Fix undefined behavior in __pointer_get_max_taggable#11
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The expression `(1 << total)` uses an `int` literal, which causes undefined behavior when `total >= 31` (i.e. when the combined tag bits equal or exceed the width of `int`). Use `1ULL` instead to ensure the shift is well-defined for any valid tag configuration. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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May 4, 2026
| { | ||
| size_t total = POINTER_TAG_BITS_HI + POINTER_TAG_BITS_LO; | ||
| return (ptrtag_t)((1 << total) - 1); | ||
| return (ptrtag_t)((1ULL << total) - 1); |
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ptrtag_t is a uint32_t, should we instead be doing (ptrtag_t)1?
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Summary
__pointer_get_max_taggable()insrc/0xc/std/pointer.cDetails
The expression
(1 << total)on line 62 uses anintliteral1. In C, shifting anintby>= 31bits is undefined behavior (per C11 §6.5.7p4: the behavior is undefined if the right operand is greater than or equal to the width of the promoted left operand).While the current default configurations use
POINTER_TAG_BITS_HI=16andPOINTER_TAG_BITS_LO=3(total=19, which avoids UB), this function is part of a public API and could be used with configurations that have more tag bits. For example, a 32-bit target could legitimately use tag bits summing to 31 or more.The fix changes
(1 << total)to(1ULL << total), ensuring the shift is well-defined for any validtotalvalue (up to 64).Test plan
make test)🤖 Generated with Claude Code