[libcxx-commits] [clang] [compiler-rt] [libcxx] Implement soft x87 `long double` for ARM64EC (PR #236406)

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Andarwinux wrote:

> The whole point of ARM64EC is to be interoperable with existing x86-64 code.

to be interoperable with existing (**MSVC**) x86-64 code.

If you change MinGW arm64ec long double to f80, this will not only be incompatible with MSVC x86-64, but also incompatible with MSVC arm64ec.

We need to consider the following facts:

1. MinGW's f80 long double is already incompatible with the Microsoft x64 (due to the well-known historical mistake)
2. MinGW's arm64ec support is a recent feature, but MSVC has supported for a very long time. This means most existing arm64ec code comes from MSVC.

https://github.com/llvm/llvm-project/pull/236406