[clang] Fix silent miscompile of template argument packs with 2^15 or more elements - #323
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[clang] Fix silent miscompile of template argument packs with 2^15 or more elements#323Cfretz244 wants to merge 1 commit into
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… more elements SubstNonTypeTemplateParmPackExpr stored its argument count in a 15-bit bitfield: a pack of 32768 elements wrapped to NumArguments == 0 and the expansion produced a bogus 'excess elements in array initializer' (counts wrapping to nonzero would silently mis-size instead). Reachable from plain C++ (a 32768-element NTTP pack via make_integer_sequence); trivially reachable through std::define_static_string / reflect_constant_string, where any string of >= 32K characters forms such a pack. Also hardens the same-family narrow pack fields: - SubstNonTypeTemplateParmExpr::PackIndex (index+1 encoding) is stored full-width; the adjacent Index:15 + Final:1 pack into 16 bits, so object size is unchanged on 64-bit hosts. - SubstTemplateTypeParmTypeBitfields::PackIndex widens 15 -> 26 bits (filling the 64-bit bitfield word) with a constructor assert. - UncommonTemplateNameStorage's 15-bit Data (pack index or stored-arg count for template template packs) asserts instead of silently wrapping. Adds a regression test exercising both sides of the old cliff (define_static_string at 32767 / 32768 / 50000). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Fixes #321.
A template argument pack of 2^15 or more elements silently miscompiled:
SubstNonTypeTemplateParmPackExprstored its argument count in a 15-bit bitfield, so a 32768-element pack wrapped toNumArguments == 0and the expansion produced a bogus "excess elements in array initializer" (nonzero wraps would silently mis-size). Reachable from plain C++ (a 32768-element NTTP pack viamake_integer_sequence); trivially reachable throughstd::define_static_string/reflect_constant_string, where any lifted string of 32K+ characters forms such a pack.Changes:
SubstNonTypeTemplateParmPackExpr::NumArgumentsstored full-width (the adjacentIndex/Finalbitfields still pack into 16 bits).SubstNonTypeTemplateParmExpr::PackIndex(index+1 encoding) full-width at no object-size cost on 64-bit hosts.SubstTemplateTypeParmTypeBitfields::PackIndexwidened 15 → 26 bits (fills the 64-bit bitfield word) + constructor assert.UncommonTemplateNameStorage's 15-bitData(pack index or stored-arg count for template template packs) asserts instead of silently wrapping.libcxx/test/std/experimental/reflection/define-static-string-large.pass.cppexercising both sides of the old cliff (32767 / 32768 / 50000 round-trips).Validated on a Release+assertions AArch64/macOS build of this branch: both repros (reflection + plain C++) fail at the base commit and pass with the fix;
clang/test/Reflectionparity with base; additionally exercised by a 36-library reflection-driven binding-generation corpus.Note for upstream-of-upstream: the narrow fields exist in
llvm/llvm-projecttoo, and the plain-C++ repro should reproduce there — reflection merely makes 32K+ packs easy to form. May be worth forwarding.