llvm: truncate non ABI sized integers on load

Closes ziglang#11263
Closes ziglang#13480

Author: Vexu

src/codegen/llvm.zig

@@ -9699,6 +9699,15 @@ pub const FuncGen = struct {
             if (isByRef(info.pointee_type)) {
                 return self.loadByRef(ptr, info.pointee_type, ptr_alignment, info.@"volatile");
             }
+            if (info.pointee_type.isAbiInt()) {
+                const abi_size_bits = info.pointee_type.abiSize(target) * 8;
+                const abi_int_ty = self.dg.context.intType(@intCast(c_uint, abi_size_bits));
+                const llvm_inst = self.builder.buildLoad(abi_int_ty, ptr, "");
+                llvm_inst.setAlignment(ptr_alignment);
+                llvm_inst.setVolatile(ptr_volatile);
+                const elem_llvm_ty = try self.dg.lowerType(info.pointee_type);
+                return self.builder.buildTrunc(llvm_inst, elem_llvm_ty, "");
+            }
             const elem_llvm_ty = try self.dg.lowerType(info.pointee_type);
             const llvm_inst = self.builder.buildLoad(elem_llvm_ty, ptr, "");
             llvm_inst.setAlignment(ptr_alignment);

src/type.zig

@@ -4560,7 +4560,7 @@ pub const Type = extern union {
     pub fn isAbiInt(ty: Type) bool {
         return switch (ty.zigTypeTag()) {
             .Int, .Enum, .ErrorSet => true,
-            .Struct => ty.containerLayout() == .Packed,
+            .Struct, .Union => ty.containerLayout() == .Packed,
             else => false,
         };
     }

test/behavior.zig

@@ -116,6 +116,7 @@ test {
     _ = @import("behavior/bugs/13171.zig");
     _ = @import("behavior/bugs/13285.zig");
     _ = @import("behavior/bugs/13435.zig");
+    _ = @import("behavior/bugs/13480.zig");
     _ = @import("behavior/byteswap.zig");
     _ = @import("behavior/byval_arg_var.zig");
     _ = @import("behavior/call.zig");

test/behavior/bugs/13480.zig

@@ -0,0 +1,44 @@
+const builtin = @import("builtin");
+const std = @import("std");
+const expect = std.testing.expect;
+
+const BitSet = packed struct {
+    mask: u7,
+};
+
+const Display = struct {
+    // This array has to be large.  Using separate padding
+    // plus a one item array doesn't work.
+    digit_masks: [10]BitSet = undefined,
+};
+
+fn smash() void {
+    var stack: [0x500]u8 = undefined;
+    @memset(&stack, 0xFF, stack.len);
+}
+
+fn stackUp(display: Display) !void {
+    var space: [0x100]u8 = undefined;
+    try part2(display);
+    _ = space;
+}
+
+fn part2(display: Display) !void {
+    var mask_a = BitSet{
+        .mask = display.digit_masks[9].mask,
+    };
+    try expect(mask_a.mask == 0b0001000);
+}
+
+test {
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+
+    var display = Display{};
+    display.digit_masks[9] = .{ .mask = 0b0001000 };
+
+    smash();
+    try stackUp(display);
+}