[PATCH] D156156: [clang] Implement constexpr evaluation for `__builtin_{add,sub}c`

[Daniel Bertalan via Phabricator via cfe-commits]

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GCC has gained support for these multiprecision arithmetic builtins in
[`r14-1896-g2b4e0415ad6`](https://gcc.gnu.org/g:2b4e0415ad6), and although they aren't explicitly specified
as such in the documentation, they are usable in a constexpr context.

      

This commit adds constexpr evaluation support to Clang to match GCC's
behavior. The implementation mirrors how the builtins are lowered to a
pair of `u{add,sub}.with.overflow` operations and the carryout is set to
1 if either of those result in an overflow.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D156156

Files:
  clang/include/clang/Basic/Builtins.def
  clang/lib/AST/ExprConstant.cpp
  clang/test/SemaCXX/builtins-multiprecision.cpp

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