Robert Allan JamesandClaude Sonnet 5 1a716c8048 Fix d_compare: use ucell_t instead of unsigned long (FABRIC-3.md §XII.4) ...

d_compare() (double_words.c), the static helper backing DMAX/DMIN/D</D=,
had the same bare-unsigned-long pattern already fixed in D+/D-/DNEGATE
(commit bea8d74) -- 32-bit unsigned long on this aarch64 target silently
truncating the low-cell comparison. Switched to ucell_t.

Verified with four cases designed specifically to expose the old
truncation: two doubles sharing the same high cell but with low cells
differing only in bits 32-63 (invisible to a 32-bit-truncated compare,
e.g. 2^32 vs 0):
  4294967296 0 0 0 D= .            -> 0  (correctly not equal)
  4294967296 0 0 0 DMIN SWAP D.    -> 0  (correctly picks the smaller)
  0 0 4294967296 0 D< .            -> -1 (correct)
  4294967296 0 0 0 D< .            -> 0  (correct)
All four pass identically on amd64, aarch64, and riscv64. These cases
were never run before this fix -- the exerciser campaign never touched
DMAX/DMIN/D</D= at all, so this is the first real evidence d_compare()
was ever exercised on any architecture. Full 24-case exerciser also
reran clean on all three architectures (no regression).

D2*/D2/ use unsigned long long (C-standard-guaranteed >=64-bit) and were
never part of this bug class. All four affected words (D+, D-, DNEGATE,
d_compare) are now fixed; M*'s separate universal DOUBLE-OVERFLOW bug
remains open (unrelated defect, out of scope here).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EXieurDfDSsDFdnSyusuWo

2026-09-11 07:44:14 -04:00

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