cbe: handle msvc struct casting quirk

MSVC can't explicitly cast a struct to a typedef of itself (ie. f128 to i128). Added a
set of macros to handle float casting, and to not produce a cast for this specific
case on MSVC. A better approach would probably be to know if the cast is redundant
and not do it.

lib/zig.h

@@ -1340,6 +1340,9 @@ static inline zig_i128 zig_sub_i128(zig_i128 lhs, zig_i128 rhs) {
 // TODO: Implement
 static zig_i128 zig_div_trunc_i128(zig_i128 lhs, zig_i128 rhs);
 
+// TODO: Implement
+static zig_u128 zig_div_trunc_u128(zig_u128 lhs, zig_u128 rhs);
+
 // TODO: Implement
 static zig_i128 zig_rem_i128(zig_i128 lhs, zig_i128 rhs);
 
@@ -1783,6 +1786,18 @@ typedef zig_i128 zig_f128;
 #define zig_as_special_f128(sign, name, arg, repr) repr
 #endif
 
+#define zig_cast_f16 (zig_f16)
+#define zig_cast_f32 (zig_f32)
+#define zig_cast_f64 (zig_f64)
+#define zig_cast_f80 (zig_f80)
+#define zig_cast_c_longdouble (zig_c_longdouble)
+
+#if _MSC_VER && !zig_has_f128
+#define zig_cast_f128
+#else
+#define zig_cast_f128 (zig_f128)
+#endif
+
 #define zig_has_c_longdouble 1
 typedef long double zig_c_longdouble;
 #define zig_as_c_longdouble(fp, repr) fp##l

src/codegen/c.zig

@@ -927,9 +927,9 @@ pub const DeclGen = struct {
                 };
                 const int_val = Value.initPayload(&int_val_pl.base);
 
-                try writer.writeByte('(');
-                try dg.renderTypecast(writer, ty);
-                try writer.writeByte(')');
+                try writer.writeAll("zig_cast_");
+                try dg.renderTypeForBuiltinFnName(writer, ty);
+                try writer.writeByte(' ');
                 if (std.math.isFinite(f128_val)) {
                     try writer.writeAll("zig_as_");
                     try dg.renderTypeForBuiltinFnName(writer, ty);
@@ -3344,16 +3344,27 @@ fn airIntCast(f: *Function, inst: Air.Inst.Index) !CValue {
         return CValue.none;
     }
 
+    const target = f.object.dg.module.getTarget();
     const operand = try f.resolveInst(ty_op.operand);
     try reap(f, inst, &.{ty_op.operand});
     const writer = f.object.writer();
     const inst_ty = f.air.typeOfIndex(inst);
     const local = try f.allocLocal(inst, inst_ty);
     try f.writeCValue(writer, local, .Other);
-    try writer.writeAll(" = (");
-    try f.renderTypecast(writer, inst_ty);
-    try writer.writeByte(')');
+    try writer.writeAll(" = ");
+    const cant_cast = inst_ty.isInt() and inst_ty.bitSize(target) > 64;
+    if (cant_cast) {
+        if (f.air.typeOf(ty_op.operand).bitSize(target) > 64) return f.fail("TODO: C backend: implement casting between types > 64 bits", .{});
+        try writer.writeAll("zig_as_");
+        try f.object.dg.renderTypeForBuiltinFnName(writer, inst_ty);
+        try writer.writeAll("(0, ");
+    } else {
+        try writer.writeByte('(');
+        try f.renderTypecast(writer, inst_ty);
+        try writer.writeByte(')');
+    }
     try f.writeCValue(writer, operand, .Other);
+    if (cant_cast) try writer.writeByte(')');
     try writer.writeAll(";\n");
     return local;
 }