[clang][bytecode] Use bitcasts to cast from integer to vector (#117547)

In C, a cast from an integer to a vector is a CK_BitCast. Implement this
using the same code we use for __builtin_bit_cast.

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -448,6 +448,10 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
 
     QualType SubExprTy = SubExpr->getType();
     std::optional<PrimType> FromT = classify(SubExprTy);
+    // Casts from integer to vectors in C.
+    if (FromT && CE->getType()->isVectorType())
+      return this->emitBuiltinBitCast(CE);
+
     std::optional<PrimType> ToT = classify(CE->getType());
     if (!FromT || !ToT)
       return false;
@@ -6494,8 +6498,23 @@ bool Compiler<Emitter>::emitBuiltinBitCast(const CastExpr *E) {
   }
 
   // Get a pointer to the value-to-cast on the stack.
-  if (!this->visit(SubExpr))
-    return false;
+  // For CK_LValueToRValueBitCast, this is always an lvalue and
+  // we later assume it to be one (i.e. a PT_Ptr). However,
+  // we call this function for other utility methods where
+  // a bitcast might be useful, so convert it to a PT_Ptr in that case.
+  if (SubExpr->isGLValue()) {
+    if (!this->visit(SubExpr))
+      return false;
+  } else if (std::optional<PrimType> FromT = classify(SubExpr)) {
+    unsigned TempOffset = allocateLocalPrimitive(
+        SubExpr, *FromT, /*IsConst=*/true, /*IsExtended=*/false);
+    if (!this->visit(SubExpr))
+      return false;
+    if (!this->emitSetLocal(*FromT, TempOffset, E))
+      return false;
+    if (!this->emitGetPtrLocal(TempOffset, E))
+      return false;
+  }
 
   if (!ToT || ToT == PT_Ptr) {
     if (!this->emitBitCastPtr(E))

clang/test/AST/ByteCode/c23.c

@@ -1,5 +1,8 @@
 // RUN: %clang_cc1 -std=c23 -fexperimental-new-constant-interpreter -verify=expected,both %s
 // RUN: %clang_cc1 -std=c23 -verify=ref,both %s
+// RUN: %clang_cc1 -std=c23 -triple=aarch64_be-linux-gnu -fexperimental-new-constant-interpreter -verify=expected,both %s
+// RUN: %clang_cc1 -std=c23 -triple=aarch64_be-linux-gnu -verify=ref,both %s
+
 
 typedef typeof(nullptr) nullptr_t;
 
@@ -23,5 +26,26 @@ char bar() {
   return ((struct S *)buffer)->c;
 }
 
-
 static_assert((nullptr_t){} == 0);
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#  define LITTLE_END 1
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#  define LITTLE_END 0
+#else
+#  error "huh?"
+#endif
+
+typedef unsigned char u8x4_t __attribute__((vector_size(4)));
+constexpr u8x4_t arg1 = (u8x4_t)0xCAFEBABE; // okay
+#if LITTLE_END
+static_assert(arg1[0] == 190);
+static_assert(arg1[1] == 186);
+static_assert(arg1[2] == 254);
+static_assert(arg1[3] == 202);
+#else
+static_assert(arg1[0] == 202);
+static_assert(arg1[1] == 254);
+static_assert(arg1[2] == 186);
+static_assert(arg1[3] == 190);
+#endif