[JSC] Add String#lastIndexOf optimizations

5efecc3

Source/WTF/wtf/SIMDHelpers.h

+ALWAYS_INLINE uint8_t findLastNonZeroIndex(SIMDRegister<uint8_t, N> mask)
+{
+    for (int i = N - 1; i >= 0; --i)
+        if (mask[i])
+            return static_cast<uint8_t>(i);
+    return UINT8_MAX;
+}

Adds JIT intrinsic support for String.prototype.lastIndexOf across DFG and FTL, mirroring the existing indexOf infrastructure. Introduces a StringLastIndexOf DFG node, FTL B3 lowering, and SIMD-accelerated reverse string search (SIMD::reverseFind, findLastNonZeroIndex) in WTF. The fromIndex semantics differ from forward search: NaN maps to +Infinity (not 0), and the search scans backward from min(fromIndex, len - searchLen).

Significance

Substantial new JIT code surface — new node semantics, new SIMD primitives, and fromIndex boundary logic that differs non-trivially from the forward-search path.

Audit directions

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a Aaaaaaaaaaaaa Aaaaaaaa Aaaaaaaaaaaaa Aaa a Aaaaaaaaa Aaaaa Aaaaaaaa a Aaaa a Aaaaaaaaaaa Aaaaaaaa Aaaaaaa Aaa Aaa Aaaa Aaaa Aaa Aaa Aaaaaaaaaaaaaa Aaaa Aaaa Aa Aaaaaaaaaaaa
a Aaaaaaaaaaaa Aaaa Aaaaaaaaaaaa Aaaaaa Aaaaaaaaaaaaaa Aaaa Aa Aaaaaaaaaaa Aaa Aaaa Aaaaaa Aaaaaaaa Aaaa Aaaaaaaaa Aaaaaaaaaaa Aaaaa Aaaaaaaaa Aaa Aaaaaa Aaaaaaa Aaaaa
a Aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa Aaaaaaaaaaaaaaa Aaaaaaaaaaaaaa Aaaaa Aaaaaaa Aa Aaa Aaaaaa a Aaaaa Aaaaaaa Aa Aa Aaaaaa Aaaaaaaaaaaaaa

🔒

New SIMD reverse-scan kernel and JIT fast path with non-trivial fromIndex boundary semantics — several edge cases warrant security investigation.

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