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simplify_count_zeroes validates DeBruijn CLZ tables by computing (1 << (data + 1)) - 1 to simulate the value produced by the OR-cascade b |= b >> 1; ... b |= b >> 32. For 64-bit input with data == 63 (the MSB bit), data + 1 equals HOST_BITS_PER_WIDE_INT, making the shift (HOST_WIDE_INT_1U << 64) undefined behavior. Hosts typically produce 0, so the check (0 * magic) >> 58 == 63 fails and check_table_array returns false. Every well-formed 64-bit DeBruijn CLZ table has an entry mapping the all-ones value to bit 63, so this UB rejected every such table -- including the magic 0x03f79d71b4cb0a89 used in Stockfish's msb(), zstd's bits.h, and cpython's pycore_bitutils.h. Fix by special-casing data + 1 == HOST_BITS_PER_WIDE_INT to use HOST_WIDE_INT_M1U. Only the 64-bit CLZ path is affected. gcc/ChangeLog: PR tree-optimization/122569 * tree-ssa-forwprop.cc (simplify_count_zeroes): Avoid shift-by-HOST_BITS_PER_WIDE_INT UB when computing the all-ones value for the CLZ validator. gcc/testsuite/ChangeLog: PR tree-optimization/122569 * gcc.dg/tree-ssa/pr122569-1.c: New test. * gcc.dg/tree-ssa/pr122569-2.c: New test.
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