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

[Richard Smith via cfe-commits]

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@@ -7441,6 +7500,28 @@ static bool handleLValueToRValueBitCast(EvalInfo &Info, APValue &DestValue,
   return true;
 }
 
+static bool handleLValueToRValueBitCast(EvalInfo &Info, APValue &DestValue,
+                                        APValue &SourceValue,
+                                        const CastExpr *BCE) {
+  assert(CHAR_BIT == 8 && Info.Ctx.getTargetInfo().getCharWidth() == 8 &&
+         "no host or target supports non 8-bit chars");
+  assert(SourceValue.isLValue() &&
+         "LValueToRValueBitcast requires an lvalue operand!");
+
+  if (!checkBitCastConstexprEligibility(&Info, Info.Ctx, BCE))
+    return false;
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zygoloid wrote:

Can we avoid doing this twice (once here and again in `handleRValueToRValueBitCast`? It does a recursive walk of the source and destination types so isn't all that cheap.

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