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

[Richard Smith via cfe-commits]

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@@ -7095,6 +7096,40 @@ class APValueToBufferConverter {
     return true;
   }
 
+  bool visitVector(const APValue &Val, QualType Ty, CharUnits Offset) {
+    const auto *VT = Ty->castAs<VectorType>();
+    unsigned VectorLength = Val.getVectorLength();
+
+    if (VT->isExtVectorBoolType()) {
+      // Special handling for OpenCL bool vectors: we need to pack the vector
+      // of 1-bit unsigned integers into a single integer with the corresponding
+      // bits set, then write out the resulting integer.
+
+      CharUnits VecWidthBits = Info.Ctx.getTypeSizeInChars(VT) * 8;
+
+      APSInt Bits(VecWidthBits.getQuantity());
+      for (unsigned I = 0; I != VectorLength; ++I) {
+        const APValue &SubObj = Val.getVectorElt(I);
+        assert(SubObj.isInt() && "Bool vector element isn't an int?");
+        Bits.setBitVal(I, !SubObj.getInt().isZero());
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zygoloid wrote:

Hm, it concerns me that the documentation says: "When <N*M> isn’t evenly divisible by the byte size the exact memory layout is unspecified (just like it is for an integral type of the same size). This is because different targets could put the padding at different positions when the type size is smaller than the type’s store size."

Can we reliably constant-evaluate a `bit_cast` from a vector of a number of bits that's not a multiple of the byte size?

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