[clang] [clang] Implement constexpr bit_cast for vectors (PR #66894)

[Richard Smith via cfe-commits]

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@@ -463,3 +463,38 @@ static_assert(bit_cast<long double>(ld539) == fivehundredandthirtynine, "");
 static_assert(round_trip<__int128_t>(34.0L));
 #endif
 }
+
+namespace test_vector {
+
+typedef unsigned uint2 __attribute__((vector_size(2 * sizeof(unsigned))));
+typedef char byte8 __attribute__((vector_size(sizeof(unsigned long long))));
+
+constexpr uint2 test_vector = { 0x0C05FEFE, 0xCAFEBABE };
+
+static_assert(bit_cast<unsigned long long>(test_vector) == (LITTLE_END
+                                                                ? 0xCAFEBABE0C05FEFE
+                                                                : 0x0C05FEFECAFEBABE), "");
+
+static_assert(round_trip<uint2>(0xCAFEBABE0C05FEFEULL), "");
+static_assert(round_trip<byte8>(0xCAFEBABE0C05FEFEULL), "");
+
+typedef bool bool8 __attribute__((ext_vector_type(8)));
+typedef bool bool9 __attribute__((ext_vector_type(9)));
+typedef bool bool17 __attribute__((ext_vector_type(17)));
+typedef bool bool32 __attribute__((ext_vector_type(32)));
+
+static_assert(bit_cast<unsigned char>(bool8{1,0,1,0,1,0,1,0}) == 0x55, "");
+static_assert(round_trip<bool8>(static_cast<unsigned char>(0)), "");
+static_assert(round_trip<bool8>(static_cast<unsigned char>(1)), "");
+static_assert(round_trip<bool8>(static_cast<unsigned char>(0x55)), "");
+
+static_assert(bit_cast<unsigned short>(bool9{1,1,0,1,0,1,0,1,0}) == 0xAB, "");
----------------
zygoloid wrote:

I believe this result should be endianness-dependent; for ppc64 I think we should give `0x1AA` instead.

https://github.com/llvm/llvm-project/pull/66894