perf(stable-api): inline encoding_get for MRI stable versions#730
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Avoid the dylib call to rb_enc_get_index for the common case by reading the encoding index directly from RBasic.flags. The inline index is stored in bits RUBY_ENCODING_SHIFT..+7 (= FL_USHIFT+10..+17). Only when the inline slot equals RUBY_ENCODING_INLINE_MAX (0x7f) — meaning the real index is stored out-of-line via an ivar — do we fall back to rb_enc_get_index. Matches CRuby's ENCODING_GET inline function semantics exactly. Tested parity with compiled C shim on ASCII/UTF-8. The C shim (compiled.c) and TruffleRuby's compiled-only path are unchanged — both continue to use ENCODING_GET macro or rb_enc_get_index respectively, since TruffleRuby's RBasic does not expose flags.
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Summary
Follow-up to #681. The initial implementation of
encoding_getdelegates to the exportedrb_enc_get_indexfunction on all calls — but for the vast majority of objects (any encoding with index < 0x7f, which covers ASCII, UTF-8, US-ASCII, and all commonly used encodings), the encoding index is stored inline inRBasic.flagsand can be read with no function call.This PR restores the fast path: read the index directly from flags, and only fall back to
rb_enc_get_indexwhen the inline slot reads asRUBY_ENCODING_INLINE_MAX (0x7f), which is the sentinel that CRuby uses to signal "look at the ivar instead."Semantics are unchanged — parity-tested against the compiled C shim.
Changes
crates/rb-sys/src/stable_api/ruby_{2_7,3_0,3_1,3_2,3_3,3_4,4_0}.rs— inline flag read + sentinel fallback.compiled.c/compiled.rs: C side already usesENCODING_GETmacro when defined, and TruffleRuby keeps itsrb_enc_get_indexfallback (noRBasic.flagsexposed).Test plan
test_encoding_get_ascii_string,test_encoding_get_utf8_string)🤖 Generated with Claude Code