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

[Richard Smith via cfe-commits]

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@@ -10517,41 +10598,19 @@ bool VectorExprEvaluator::VisitCastExpr(const CastExpr *E) {
     return Success(Elts, E);
   }
   case CK_BitCast: {
-    // Evaluate the operand into an APInt we can extract from.
-    llvm::APInt SValInt;
-    if (!EvalAndBitcastToAPInt(Info, SE, SValInt))
+    APValue SVal;
+    if (!Evaluate(SVal, Info, SE))
       return false;
-    // Extract the elements
-    QualType EltTy = VTy->getElementType();
-    unsigned EltSize = Info.Ctx.getTypeSize(EltTy);
-    bool BigEndian = Info.Ctx.getTargetInfo().isBigEndian();
-    SmallVector<APValue, 4> Elts;
-    if (EltTy->isRealFloatingType()) {
-      const llvm::fltSemantics &Sem = Info.Ctx.getFloatTypeSemantics(EltTy);
-      unsigned FloatEltSize = EltSize;
-      if (&Sem == &APFloat::x87DoubleExtended())
-        FloatEltSize = 80;
-      for (unsigned i = 0; i < NElts; i++) {
-        llvm::APInt Elt;
-        if (BigEndian)
-          Elt = SValInt.rotl(i * EltSize + FloatEltSize).trunc(FloatEltSize);
-        else
-          Elt = SValInt.rotr(i * EltSize).trunc(FloatEltSize);
-        Elts.push_back(APValue(APFloat(Sem, Elt)));
-      }
-    } else if (EltTy->isIntegerType()) {
-      for (unsigned i = 0; i < NElts; i++) {
-        llvm::APInt Elt;
-        if (BigEndian)
-          Elt = SValInt.rotl(i*EltSize+EltSize).zextOrTrunc(EltSize);
-        else
-          Elt = SValInt.rotr(i*EltSize).zextOrTrunc(EltSize);
-        Elts.push_back(APValue(APSInt(Elt, !EltTy->isSignedIntegerType())));
-      }
-    } else {
+
+    if (!SVal.isInt() && !SVal.isFloat() && !SVal.isVector())
+      // Give up if the input isn't an int, float, or vector.  For example, we
+      // reject "(v4i16)(intptr_t)&a".
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zygoloid wrote:

Do we need this check? I think `handleRValueToRValueBitCast` rejects the same cases, and produces a useful diagnostic when doing so.

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