Switch case with an inline if never evaluates to false

[musiKk]

Zig Version

0.9.0-dev.2004+1fa5a1959

Steps to Reproduce

Create a project with the following code in src/main.zig, then run with zig run src/main.zig.

const std = @import("std");

pub fn main() !void {
    var n: u8 = 0;
    while (n < 3) : (n += 1) {
        const r = switch (n) {
            0 => @as(u8, if (one() > 2) 10 else 20),
            1 => blk: {
                break :blk @as(u8, if (one() > 2) 10 else 20);
            },
            2 => blk: {
                const v = @as(u8, if (one() > 2) 10 else 20);
                break :blk v;
            },
            else => unreachable
        };
        std.debug.print("{}\n", .{r});
    }

}

fn one() u8 {
    return 1;
}

Expected Behavior

I expect the output

20
20
20

Actual Behavior

The program prints instead

10
10
20

This only happens if n is var. If n is a const value, it works as expected. Also the if expression needs to be evaluated at runtime, that's why there is a one() function.

The third case is my workaround. I don't know if there is anything better.

[SpexGuy]

It looks like the compiler generates the correct branch, but then both paths jump to a basic block (EndIf) that overwrites the value with 10.

define void @foo() #1 !dbg !2680 {
Entry:
  %n = alloca i8, align 1
  %v = alloca i8, align 1
  %r = alloca i8, align 1
  store i8 0, i8* %n, align 1, !dbg !2688
  br label %WhileCond, !dbg !2689

WhileCond:                                        ; preds = %OverflowOk, %Entry
  %0 = load i8, i8* %n, align 1, !dbg !2690
  %1 = icmp ult i8 %0, 3, !dbg !2691
  br i1 %1, label %WhileBody, label %WhileEnd, !dbg !2691

WhileBody:                                        ; preds = %WhileCond
  %2 = load i8, i8* %n, align 1, !dbg !2692
  switch i8 %2, label %SwitchElse [
    i8 0, label %SwitchProng
    i8 1, label %SwitchProng1
    i8 2, label %SwitchProng2
  ], !dbg !2692

SwitchElse:                                       ; preds = %WhileBody
  call fastcc void @panic(%"[]u8"* @71, %std.builtin.StackTrace* null), !dbg !2693
  unreachable, !dbg !2693

SwitchProng:                                      ; preds = %WhileBody
  %3 = call fastcc i8 @one(), !dbg !2694
  %4 = icmp ugt i8 %3, 2, !dbg !2695
  br i1 %4, label %Then3, label %Else8, !dbg !2696

SwitchProng1:                                     ; preds = %WhileBody
  %5 = call fastcc i8 @one(), !dbg !2697
  %6 = icmp ugt i8 %5, 2, !dbg !2699
  br i1 %6, label %Then4, label %Else9, !dbg !2700

SwitchProng2:                                     ; preds = %WhileBody
  %7 = call fastcc i8 @one(), !dbg !2701
  %8 = icmp ugt i8 %7, 2, !dbg !2702
  br i1 %8, label %Then, label %Else, !dbg !2703

Then:                                             ; preds = %SwitchProng2
  store i8 10, i8* %v, align 1, !dbg !2704
  br label %EndIf6, !dbg !2703

Else:                                             ; preds = %SwitchProng2
  store i8 20, i8* %v, align 1, !dbg !2705
  br label %EndIf6, !dbg !2703

Then3:                                            ; preds = %SwitchProng
  store i8 10, i8* %r, align 1, !dbg !2706
  br label %EndIf, !dbg !2696

EndIf:                                            ; preds = %Else8, %Then3
  store i8 10, i8* %r, align 1, !dbg !2707 ; bug here! This follows after Else8
  br label %SwitchEnd, !dbg !2692

Then4:                                            ; preds = %SwitchProng1
  store i8 10, i8* %r, align 1, !dbg !2708
  br label %EndIf5, !dbg !2700

EndIf5:                                           ; preds = %Else9, %Then4
  store i8 10, i8* %r, align 1, !dbg !2709
  br label %BlockEnd, !dbg !2710

BlockEnd:                                         ; preds = %EndIf5
  store i8 10, i8* %r, align 1, !dbg !2711
  br label %SwitchEnd, !dbg !2692

EndIf6:                                           ; preds = %Else, %Then
  call void @llvm.dbg.declare(metadata i8* %v, metadata !2684, metadata !DIExpression()), !dbg !2712
  %9 = load i8, i8* %v, align 1, !dbg !2713
  store i8 %9, i8* %r, align 1, !dbg !2713
  br label %BlockEnd7, !dbg !2714

BlockEnd7:                                        ; preds = %EndIf6
  store i8 %9, i8* %r, align 1, !dbg !2715
  br label %SwitchEnd, !dbg !2692

SwitchEnd:                                        ; preds = %BlockEnd7, %BlockEnd, %EndIf
  call void @llvm.dbg.declare(metadata i8* %r, metadata !2687, metadata !DIExpression()), !dbg !2716
  %10 = load i8, i8* %r, align 1, !dbg !2717
  call void @clobber(i8 %10), !dbg !2718
  %11 = load i8, i8* %n, align 1, !dbg !2719
  %12 = call { i8, i1 } @llvm.uadd.with.overflow.i8(i8 %11, i8 1), !dbg !2720
  %13 = extractvalue { i8, i1 } %12, 0, !dbg !2720
  %14 = extractvalue { i8, i1 } %12, 1, !dbg !2720
  br i1 %14, label %OverflowFail, label %OverflowOk, !dbg !2720

WhileEnd:                                         ; preds = %WhileCond
  ret void, !dbg !2721

Else8:                                            ; preds = %SwitchProng
  store i8 20, i8* %r, align 1, !dbg !2722
  br label %EndIf, !dbg !2696

Else9:                                            ; preds = %SwitchProng1
  store i8 20, i8* %r, align 1, !dbg !2723
  br label %EndIf5, !dbg !2700

OverflowFail:                                     ; preds = %SwitchEnd
  call fastcc void @panic(%"[]u8"* @73, %std.builtin.StackTrace* null), !dbg !2720
  unreachable, !dbg !2720

OverflowOk:                                       ; preds = %SwitchEnd
  store i8 %13, i8* %n, align 1, !dbg !2720
  br label %WhileCond, !dbg !2689
}

[FiniteNick]

Zig version
0.10.0-dev.390+0866fa9d1

I've found another case for this when creating anonymous struct literals:

const std = @import("std");

pub fn main() anyerror!void {}

fn one() u8 {
    return 1;
}

test "basic test" {
    const a = @as(u32, if (one() > 2) 1 else 2);

    const b = .{
        .val = @as(u32, if (one() > 2) 1 else 2),
    };

    const c = .{
        .val = blk: {
            const ret = @as(u32, if (one() > 2) 1 else 2);
            break :blk ret;
        },
    };

    try std.testing.expectEqual(a, 2);
    try std.testing.expectEqual(b.val, 2); // *********** Fails ***********
    try std.testing.expectEqual(c.val, 2);
}

[Vexu]

Works in self-hosted, tested in test/behavior/eval.zig:test "if inside a switch" {-