Make powl() thread-safe (#222)#355
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powl() in ld80/e_powl.c kept its working scratch values (z, w, W, Wa, Wb, ya, yb, u) in file-scope `static` variables. Two threads calling powl() concurrently clobbered each other's scratch, so a call could return arbitrary garbage -- e.g. the reproducer from the issue, powl(0x8.779021e7c2f81b2p+14982L, -0x8.0021b03e1f821c9p-15097L), which is exactly 1, would instead come back as huge values or inf. Move that scratch to local automatic variables inside powl(). The `volatile` on z is preserved (as a local volatile) because it is load-bearing for the rounding/underflow behaviour, exactly like the twom10000 constant. The math is unchanged. reducl() and powil() already used local automatics and were unaffected. Add test/regression/test-222.c: it spawns 8 threads, each repeatedly computing a different power with a known-exact result, and fails if any call ever deviates. Before the fix this fails essentially every run; after it, it is solid. It skips (77) when long double is not the 80-bit type or when threads cannot be started. The regression Makefile rule gains -pthread (harmless for the single-threaded tests). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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good work! I'll check that with the next release |
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Fixes #222.
powlreturned garbage under concurrent calls — e.g. two threads computingpowl(0x8.779021e7c2f81b2p+14982L, -0x8.0021b03e1f821c9p-15097L)(exactly 1) got different huge/Inf values.Root cause
ld80/e_powl.ckeptpowl()'s working scratch in file-scopestaticmutable variables:Concurrent calls race on this shared state. (
reducl()/powil()already used locals; the read-only coefficient tables are fine.)Fix
Move that scratch to local automatic variables inside
powl()(preserving the load-bearingvolatileonzas a local). No math changes.Verification
make testpasses.test/regression/test-222.c(pthreads; ld80-only via#if LDBL_MANT_DIG == 64, so it builds/runs natively and compile-skips elsewhere). Proven to fail before / pass after.Note: touches
ld80/e_powl.cin a different region than #351 (#334's overflow guard); no conflict.