implement defining C variadic functions

doc/langref.html.in

@@ -8088,6 +8088,35 @@ test "main" {
       {#see_also|Import from C Header File|@cImport|@cDefine|@cInclude#}
       {#header_close#}
 
+      {#header_open|@cVaArg#}
+      <pre>{#syntax#}@cVaArg(operand: *std.builtin.VaList, comptime T: type) T{#endsyntax#}</pre>
+      <p>
+      Implements the C macro {#syntax#}va_arg{#endsyntax#}.
+      </p>
+      {#see_also|@cVaCopy|@cVaEnd|@cVaStart#}
+      {#header_close#}
+      {#header_open|@cVaCopy#}
+      <pre>{#syntax#}@cVaCopy(src: *std.builtin.VaList) std.builtin.VaList{#endsyntax#}</pre>
+      <p>
+      Implements the C macro {#syntax#}va_copy{#endsyntax#}.
+      </p>
+      {#see_also|@cVaArg|@cVaEnd|@cVaStart#}
+      {#header_close#}
+      {#header_open|@cVaEnd#}
+      <pre>{#syntax#}@cVaEnd(src: *std.builtin.VaList) void{#endsyntax#}</pre>
+      <p>
+      Implements the C macro {#syntax#}va_end{#endsyntax#}.
+      </p>
+      {#see_also|@cVaArg|@cVaCopy|@cVaStart#}
+      {#header_close#}
+      {#header_open|@cVaStart#}
+      <pre>{#syntax#}@cVaStart() std.builtin.VaList{#endsyntax#}</pre>
+      <p>
+      Implements the C macro {#syntax#}va_start{#endsyntax#}. Only valid inside a variadic function.
+      </p>
+      {#see_also|@cVaArg|@cVaCopy|@cVaEnd#}
+      {#header_close#}
+
       {#header_open|@divExact#}
       <pre>{#syntax#}@divExact(numerator: T, denominator: T) T{#endsyntax#}</pre>
       <p>
@@ -10802,14 +10831,32 @@ test "variadic function" {
 }
       {#code_end#}
       <p>
-        Non extern variadic functions are currently not implemented, but there
-        is an accepted proposal. See <a href="https://github.com/ziglang/zig/issues/515">#515</a>.
+        Variadic functions can be implemented using {#link|@cVaStart#}, {#link|@cVaEnd#}, {#link|@cVaArg#} and {#link|@cVaCopy#}
       </p>
-      {#code_begin|obj_err|non-extern function is variadic#}
-export fn printf(format: [*:0]const u8, ...) c_int {
-    _ = format;
+      {#code_begin|test|defining_variadic_function#}
+const std = @import("std");
+const testing = std.testing;
+const builtin = @import("builtin");
 
-    return 0;
+fn add(count: c_int, ...) callconv(.C) c_int {
+    var ap = @cVaStart();
+    defer @cVaEnd(&ap);
+    var i: usize = 0;
+    var sum: c_int = 0;
+    while (i < count) : (i += 1) {
+        sum += @cVaArg(&ap, c_int);
+    }
+    return sum;
+}
+
+test "defining a variadic function" {
+    // Variadic functions are currently disabled on some targets due to miscompilations.
+    if (builtin.cpu.arch == .aarch64 and builtin.os.tag != .windows and builtin.os.tag != .macos) return error.SkipZigTest;
+    if (builtin.cpu.arch == .x86_64 and builtin.os.tag == .windows) return error.SkipZigTest;
+
+    try std.testing.expectEqual(@as(c_int, 0), add(0));
+    try std.testing.expectEqual(@as(c_int, 1), add(1, @as(c_int, 1)));
+    try std.testing.expectEqual(@as(c_int, 3), add(2, @as(c_int, 1), @as(c_int, 2)));
 }
       {#code_end#}
       {#header_close#}

lib/std/builtin.zig

@@ -623,6 +623,87 @@ pub const CallModifier = enum {
     compile_time,
 };
 
+/// This data structure is used by the Zig language code generation and
+/// therefore must be kept in sync with the compiler implementation.
+pub const VaListAarch64 = extern struct {
+    __stack: *anyopaque,
+    __gr_top: *anyopaque,
+    __vr_top: *anyopaque,
+    __gr_offs: c_int,
+    __vr_offs: c_int,
+};
+
+/// This data structure is used by the Zig language code generation and
+/// therefore must be kept in sync with the compiler implementation.
+pub const VaListHexagon = extern struct {
+    __gpr: c_long,
+    __fpr: c_long,
+    __overflow_arg_area: *anyopaque,
+    __reg_save_area: *anyopaque,
+};
+
+/// This data structure is used by the Zig language code generation and
+/// therefore must be kept in sync with the compiler implementation.
+pub const VaListPowerPc = extern struct {
+    gpr: u8,
+    fpr: u8,
+    reserved: c_ushort,
+    overflow_arg_area: *anyopaque,
+    reg_save_area: *anyopaque,
+};
+
+/// This data structure is used by the Zig language code generation and
+/// therefore must be kept in sync with the compiler implementation.
+pub const VaListS390x = extern struct {
+    __current_saved_reg_area_pointer: *anyopaque,
+    __saved_reg_area_end_pointer: *anyopaque,
+    __overflow_area_pointer: *anyopaque,
+};
+
+/// This data structure is used by the Zig language code generation and
+/// therefore must be kept in sync with the compiler implementation.
+pub const VaListX86_64 = extern struct {
+    gp_offset: c_uint,
+    fp_offset: c_uint,
+    overflow_arg_area: *anyopaque,
+    reg_save_area: *anyopaque,
+};
+
+/// This data structure is used by the Zig language code generation and
+/// therefore must be kept in sync with the compiler implementation.
+pub const VaList = switch (builtin.cpu.arch) {
+    .aarch64 => switch (builtin.os.tag) {
+        .windows => *u8,
+        .ios, .macos, .tvos, .watchos => *u8,
+        else => @compileError("disabled due to miscompilations"), // VaListAarch64,
+    },
+    .arm => switch (builtin.os.tag) {
+        .ios, .macos, .tvos, .watchos => *u8,
+        else => *anyopaque,
+    },
+    .amdgcn => *u8,
+    .avr => *anyopaque,
+    .bpfel, .bpfeb => *anyopaque,
+    .hexagon => if (builtin.target.isMusl()) VaListHexagon else *u8,
+    .mips, .mipsel, .mips64, .mips64el => *anyopaque,
+    .riscv32, .riscv64 => *anyopaque,
+    .powerpc, .powerpcle => switch (builtin.os.tag) {
+        .ios, .macos, .tvos, .watchos, .aix => *u8,
+        else => VaListPowerPc,
+    },
+    .powerpc64, .powerpc64le => *u8,
+    .sparc, .sparcel, .sparc64 => *anyopaque,
+    .spirv32, .spirv64 => *anyopaque,
+    .s390x => VaListS390x,
+    .wasm32, .wasm64 => *anyopaque,
+    .x86 => *u8,
+    .x86_64 => switch (builtin.os.tag) {
+        .windows => @compileError("disabled due to miscompilations"), // *u8,
+        else => VaListX86_64,
+    },
+    else => @compileError("VaList not supported for this target yet"),
+};
+
 /// This data structure is used by the Zig language code generation and
 /// therefore must be kept in sync with the compiler implementation.
 pub const PrefetchOptions = struct {

src/Air.zig

@@ -741,6 +741,19 @@ pub const Inst = struct {
         /// Uses the `vector_store_elem` field.
         vector_store_elem,
 
+        /// Implements @cVaArg builtin.
+        /// Uses the `ty_op` field.
+        c_va_arg,
+        /// Implements @cVaCopy builtin.
+        /// Uses the `ty_op` field.
+        c_va_copy,
+        /// Implements @cVaEnd builtin.
+        /// Uses the `un_op` field.
+        c_va_end,
+        /// Implements @cVaStart builtin.
+        /// Uses the `ty` field.
+        c_va_start,
+
         pub fn fromCmpOp(op: std.math.CompareOperator, optimized: bool) Tag {
             switch (op) {
                 .lt => return if (optimized) .cmp_lt_optimized else .cmp_lt,
@@ -1092,6 +1105,7 @@ pub fn typeOfIndex(air: Air, inst: Air.Inst.Index) Type {
         .ret_ptr,
         .arg,
         .err_return_trace,
+        .c_va_start,
         => return datas[inst].ty,
 
         .assembly,
@@ -1156,6 +1170,8 @@ pub fn typeOfIndex(air: Air, inst: Air.Inst.Index) Type {
         .byte_swap,
         .bit_reverse,
         .addrspace_cast,
+        .c_va_arg,
+        .c_va_copy,
         => return air.getRefType(datas[inst].ty_op.ty),
 
         .loop,
@@ -1187,6 +1203,7 @@ pub fn typeOfIndex(air: Air, inst: Air.Inst.Index) Type {
         .prefetch,
         .set_err_return_trace,
         .vector_store_elem,
+        .c_va_end,
         => return Type.void,
 
         .ptrtoint,

src/AstGen.zig

@@ -42,6 +42,7 @@ string_table: std.HashMapUnmanaged(u32, void, StringIndexContext, std.hash_map.d
 compile_errors: ArrayListUnmanaged(Zir.Inst.CompileErrors.Item) = .{},
 /// The topmost block of the current function.
 fn_block: ?*GenZir = null,
+fn_var_args: bool = false,
 /// Maps string table indexes to the first `@import` ZIR instruction
 /// that uses this string as the operand.
 imports: std.AutoArrayHashMapUnmanaged(u32, Ast.TokenIndex) = .{},
@@ -3892,17 +3893,17 @@ fn fnDecl(
             .noalias_bits = noalias_bits,
         });
     } else func: {
-        if (is_var_args) {
-            return astgen.failTok(fn_proto.ast.fn_token, "non-extern function is variadic", .{});
-        }
-
         // as a scope, fn_gz encloses ret_gz, but for instruction list, fn_gz stacks on ret_gz
         fn_gz.instructions_top = ret_gz.instructions.items.len;
 
         const prev_fn_block = astgen.fn_block;
         astgen.fn_block = &fn_gz;
         defer astgen.fn_block = prev_fn_block;
 
+        const prev_var_args = astgen.fn_var_args;
+        astgen.fn_var_args = is_var_args;
+        defer astgen.fn_var_args = prev_var_args;
+
         astgen.advanceSourceCursorToNode(body_node);
         const lbrace_line = astgen.source_line - decl_gz.decl_line;
         const lbrace_column = astgen.source_column;
@@ -6071,15 +6072,15 @@ fn whileExpr(
             const tag: Zir.Inst.Tag = if (payload_is_ref) .is_non_err_ptr else .is_non_err;
             break :c .{
                 .inst = err_union,
-                .bool_bit = try cond_scope.addUnNode(tag, err_union, while_full.ast.then_expr),
+                .bool_bit = try cond_scope.addUnNode(tag, err_union, while_full.ast.cond_expr),
             };
         } else if (while_full.payload_token) |_| {
             const cond_ri: ResultInfo = .{ .rl = if (payload_is_ref) .ref else .none };
             const optional = try expr(&cond_scope, &cond_scope.base, cond_ri, while_full.ast.cond_expr);
             const tag: Zir.Inst.Tag = if (payload_is_ref) .is_non_null_ptr else .is_non_null;
             break :c .{
                 .inst = optional,
-                .bool_bit = try cond_scope.addUnNode(tag, optional, while_full.ast.then_expr),
+                .bool_bit = try cond_scope.addUnNode(tag, optional, while_full.ast.cond_expr),
             };
         } else {
             const cond = try expr(&cond_scope, &cond_scope.base, bool_ri, while_full.ast.cond_expr);
@@ -8384,6 +8385,46 @@ fn builtinCall(
             });
             return rvalue(gz, ri, result, node);
         },
+        .c_va_arg => {
+            if (astgen.fn_block == null) {
+                return astgen.failNode(node, "'@cVaArg' outside function scope", .{});
+            }
+            const result = try gz.addExtendedPayload(.c_va_arg, Zir.Inst.BinNode{
+                .node = gz.nodeIndexToRelative(node),
+                .lhs = try expr(gz, scope, .{ .rl = .none }, params[0]),
+                .rhs = try typeExpr(gz, scope, params[1]),
+            });
+            return rvalue(gz, ri, result, node);
+        },
+        .c_va_copy => {
+            if (astgen.fn_block == null) {
+                return astgen.failNode(node, "'@cVaCopy' outside function scope", .{});
+            }
+            const result = try gz.addExtendedPayload(.c_va_copy, Zir.Inst.UnNode{
+                .node = gz.nodeIndexToRelative(node),
+                .operand = try expr(gz, scope, .{ .rl = .none }, params[0]),
+            });
+            return rvalue(gz, ri, result, node);
+        },
+        .c_va_end => {
+            if (astgen.fn_block == null) {
+                return astgen.failNode(node, "'@cVaEnd' outside function scope", .{});
+            }
+            const result = try gz.addExtendedPayload(.c_va_end, Zir.Inst.UnNode{
+                .node = gz.nodeIndexToRelative(node),
+                .operand = try expr(gz, scope, .{ .rl = .none }, params[0]),
+            });
+            return rvalue(gz, ri, result, node);
+        },
+        .c_va_start => {
+            if (astgen.fn_block == null) {
+                return astgen.failNode(node, "'@cVaStart' outside function scope", .{});
+            }
+            if (!astgen.fn_var_args) {
+                return astgen.failNode(node, "'@cVaStart' in a non-variadic function", .{});
+            }
+            return rvalue(gz, ri, try gz.addNodeExtended(.c_va_start, node), node);
+        },
     }
 }
 

src/BuiltinFn.zig

@@ -30,6 +30,10 @@ pub const Tag = enum {
     compile_log,
     ctz,
     c_undef,
+    c_va_arg,
+    c_va_copy,
+    c_va_end,
+    c_va_start,
     div_exact,
     div_floor,
     div_trunc,
@@ -354,6 +358,30 @@ pub const list = list: {
                 .param_count = 1,
             },
         },
+        .{
+            "@cVaArg", .{
+                .tag = .c_va_arg,
+                .param_count = 2,
+            },
+        },
+        .{
+            "@cVaCopy", .{
+                .tag = .c_va_copy,
+                .param_count = 1,
+            },
+        },
+        .{
+            "@cVaEnd", .{
+                .tag = .c_va_end,
+                .param_count = 1,
+            },
+        },
+        .{
+            "@cVaStart", .{
+                .tag = .c_va_start,
+                .param_count = 0,
+            },
+        },
         .{
             "@divExact",
             .{

src/Liveness.zig

@@ -238,6 +238,7 @@ pub fn categorizeOperand(
         .wasm_memory_size,
         .err_return_trace,
         .save_err_return_trace_index,
+        .c_va_start,
         => return .none,
 
         .fence => return .write,
@@ -279,6 +280,8 @@ pub fn categorizeOperand(
         .splat,
         .error_set_has_value,
         .addrspace_cast,
+        .c_va_arg,
+        .c_va_copy,
         => {
             const o = air_datas[inst].ty_op;
             if (o.operand == operand_ref) return matchOperandSmallIndex(l, inst, 0, .none);
@@ -322,6 +325,7 @@ pub fn categorizeOperand(
         .trunc_float,
         .neg,
         .cmp_lt_errors_len,
+        .c_va_end,
         => {
             const o = air_datas[inst].un_op;
             if (o == operand_ref) return matchOperandSmallIndex(l, inst, 0, .none);
@@ -857,6 +861,7 @@ fn analyzeInst(
         .wasm_memory_size,
         .err_return_trace,
         .save_err_return_trace_index,
+        .c_va_start,
         => return trackOperands(a, new_set, inst, main_tomb, .{ .none, .none, .none }),
 
         .not,
@@ -898,6 +903,8 @@ fn analyzeInst(
         .splat,
         .error_set_has_value,
         .addrspace_cast,
+        .c_va_arg,
+        .c_va_copy,
         => {
             const o = inst_datas[inst].ty_op;
             return trackOperands(a, new_set, inst, main_tomb, .{ o.operand, .none, .none });
@@ -936,6 +943,7 @@ fn analyzeInst(
         .neg_optimized,
         .cmp_lt_errors_len,
         .set_err_return_trace,
+        .c_va_end,
         => {
             const operand = inst_datas[inst].un_op;
             return trackOperands(a, new_set, inst, main_tomb, .{ operand, .none, .none });