fix(bb): unaligned SIMD store in Constantine x4 recoder (nightly debug segfault)

[AztecBot]

Summary

The Nightly Debug Build failed (exit 139 / SIGSEGV) in ecc_tests:

FAILED: barretenberg/cpp/scripts/run_test.sh ecc_tests PippengerConstantine.SimdX4MatchesScalarPathLanewise (code: 139)
timeout: the monitored command dumped core

The test crashes immediately at [ RUN ], in the debug build only.

Root cause

simd_u32x4_store in pippenger_constantine.hpp (introduced in #23562) stored the 4-wide vector with:

*reinterpret_cast<SimdU32x4*>(dst) = v;

SimdU32x4 is a uint32_t __attribute__((vector_size(16))) type with 16-byte natural alignment, so this is an aligned vector store (movdqa on SSE2). But dst comes from a std::array<uint32_t, 4> in the test/fuzzer, which is only 4-byte aligned. Storing 16 aligned bytes to a 4-byte-aligned address faults.

The helper's own comment already documented the intended lowering as the unaligned movups / st1 — the implementation just didn't match it. In optimized builds the stack layout happened to land the array on a 16-byte boundary (or the store got reassociated), so the fault only surfaced reliably at -O0 in the nightly debug build.

Fix

Use __builtin_memcpy, which does not assume over-alignment of dst. Codegen verified:

• -O2 x86 SSE2: a single movups %xmm0, (%rdi) — same intended unaligned store, no perf regression.
• -O0: spills via aligned stack slot then copies — no aligned store to dst, no fault.

The WASM path (wasm_v128_store, which is alignment-agnostic) is unchanged.

Verification (red/green, debug preset)

Reproduced and fixed in build-debug on the exact failing commit:

• Buggy aligned store → PippengerConstantine.SimdX4MatchesScalarPathLanewise exits 139 at [ RUN ] (matches CI).
• With the fix → all 6 PippengerConstantine.* tests pass.

The SIMD x4 helpers are currently only exercised by the unit test and fuzzer (not yet wired into scalar_multiplication.cpp), so this is a test/codegen correctness fix with no behavioural change to the MSM.

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Created by claudebox · group: slackbot