在 2025-02-10 22:14, Jacek Caban 写道:
The MinGW variant of ARM64EC is still undefined, meaning we have the flexibility to decide how it should work. There is an argument that ARM64EC should support 80-bit long double, as full interoperability is a core principle of ARM64EC, and lacking it goes against that.What you're seeing with x87 FPU emulation is a function of the emulator itself. See also [1] for some interesting challenges that FEX had to address. In MSVC ARM64EC, this isn't directly exposed to ARM64EC code, at least from the compiler's perspective. Register mapping is well-defined [2], meaning it would theoretically be possible to access x87 FPU values in ARM64EC assembly by combining registers (e.g., R1 could be derived from x6 and a subset of x16).
On x86-32 (a.k.a. i686-w64-mingw32), the x87 FPU is used to return floating-point values from functions. And more importantly, it is the only hardware way to perform conversion between `long long` and `double`, and to load or store a `long long` [1].
This requires x87 to have been set up in 64-bit extended precision. However Windows initializes x87 to 63-bit double precision, so with MSVC such operations are subject to precision loss, and therefore have to be implemented in software.
We cannot give up x87 unless we make GCC and Clang stop generating such code. [1] https://gcc.godbolt.org/z/M6fYqarxc -- Best regards, LIU Hao
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