[mypyc] Specialize s[i] == 'x' to a codepoint int compare#21579
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VaggelisD wants to merge 1 commit into
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[mypyc] Specialize s[i] == 'x' to a codepoint int compare#21579VaggelisD wants to merge 1 commit into
s[i] == 'x' to a codepoint int compare#21579VaggelisD wants to merge 1 commit into
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Recognizes the AST shape `IndexExpr(str) == StrLiteral` (and the symmetric `StrLiteral == IndexExpr(str)`, plus the `!=` variants) and lowers it to an int compare of codepoints reusing the existing CPyStr_GetItemUnsafeAsInt primitive. Today the pattern lowers to CPyStr_GetItem + CPyStr_EqualLiteral, which allocates or looks up a 1-character PyUnicode object per iteration and goes through a generic string-equality call. After specialization it becomes an inlined PyUnicode_READ plus an int compare -- about 4x faster on bench_str_compare with a 3-compares-per-iteration workload, and closer to ~9x with the more typical 1-compare-per-iteration shape. No annotations required; benefits any code that compares a string index against a 1-character literal. Multi-character / empty literals fall through to the generic path (which still correctly returns False). Bounds checking is preserved -- the helper raises IndexError for out-of-range indices, same as the unspecialized path. Stack: builds on the `ord(s[i])` primitive (python#20578) and the librt.strings codepoint helpers (python#21462, python#21504, python#21509, python#21521, python#21522, python#21553).
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7th PR of #21418
Lowers
s[i] == 'x'(and the symmetric==/!=forms) down to a bounds-checked codepoint read + int compare, instead ofCPyStr_GetItem+CPyStr_EqualLiteralwhich (may) allocate a 1-characterPyUnicodeper iteration. No annotations are required for this optimization.On microbenchmarks (1-compare-per-iter hot loop, ~2.5M-codepoint SQL-like string) the comparison is ~3.6x times faster.
Some follow up optimizations that might be worth it I can work on:
s[i] in ('a', 'b', 'c')--> Fuse to one check with N int comparisonss[i] < 'x'--> Need to expand the op set