sha2: add soft-compact backend#686
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…to#687) This PR is a slightly modified backport of [RustCrypto#686]. Instead of the configuration flag it introduces the `force-soft-compact` crate feature. [RustCrypto#686]: RustCrypto#686
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### Changed - Edition changed to 2024 and MSRV bumped to 1.85 ([#652]) - Relax MSRV policy and allow MSRV bumps in patch releases - Update to `digest` v0.11 - Replace type aliases with newtypes ([#678]) - `compress256` and `compress512` are moved to the `block_api` module ([#678]) - Implementation of the `SerializableState` trait ([#716]) - Configuration flags for backend selection ([#614], [#615], [#686], [#802]) ### Added - `alloc` crate feature ([#678]) - RISC-V scalar crypto extension support ([#614]) ### Removed - `asm`, `asm-aarch64`, `loongarch64_asm`, and `compress` crate features ([#542]) - `soft` crate feature ([#615]) - `force-soft-compact` crate feature ([#686]) - `std` crate feature ([#678]) [#542]: #542 [#614]: #614 [#615]: #615 [#652]: #652 [#678]: #678 [#686]: #686 [#716]: #716 [#802]: #802
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) sha2 0.11 moved the compact backend from the force-soft-compact cargo feature to a rustc cfg (RustCrypto/hashes#686), so the suites have been silently measuring the speed-oriented default since the 0.11 bump. Set sha2_backend_soft="compact" in all three harness configs — the right tradeoff for a size-measurement harness. Measured effect on the verify-minus-baseline .text deltas: - Cortex-M3, SHA-256 configs: ~13.1 -> ~6.5 KiB - RV32, 2048/SHA-256: 21.7 -> 9.5 KiB SHA-1 rows unchanged (sha1 0.11 has no compact soft variant). The README resource table predates this and the 0.4.0 stack; needs a refresh pass from a full suite run.
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The new backend results in a much smaller binary code (e.g. for SHA-256 on
thumbv7em-none-eabiwe get ~150 instructions vs ~1680 instructions, for SHA-512 it's ~330 vs ~7950) while being somewhat slower on non-embedded CPUs (e.g. for SHA-256 on my x86-64 laptop withopt-level=sit results in 289 MB/s vs 315 MB/s).We probably should also backport this PR to the
backports/digest-v0.10branch and release sha2 v0.10.9 with this addition.Closes #561