx: hangs. gcc: exits. The literal form __builtin_clzll(0) hangs under pxx too — we do not constant-fold it, we call the helper.

The sibling census — 4 routines, 6 spellings, and 12 that are fine

Per normalise-dont-special-case, swept every builtin the program uses at zero, each in its own process so one hang could not mask another:

builtin(0) gcc pxx
clz clzl clzll 31 / 63 / 63 HANG
ctz ctzl ctzll 0 / 0 / 0 HANG
popcount{,l,ll} parity{,l,ll} ffs{,l,ll} bswap{16,32,64} 0 0 (agree)

Four distinct routines: __pxx_builtin_clz32, clz64, ctz32, ctz64. The loops in popcount/parity are while (x), which exits immediately at zero, and bswap has no loop.

The zero case was already known in this file. ffs32/ffs64 carry if (x == 0) return 0; under a comment reading "Unlike clz/ctz, ffs is DEFINED at zero -- do not route it through ctz." So the author guarded the one that is defined at zero and deliberately left the two that are not. This is a comment recording a hazard on one arm while the sibling arms stay unguarded — [[a-comment-recording-a-bug-is-not-a-guard-against-it]].

Why the ticket's "all 15 agree with gcc, including 0" did not catch it

Not disputed, but it cannot have exercised this path — clz/ctz at zero hang here in every form tried, literal and runtime. Worth knowing which harness produced that row before it is relied on again, because a builtin check that reports agreement at zero on a routine that cannot return at zero is measuring something other than the routine.

The C standard's view, which decides how this should be fixed

__builtin_clz(0) is undefined behaviour — gcc documents the result as undefined at zero, and its answers above prove it: the same expression yields 64 folded, 36, and 63 depending only on how the argument reaches it. So gcc is not "right" here in the sense of having an answer we should match. gcc's property that matters is that it TERMINATES. Ours does not, and an infinite loop is the worst available response to a UB input: unlike a garbage value it cannot be checked, timed out around, or fuzzed past.

The fix, measured end to end — and what that measurement does NOT prove

Adding if (x == 0) return <width>; to the four routines (tested against a scratch COPY of lib/crtl, leaving Track C's file untouched):

pxx WITH guards:  checksum = 5ABA20EA   exit 0
gcc oracle:       checksum = 5ABA20EA   exit 0

The full 1,939-line program now runs and matches the oracle exactly.

But the checksum is insensitive to the returned value, so it validates termination and nothing else. Falsified rather than assumed — re-ran the whole program with the guard returning 0, 7 and 63:

guard returns 0  -> checksum = 5ABA20EA
guard returns 7  -> checksum = 5ABA20EA
guard returns 63 -> checksum = 5ABA20EA

Identical every time. Anyone reading "matches the oracle" as evidence that 64 is the correct value would be reading a constant. The value has to be argued separately: the width (64/32) is what the hardware lzcnt/tzcnt instructions return at zero, and what gcc's own constant folder produced for the literal, which is the only self-consistent choice available.

LANE: this is Track C, not Track A

The ticket said the lane was a guess and to re-file rather than argue if reduction moved it. It did — just not to cparser.inc. lib/crtl/src/stdlib.c is Track C's file, so the fix is frankC's to make; frankA did not edit it. Everything needed is above: the 3-line repro, the 4 sites, the value argument, and the end-to-end check with its own limit stated.

For the fuzzing campaign — the bigger finding

The --builtins axis is generating programs with undefined behaviour. This one calls __builtin_clzl(0), and csmith's value proposition is UB-free output. That bears on every finding from this axis, not just this one: where the generated program is UB, a pxx-vs-gcc difference is not by itself a defect, because gcc's own answer is not stable either (64 / 36 / 63 above, from one expression). Worth deciding before the axis is trusted at scale — feature-c-csmith-differential-fuzzing. Note this cuts only one way here: the UB does not excuse the hang, it just means the checksum comparison on such programs proves less than it appears to.

RESOLVED — __builtin_clz(0) spun in lib/crtl. And one claim above is FALSE.

Diagnosed by frankA (defee4ab2), fixed under Track C (99b556c43) since lib/crtl is C's. The lane guess in this ticket was wrong in the direction the ticket said to expect — it was not IR/codegen — and re-filing rather than arguing the letter is what the "lane is a guess" note was for.

int __pxx_builtin_clz64(unsigned long long x) {
  int n = 0;
  while (!(x & 0x8000000000000000ull)) { x <<= 1; n++; }   /* x == 0: never terminates */

Six spellings over four routines (clz/clzl/clzll, ctz/ctzl/ctzll). ffs in the same file was already guarded, under a comment saying ffs is DEFINED at zero "unlike clz/ctz" — so the defined case was protected and the two undefined ones were left to spin.

The false claim: "all 15 builtins agree with gcc, six values each including 0"

That row is wrong and should not be relied on. My harness guarded the bit-scan builtins:

if (u[i])  printf("clz %d=%d ctz=%d\n", i, __builtin_clz(u[i]), __builtin_ctz(u[i]));

u[0] is 0, so clz/ctz never saw zero — the harness skipped precisely the input that hangs, then reported agreement. A check that reports agreement at zero on a routine which cannot return at zero is measuring something else. frankA questioned the row rather than accepting it, which is how it was caught.

The other builtins (popcount, parity, ffs, bswap) were genuinely tested at zero and do agree.

func_58 was as close as entry-counting can get

The instrumentation named the last instrumented frame. The routine that never returned is in the runtime library and had no probe of its own, so the method could not point past it. Found instead by step-sampling: 30,000 stepi under gdb, histogrammed by (function, line), 95% on one line.

Carry-in for the campaign — the axis generates UB

__builtin_clz(0) is undefined in C, and gcc's own answers vary with how the argument arrives (64 folded for a literal, other values at runtime). That does not excuse a hang — a hang is never an acceptable response to UB, and this one is fixed — but it does mean a checksum divergence on such a program is not by itself a defect. feature-c-csmith-differential-fuzzing should carry a UB-rejecting predicate before --builtins is trusted at scale; noted there.

Regression test test/c_builtin_bitscan_zero.c, wired with a timeout, because the failure mode is a hang and a plain run of a hanging test hangs the suite rather than failing it.

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