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

[via cfe-commits]

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@@ -7304,6 +7382,21 @@ class BufferToAPValueConverter {
     return ArrayValue;
   }
 
+  std::optional<APValue> visit(const VectorType *Ty, CharUnits Offset) {
+    SmallVector<uint8_t, 8> Bytes;
+    if (!Buffer.readObject(Offset, Info.Ctx.getTypeSizeInChars(Ty), Bytes))
+      return std::nullopt;
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DaMatrix wrote:

Hrm, this is indeed awkward: I obviously can't load the entire f80 vector at once, but loading individual elements at a time would not only force me to duplicate much of the code again, but would also make bool vectors break again without once again adding special handling (since we'd have to load individual bytes at a time again).

Can we simply assume the uninitialized padding bits are always zero? This test in `CodeGen/const-init.c` (the case `b`):  
https://github.com/llvm/llvm-project/blob/8768741800aae37a825864e2ee782484ed073ce9/clang/test/CodeGen/const-init.c#L138-L148  
which uses `CK_BitCast` seems to make that assumption, while a variety of the existing tests in `SemaCXX/constexpr-builtin-bit-cast.cpp`:  
https://github.com/llvm/llvm-project/blob/8768741800aae37a825864e2ee782484ed073ce9/clang/test/SemaCXX/constexpr-builtin-bit-cast.cpp#L332-L351  
assume that bit-casting padding space should return uninitialized values (or fail when the target type isn't allowed to be uninitialized in a constexpr context).

Is the `CK_BitCast` assumption simply wrong, or do these two different ways of triggering a bit cast really have such different semantics? I can see a few places in the actual LLVM constant folding code where vector bit casts containing uninitialized elements are, in fact, initialized to zero, but I feel like that makes sense there since it doesn't have to adhere to the rules of `constexpr`. Should I rewrite all the bit cast conversion code in `APValueToBufferConverter`/`BufferToAPValueConverter` again while taking uninitialized padding bytes into account, and redirect the existing `CK_BitCast` code to point to that?

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