stage2: LLVM backend: implement `@tagName` for enums

Introduced a new AIR instruction: `tag_name`. Reasons to do this
instead of lowering it in Sema to a switch, function call, array
lookup, or if-else tower:
 * Sema is a bottleneck; do less work in Sema whenever possible.
 * If any optimization passes run, and the operand to becomes
   comptime-known, then it could change to have a comptime result
   value instead of lowering to a function or array or something which
   would then have to be garbage-collected.
 * Backends may want to choose to use a function and a switch branch,
   or they may want to use a different strategy.

Codegen for `@tagName` is implemented for the LLVM backend but not any
others yet.

Introduced some new `Type` tags:
 * `const_slice_u8_sentinel_0`
 * `manyptr_const_u8_sentinel_0`

The motivation for this was to make typeof() on the tag_name AIR
instruction non-allocating.

A bunch more enum tests are passing now.

1 files changed, 138 insertions, 127 deletions

diff --git a/src/arch/aarch64/CodeGen.zig b/src/arch/aarch64/CodeGen.zig
index 0142e6abf8..1a7105da31 100644
--- a/src/arch/aarch64/CodeGen.zig
+++ b/src/arch/aarch64/CodeGen.zig
@@ -504,133 +504,134 @@ fn genBody(self: *Self, body: []const Air.Inst.Index) InnerError!void {
 
         switch (air_tags[inst]) {
             // zig fmt: off
-                    .add, .ptr_add   => try self.airAdd(inst),
-                    .addwrap         => try self.airAddWrap(inst),
-                    .add_sat         => try self.airAddSat(inst),
-                    .sub, .ptr_sub   => try self.airSub(inst),
-                    .subwrap         => try self.airSubWrap(inst),
-                    .sub_sat         => try self.airSubSat(inst),
-                    .mul             => try self.airMul(inst),
-                    .mulwrap         => try self.airMulWrap(inst),
-                    .mul_sat         => try self.airMulSat(inst),
-                    .rem             => try self.airRem(inst),
-                    .mod             => try self.airMod(inst),
-                    .shl, .shl_exact => try self.airShl(inst),
-                    .shl_sat         => try self.airShlSat(inst),
-                    .min             => try self.airMin(inst),
-                    .max             => try self.airMax(inst),
-                    .slice           => try self.airSlice(inst),
-
-                    .add_with_overflow => try self.airAddWithOverflow(inst),
-                    .sub_with_overflow => try self.airSubWithOverflow(inst),
-                    .mul_with_overflow => try self.airMulWithOverflow(inst),
-                    .shl_with_overflow => try self.airShlWithOverflow(inst),
-
-                    .div_float, .div_trunc, .div_floor, .div_exact => try self.airDiv(inst),
-
-                    .cmp_lt  => try self.airCmp(inst, .lt),
-                    .cmp_lte => try self.airCmp(inst, .lte),
-                    .cmp_eq  => try self.airCmp(inst, .eq),
-                    .cmp_gte => try self.airCmp(inst, .gte),
-                    .cmp_gt  => try self.airCmp(inst, .gt),
-                    .cmp_neq => try self.airCmp(inst, .neq),
-
-                    .bool_and => try self.airBoolOp(inst),
-                    .bool_or  => try self.airBoolOp(inst),
-                    .bit_and  => try self.airBitAnd(inst),
-                    .bit_or   => try self.airBitOr(inst),
-                    .xor      => try self.airXor(inst),
-                    .shr      => try self.airShr(inst),
-
-                    .alloc           => try self.airAlloc(inst),
-                    .ret_ptr         => try self.airRetPtr(inst),
-                    .arg             => try self.airArg(inst),
-                    .assembly        => try self.airAsm(inst),
-                    .bitcast         => try self.airBitCast(inst),
-                    .block           => try self.airBlock(inst),
-                    .br              => try self.airBr(inst),
-                    .breakpoint      => try self.airBreakpoint(),
-                    .ret_addr        => try self.airRetAddr(),
-                    .fence           => try self.airFence(),
-                    .call            => try self.airCall(inst),
-                    .cond_br         => try self.airCondBr(inst),
-                    .dbg_stmt        => try self.airDbgStmt(inst),
-                    .fptrunc         => try self.airFptrunc(inst),
-                    .fpext           => try self.airFpext(inst),
-                    .intcast         => try self.airIntCast(inst),
-                    .trunc           => try self.airTrunc(inst),
-                    .bool_to_int     => try self.airBoolToInt(inst),
-                    .is_non_null     => try self.airIsNonNull(inst),
-                    .is_non_null_ptr => try self.airIsNonNullPtr(inst),
-                    .is_null         => try self.airIsNull(inst),
-                    .is_null_ptr     => try self.airIsNullPtr(inst),
-                    .is_non_err      => try self.airIsNonErr(inst),
-                    .is_non_err_ptr  => try self.airIsNonErrPtr(inst),
-                    .is_err          => try self.airIsErr(inst),
-                    .is_err_ptr      => try self.airIsErrPtr(inst),
-                    .load            => try self.airLoad(inst),
-                    .loop            => try self.airLoop(inst),
-                    .not             => try self.airNot(inst),
-                    .ptrtoint        => try self.airPtrToInt(inst),
-                    .ret             => try self.airRet(inst),
-                    .ret_load        => try self.airRetLoad(inst),
-                    .store           => try self.airStore(inst),
-                    .struct_field_ptr=> try self.airStructFieldPtr(inst),
-                    .struct_field_val=> try self.airStructFieldVal(inst),
-                    .array_to_slice  => try self.airArrayToSlice(inst),
-                    .int_to_float    => try self.airIntToFloat(inst),
-                    .float_to_int    => try self.airFloatToInt(inst),
-                    .cmpxchg_strong  => try self.airCmpxchg(inst),
-                    .cmpxchg_weak    => try self.airCmpxchg(inst),
-                    .atomic_rmw      => try self.airAtomicRmw(inst),
-                    .atomic_load     => try self.airAtomicLoad(inst),
-                    .memcpy          => try self.airMemcpy(inst),
-                    .memset          => try self.airMemset(inst),
-                    .set_union_tag   => try self.airSetUnionTag(inst),
-                    .get_union_tag   => try self.airGetUnionTag(inst),
-                    .clz             => try self.airClz(inst),
-                    .ctz             => try self.airCtz(inst),
-                    .popcount        => try self.airPopcount(inst),
-
-                    .atomic_store_unordered => try self.airAtomicStore(inst, .Unordered),
-                    .atomic_store_monotonic => try self.airAtomicStore(inst, .Monotonic),
-                    .atomic_store_release   => try self.airAtomicStore(inst, .Release),
-                    .atomic_store_seq_cst   => try self.airAtomicStore(inst, .SeqCst),
-
-                    .struct_field_ptr_index_0 => try self.airStructFieldPtrIndex(inst, 0),
-                    .struct_field_ptr_index_1 => try self.airStructFieldPtrIndex(inst, 1),
-                    .struct_field_ptr_index_2 => try self.airStructFieldPtrIndex(inst, 2),
-                    .struct_field_ptr_index_3 => try self.airStructFieldPtrIndex(inst, 3),
-
-                    .switch_br       => try self.airSwitch(inst),
-                    .slice_ptr       => try self.airSlicePtr(inst),
-                    .slice_len       => try self.airSliceLen(inst),
-
-                    .ptr_slice_len_ptr => try self.airPtrSliceLenPtr(inst),
-                    .ptr_slice_ptr_ptr => try self.airPtrSlicePtrPtr(inst),
-
-                    .array_elem_val      => try self.airArrayElemVal(inst),
-                    .slice_elem_val      => try self.airSliceElemVal(inst),
-                    .slice_elem_ptr      => try self.airSliceElemPtr(inst),
-                    .ptr_elem_val        => try self.airPtrElemVal(inst),
-                    .ptr_elem_ptr        => try self.airPtrElemPtr(inst),
-
-                    .constant => unreachable, // excluded from function bodies
-                    .const_ty => unreachable, // excluded from function bodies
-                    .unreach  => self.finishAirBookkeeping(),
-
-                    .optional_payload           => try self.airOptionalPayload(inst),
-                    .optional_payload_ptr       => try self.airOptionalPayloadPtr(inst),
-                    .optional_payload_ptr_set   => try self.airOptionalPayloadPtrSet(inst),
-                    .unwrap_errunion_err        => try self.airUnwrapErrErr(inst),
-                    .unwrap_errunion_payload    => try self.airUnwrapErrPayload(inst),
-                    .unwrap_errunion_err_ptr    => try self.airUnwrapErrErrPtr(inst),
-                    .unwrap_errunion_payload_ptr=> try self.airUnwrapErrPayloadPtr(inst),
-
-                    .wrap_optional         => try self.airWrapOptional(inst),
-                    .wrap_errunion_payload => try self.airWrapErrUnionPayload(inst),
-                    .wrap_errunion_err     => try self.airWrapErrUnionErr(inst),
-                    // zig fmt: on
+            .add, .ptr_add   => try self.airAdd(inst),
+            .addwrap         => try self.airAddWrap(inst),
+            .add_sat         => try self.airAddSat(inst),
+            .sub, .ptr_sub   => try self.airSub(inst),
+            .subwrap         => try self.airSubWrap(inst),
+            .sub_sat         => try self.airSubSat(inst),
+            .mul             => try self.airMul(inst),
+            .mulwrap         => try self.airMulWrap(inst),
+            .mul_sat         => try self.airMulSat(inst),
+            .rem             => try self.airRem(inst),
+            .mod             => try self.airMod(inst),
+            .shl, .shl_exact => try self.airShl(inst),
+            .shl_sat         => try self.airShlSat(inst),
+            .min             => try self.airMin(inst),
+            .max             => try self.airMax(inst),
+            .slice           => try self.airSlice(inst),
+
+            .add_with_overflow => try self.airAddWithOverflow(inst),
+            .sub_with_overflow => try self.airSubWithOverflow(inst),
+            .mul_with_overflow => try self.airMulWithOverflow(inst),
+            .shl_with_overflow => try self.airShlWithOverflow(inst),
+
+            .div_float, .div_trunc, .div_floor, .div_exact => try self.airDiv(inst),
+
+            .cmp_lt  => try self.airCmp(inst, .lt),
+            .cmp_lte => try self.airCmp(inst, .lte),
+            .cmp_eq  => try self.airCmp(inst, .eq),
+            .cmp_gte => try self.airCmp(inst, .gte),
+            .cmp_gt  => try self.airCmp(inst, .gt),
+            .cmp_neq => try self.airCmp(inst, .neq),
+
+            .bool_and => try self.airBoolOp(inst),
+            .bool_or  => try self.airBoolOp(inst),
+            .bit_and  => try self.airBitAnd(inst),
+            .bit_or   => try self.airBitOr(inst),
+            .xor      => try self.airXor(inst),
+            .shr      => try self.airShr(inst),
+
+            .alloc           => try self.airAlloc(inst),
+            .ret_ptr         => try self.airRetPtr(inst),
+            .arg             => try self.airArg(inst),
+            .assembly        => try self.airAsm(inst),
+            .bitcast         => try self.airBitCast(inst),
+            .block           => try self.airBlock(inst),
+            .br              => try self.airBr(inst),
+            .breakpoint      => try self.airBreakpoint(),
+            .ret_addr        => try self.airRetAddr(),
+            .fence           => try self.airFence(),
+            .call            => try self.airCall(inst),
+            .cond_br         => try self.airCondBr(inst),
+            .dbg_stmt        => try self.airDbgStmt(inst),
+            .fptrunc         => try self.airFptrunc(inst),
+            .fpext           => try self.airFpext(inst),
+            .intcast         => try self.airIntCast(inst),
+            .trunc           => try self.airTrunc(inst),
+            .bool_to_int     => try self.airBoolToInt(inst),
+            .is_non_null     => try self.airIsNonNull(inst),
+            .is_non_null_ptr => try self.airIsNonNullPtr(inst),
+            .is_null         => try self.airIsNull(inst),
+            .is_null_ptr     => try self.airIsNullPtr(inst),
+            .is_non_err      => try self.airIsNonErr(inst),
+            .is_non_err_ptr  => try self.airIsNonErrPtr(inst),
+            .is_err          => try self.airIsErr(inst),
+            .is_err_ptr      => try self.airIsErrPtr(inst),
+            .load            => try self.airLoad(inst),
+            .loop            => try self.airLoop(inst),
+            .not             => try self.airNot(inst),
+            .ptrtoint        => try self.airPtrToInt(inst),
+            .ret             => try self.airRet(inst),
+            .ret_load        => try self.airRetLoad(inst),
+            .store           => try self.airStore(inst),
+            .struct_field_ptr=> try self.airStructFieldPtr(inst),
+            .struct_field_val=> try self.airStructFieldVal(inst),
+            .array_to_slice  => try self.airArrayToSlice(inst),
+            .int_to_float    => try self.airIntToFloat(inst),
+            .float_to_int    => try self.airFloatToInt(inst),
+            .cmpxchg_strong  => try self.airCmpxchg(inst),
+            .cmpxchg_weak    => try self.airCmpxchg(inst),
+            .atomic_rmw      => try self.airAtomicRmw(inst),
+            .atomic_load     => try self.airAtomicLoad(inst),
+            .memcpy          => try self.airMemcpy(inst),
+            .memset          => try self.airMemset(inst),
+            .set_union_tag   => try self.airSetUnionTag(inst),
+            .get_union_tag   => try self.airGetUnionTag(inst),
+            .clz             => try self.airClz(inst),
+            .ctz             => try self.airCtz(inst),
+            .popcount        => try self.airPopcount(inst),
+            .tag_name        => try self.airTagName(inst),
+
+            .atomic_store_unordered => try self.airAtomicStore(inst, .Unordered),
+            .atomic_store_monotonic => try self.airAtomicStore(inst, .Monotonic),
+            .atomic_store_release   => try self.airAtomicStore(inst, .Release),
+            .atomic_store_seq_cst   => try self.airAtomicStore(inst, .SeqCst),
+
+            .struct_field_ptr_index_0 => try self.airStructFieldPtrIndex(inst, 0),
+            .struct_field_ptr_index_1 => try self.airStructFieldPtrIndex(inst, 1),
+            .struct_field_ptr_index_2 => try self.airStructFieldPtrIndex(inst, 2),
+            .struct_field_ptr_index_3 => try self.airStructFieldPtrIndex(inst, 3),
+
+            .switch_br       => try self.airSwitch(inst),
+            .slice_ptr       => try self.airSlicePtr(inst),
+            .slice_len       => try self.airSliceLen(inst),
+
+            .ptr_slice_len_ptr => try self.airPtrSliceLenPtr(inst),
+            .ptr_slice_ptr_ptr => try self.airPtrSlicePtrPtr(inst),
+
+            .array_elem_val      => try self.airArrayElemVal(inst),
+            .slice_elem_val      => try self.airSliceElemVal(inst),
+            .slice_elem_ptr      => try self.airSliceElemPtr(inst),
+            .ptr_elem_val        => try self.airPtrElemVal(inst),
+            .ptr_elem_ptr        => try self.airPtrElemPtr(inst),
+
+            .constant => unreachable, // excluded from function bodies
+            .const_ty => unreachable, // excluded from function bodies
+            .unreach  => self.finishAirBookkeeping(),
+
+            .optional_payload           => try self.airOptionalPayload(inst),
+            .optional_payload_ptr       => try self.airOptionalPayloadPtr(inst),
+            .optional_payload_ptr_set   => try self.airOptionalPayloadPtrSet(inst),
+            .unwrap_errunion_err        => try self.airUnwrapErrErr(inst),
+            .unwrap_errunion_payload    => try self.airUnwrapErrPayload(inst),
+            .unwrap_errunion_err_ptr    => try self.airUnwrapErrErrPtr(inst),
+            .unwrap_errunion_payload_ptr=> try self.airUnwrapErrPayloadPtr(inst),
+
+            .wrap_optional         => try self.airWrapOptional(inst),
+            .wrap_errunion_payload => try self.airWrapErrUnionPayload(inst),
+            .wrap_errunion_err     => try self.airWrapErrUnionErr(inst),
+            // zig fmt: on
         }
         if (std.debug.runtime_safety) {
             if (self.air_bookkeeping < old_air_bookkeeping + 1) {
@@ -2546,6 +2547,16 @@ fn airMemcpy(self: *Self, inst: Air.Inst.Index) !void {
     return self.fail("TODO implement airMemcpy for {}", .{self.target.cpu.arch});
 }
 
+fn airTagName(self: *Self, inst: Air.Inst.Index) !void {
+    const un_op = self.air.instructions.items(.data)[inst].un_op;
+    const operand = try self.resolveInst(un_op);
+    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else {
+        _ = operand;
+        return self.fail("TODO implement airTagName for aarch64", .{});
+    };
+    return self.finishAir(inst, result, .{ un_op, .none, .none });
+}
+
 fn resolveInst(self: *Self, inst: Air.Inst.Ref) InnerError!MCValue {
     // First section of indexes correspond to a set number of constant values.
     const ref_int = @enumToInt(inst);