`string_hash`'s tail-byte handling reads bytes through plain `char`,
whose signedness is implementation-defined. On signed-char platforms
(x86_64 Linux, macOS) bytes ≥ `0x80` are sign-extended before entering
the hash state (e.g. `c+=((unsigned)data[10]<<24);` becomes `c +=
0xFF80xxxx` instead of `c += 0x0080xxxx`); on unsigned-char platforms
(Linux aarch64) they are not. Hashes of byte strings containing such
bytes therefore differ across platforms.
This is not just cosmetic: `mpz_manager::hash` feeds the digit arrays of
large numerals through `string_hash`, so AST hashes of bit-vector
constants like `(_ bv36028797018963968 64)` (= 2^55, whose digit bytes
include `0x80`) differ between architectures. AST hashes determine hash
table layouts throughout the solver, so preprocessing and search take
platform-dependent paths for byte-identical input.
Observed impact (Z3 4.15.3 release binaries as well as local gcc-13
builds, identical `.smt2` input generated by CBMC from the
[mldsa-native](https://github.com/pq-code-package/mldsa-native)
verification suite — quantifiers + arrays + bit-vectors, ~120k lines):
| instance | x86_64 | aarch64 |
|---|---|---|
| instance A | 643 s | 11.7 s |
| instance B | 22.8 s | 1578 s |
Tracing AST construction on both hosts showed the first divergence at
the registration of the numeral `2^55`, whose node hash was `2587296535`
on x86_64 vs `808470355` on aarch64, with identical mpz digit arrays;
from that point on, hash table iteration orders (and consequently
`ctx-simplify` steps, quantifier instantiation order, and case-split
order) diverge. With this patch, instance A solves in ~21 s on x86_64
(down from 643 s), matching the aarch64 behaviour class.
The fix casts the tail bytes to `unsigned char`, matching the
4-byte-chunk path (which is already signedness-independent via `memcpy`
into `unsigned`). Note that hash values on signed-char platforms change
for inputs containing bytes ≥ `0x80` (pure-ASCII symbol names are
unaffected).
(Found while investigating cross-platform proof-time instability
reported in diffblue/cbmc#8991.)
Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>
* Initial plan
* Update z3 codebase to use std::string_view (except z3++.h)
- Updated params.cpp/h to use string_view internally for parameter descriptions
- Updated trace.h/cpp to accept string_view for trace tag functions
- Updated hash.h/cpp to use string_view for string_hash function
- Updated all callers of string_hash to use string_view
- Properly handled nullptr to empty string_view conversions
- All tests passing
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>
* Add missing string_view includes to headers
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>
---------
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>
