C backend: implement signed trunc

Author: rainbowbismuth

src/codegen/c.zig

@@ -1390,7 +1390,14 @@ fn airTrunc(f: *Function, inst: Air.Inst.Index) !CValue {
             return local;
         },
         .signed => {
-            return f.fail("TODO: C backend: implement trunc for signed integers", .{});
+            const operand_ty = f.air.typeOf(ty_op.operand);
+            const c_bits = toCIntBits(operand_ty.intInfo(target).bits) orelse
+                return f.fail("TODO: C backend: implement integer types larger than 128 bits", .{});
+            const shift_rhs = c_bits - dest_bits;
+            try writer.print("(int{d}_t)((uint{d}_t)", .{ c_bits, c_bits });
+            try f.writeCValue(writer, operand);
+            try writer.print(" << {d}) >> {d};\n", .{ shift_rhs, shift_rhs });
+            return local;
         },
     }
 }

test/behavior.zig

@@ -5,6 +5,7 @@ test {
     _ = @import("behavior/basic.zig");
     _ = @import("behavior/bool.zig");
     _ = @import("behavior/if.zig");
+    _ = @import("behavior/truncate.zig");
 
     if (builtin.object_format != .c) {
         // Tests that pass for stage1 and stage2 but not the C backend.
@@ -60,7 +61,6 @@ test {
         _ = @import("behavior/switch.zig");
         _ = @import("behavior/this.zig");
         _ = @import("behavior/translate_c_macros.zig");
-        _ = @import("behavior/truncate.zig");
         _ = @import("behavior/underscore.zig");
         _ = @import("behavior/union.zig");
         _ = @import("behavior/usingnamespace.zig");

test/behavior/basic.zig

@@ -27,8 +27,11 @@ test "truncate to non-power-of-two integers" {
     try testTrunc(u32, u1, 0b10110, 0b0);
     try testTrunc(u32, u2, 0b10101, 0b01);
     try testTrunc(u32, u2, 0b10110, 0b10);
-    // TODO add test coverage for this!
-    // try testTrunc(i32, i3, -4, -4);
+    try testTrunc(i32, i5, -4, -4);
+    try testTrunc(i32, i5, 4, 4);
+    try testTrunc(i32, i5, -28, 4);
+    try testTrunc(i32, i5, 28, -4);
+    try testTrunc(i32, i5, std.math.maxInt(i32), -1);
 }
 
 fn testTrunc(comptime Big: type, comptime Little: type, big: Big, little: Little) !void {