fix union initialization of type void

[mikdusan]

• related result location mechanism (part of no-copy semantics) #2602
• note function make1() is where the issue presents
• note function make2() is added for comparison purposes

reduction

const State = union(enum) {
    one: void,
    two: u32,
};

fn make1() State {
    return State{ .one = {} }; // regression: copy-elision-3
}

fn make2() State {
    return State{ .two = 99 };
}

export fn foobar() void {
    _ = make1(); 
    _ = make2();
}

const builtin = @import("builtin");
pub fn panic(msg: []const u8, error_return_trace: ?*builtin.StackTrace) noreturn {
    while (true) {}
}

broken IR (branch: copy-elision-3)

fn make1() { // (analyzed)
Entry_0:
    #8  | *State      | 3 | @ReturnPtr
    #9  | *void       | 0 | @UnionFieldPtr(&#8.one)
    #14 | State       | 0 | loadptr(#8)result=(null)
    #16 | State       | 0 | loadptr(#8)result=(null)
    #20 | noreturn    | - | return 
}

fn make2() { // (analyzed)
Entry_0:
    #8  | *State      | 3 | @ReturnPtr
    #9  | *u32        | 1 | @UnionFieldPtr(&#8.two)
    #13 | void        | - | *#9 = 99
    #15 | State       | 0 | loadptr(#8)result=(null)
    #17 | State       | 0 | loadptr(#8)result=(null)
    #21 | noreturn    | - | return 
}

broken LLVM-IR (branch: copy-elision-3)

; Function Attrs: nobuiltin nounwind sspstrong
define internal fastcc void @make1(%State* nonnull sret) unnamed_addr #2 !dbg !48 {
Entry:
  ret void, !dbg !60
}

; Function Attrs: nobuiltin nounwind sspstrong
define internal fastcc void @make2(%State* nonnull sret) unnamed_addr #2 !dbg !62 {
Entry:
  %1 = getelementptr inbounds %State, %State* %0, i32 0, i32 1, !dbg !63
  store i1 true, i1* %1, !dbg !63
  %2 = getelementptr inbounds %State, %State* %0, i32 0, i32 0, !dbg !63
  store i32 99, i32* %2, align 4, !dbg !63
  ret void, !dbg !65
}

patched IR

fn make1() { // (analyzed)
Entry_0:
    #8  | *State      | 3 | @ReturnPtr
    #9  | *void       | 1 | @UnionFieldPtr(&#8.one)
    #12 | void        | - | *#9 = {}
    #14 | State       | 0 | loadptr(#8)result=(null)
    #16 | State       | 0 | loadptr(#8)result=(null)
    #20 | noreturn    | - | return 
}

fn make2() { // (analyzed)
Entry_0:
    #8  | *State      | 3 | @ReturnPtr
    #9  | *u32        | 1 | @UnionFieldPtr(&#8.two)
    #13 | void        | - | *#9 = 99
    #15 | State       | 0 | loadptr(#8)result=(null)
    #17 | State       | 0 | loadptr(#8)result=(null)
    #21 | noreturn    | - | return 
}

patched LLVM-IR

; Function Attrs: nobuiltin nounwind sspstrong
define internal fastcc void @make1(%State* nonnull sret) unnamed_addr #2 !dbg !48 {
Entry:
  %1 = getelementptr inbounds %State, %State* %0, i32 0, i32 1, !dbg !60
  store i1 false, i1* %1, !dbg !60
  ret void, !dbg !62
}

; Function Attrs: nobuiltin nounwind sspstrong
define internal fastcc void @make2(%State* nonnull sret) unnamed_addr #2 !dbg !63 {
Entry:
  %1 = getelementptr inbounds %State, %State* %0, i32 0, i32 1, !dbg !64
  store i1 true, i1* %1, !dbg !64
  %2 = getelementptr inbounds %State, %State* %0, i32 0, i32 0, !dbg !64
  store i32 99, i32* %2, align 4, !dbg !64
  ret void, !dbg !66
}

reference IR (branch: master)

fn make1() { // (analyzed)
Entry_0:
    #8  | noreturn    | - | return State { .one = {}}
}

fn make2() { // (analyzed)
Entry_0:
    #9  | noreturn    | - | return State { .two = 99}
}

reference LLVM-IR (branch: master)

@0 = internal unnamed_addr constant %State { i32 undef, i1 false }, align 4
@1 = internal unnamed_addr constant %State { i32 99, i1 true }, align 4

; Function Attrs: nobuiltin nounwind sspstrong
define internal fastcc void @make1(%State* nonnull sret) unnamed_addr #2 !dbg !48 {
Entry:
  %1 = bitcast %State* %0 to i8*, !dbg !60
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %1, i8* align 4 bitcast (%State* @0 to i8*), i64 8, i1 false), !dbg !60
  ret void, !dbg !60
}

; Function Attrs: nobuiltin nounwind sspstrong
define internal fastcc void @make2(%State* nonnull sret) unnamed_addr #2 !dbg !62 {
Entry:
  %1 = bitcast %State* %0 to i8*, !dbg !63
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %1, i8* align 4 bitcast (%State* @1 to i8*), i64 8, i1 false), !dbg !63
  ret void, !dbg !63
}

[mikdusan]

closing issue: better fix committed c61e0a0

[andrewrk]

Ah, sorry, I actually hadn't seen this yet when I coded my solution.