fails to compile return of optional in certain conditionals

[MichaelBelousov]

Zig Version

0.10.0-dev.290+3901b6fb0

Steps to Reproduce

1. write the following file (NOTE: an edit below contains a smaller reproduction):

// works if I make it return not optional
fn f(use: bool, param: u32) ?u32 {
    var val: u32 = 0;
    return switch (use) {
        true => (               // SwitchProng
            if (param == 1)
                @as(u32, 2)     // Then
            else if (val == 3)  // Else
                    @as(u32, 4) // Then1 ; here is where a return value address is created
                else
                    @as(u32, 5) // Else2 ; here is where the return value address from the other branch is mistakenly consumed
        ),
        else =>                 // SwitchElse
            @as(u32, 6)
    };
}

pub fn main() void {
    _ = f(true, 7);
}

2. build it, I use:

zig build-exe file.zig

Expected Behavior

for it to compile as valid code

Actual Behavior

the compiler crashes:

broken LLVM module found: Instruction does not dominate all uses!
  %19 = getelementptr inbounds %"?u32", %"?u32"* %0, i32 0, i32 0
  store i32 5, i32* %19, align 4
Instruction does not dominate all uses!
  %19 = getelementptr inbounds %"?u32", %"?u32"* %0, i32 0, i32 0
  store i32 5, i32* %19, align 4
Instruction does not dominate all uses!
  %19 = getelementptr inbounds %"?u32", %"?u32"* %0, i32 0, i32 0
  store i32 %20, i32* %19, align 4

This is a bug in the Zig compiler.thread 2653385 panic: 
???:?:?: 0x55c02598cad7 in ??? (???)
???:?:?: 0x55c026924719 in ??? (???)
???:?:?: 0x55c02694b9e9 in ??? (???)
???:?:?: 0x55c026950b6f in ??? (???)
???:?:?: 0x55c026920273 in ??? (???)
???:?:?: 0x55c025d8fde4 in ??? (???)
???:?:?: 0x55c025d5068a in ??? (???)
???:?:?: 0x55c025b24205 in ??? (???)
???:?:?: 0x55c025b17432 in ??? (???)
???:?:?: 0x55c025b12fd1 in ??? (???)
???:?:?: 0x55c025aa15ff in ??? (???)
???:?:?: 0x55c0259bb486 in ??? (???)
???:?:?: 0x55c02598b097 in ??? (???)
???:?:?: 0x55c02598aaa9 in ??? (???)

EDIT: smaller reproduction:

fn f(param: u32) ?u32 {
    return if (param == 1) @as(u32, 2)
    else if (param == 2) @as(u32, 4)
    else if (param == 3) @as(u32, 5)
    else @as(u32, 6);
}

pub fn main() void { _ = f(7); }

[MichaelBelousov]

I analyzed the llvm IR, but failed to debug further than that. Here is the relevant IR logs:

ZIR:

// same as "Then1" in in llvm-ir
Then_74:
    #79 | SrcResetResult        | (unknown)   | - | ResetResult(none)
    #80 | SrcConst              | type        | 2 | u32
    #81 | SrcEndExpr            | (unknown)   | - | EndExpr(result=none,value=#80)
    #82 | SrcResetResult        | (unknown)   | - | ResetResult(cast(ty=#80))
    #83 | SrcConst              | comptime_int| 2 | 4
    #84 | SrcEndExpr            | (unknown)   | - | EndExpr(result=cast(ty=#80),value=#83)
    #85 | SrcImplicitCast       | (unknown)   | 2 | @implicitCast(#83)result=cast(ty=#80)
    #86 | SrcEndExpr            | (unknown)   | - | EndExpr(result=peer(next=$Else_75),value=#85)
    #87 | SrcBr                 | (unknown)   | - | goto $EndIf_76 // comptime = #73
// same as "Else2" in in llvm-ir
// seems to be exactly the same as the then clause, so the issue appears to be
// in generating the 'optional return' AIR, aka must be in an ir_analyze function
// it's in analyze_fn_ir/
Else_75:
    #88 | SrcResetResult        | (unknown)   | - | ResetResult(none)
    #89 | SrcConst              | type        | 2 | u32
    #90 | SrcEndExpr            | (unknown)   | - | EndExpr(result=none,value=#89)
    #91 | SrcResetResult        | (unknown)   | - | ResetResult(cast(ty=#89))
    #92 | SrcConst              | comptime_int| 2 | 5
    #93 | SrcEndExpr            | (unknown)   | - | EndExpr(result=cast(ty=#89),value=#92)
    #94 | SrcImplicitCast       | (unknown)   | 2 | @implicitCast(#92)result=cast(ty=#89)
    #95 | SrcEndExpr            | (unknown)   | - | EndExpr(result=peer(next=$EndIf_76),value=#94)
    #96 | SrcBr                 | (unknown)   | - | goto $EndIf_76 // comptime = #73

resumed portion of analysis log:

~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:9:30
~ #84 | SrcEndExpr            | (unknown)   | - | EndExpr(result=cast(ty=#80),value=#83)
-> #118| GenConst              | void        | 0 | {}
~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:9:21
~ #85 | SrcImplicitCast       | (unknown)   | 2 | @implicitCast(#83)result=cast(ty=#80)
-> #119| GenConst              | u32         | 0 | 4
~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:9:21
~ #86 | SrcEndExpr            | (unknown)   | - | EndExpr(result=peer(next=$Else_75),value=#85)
-> #121| GenConst              | void        | 0 | {}
~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:8:18
~ #87 | SrcBr                 | (unknown)   | - | goto $EndIf_76 // comptime = #73
append new bb Then_85
resume (6,5) Else_75 #93
-> (noreturn)
~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:11:30
~ #93 | SrcEndExpr            | (unknown)   | - | EndExpr(result=cast(ty=#89),value=#92)
-> #126| GenConst              | void        | 0 | {}
~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:11:21
~ #94 | SrcImplicitCast       | (unknown)   | 2 | @implicitCast(#92)result=cast(ty=#89)
-> #127| GenConst              | u32         | 0 | 5
~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:11:21
~ #95 | SrcEndExpr            | (unknown)   | - | EndExpr(result=peer(next=$EndIf_76),value=#94)
-> #129| GenConst              | void        | 0 | {}
~ /home/mike/personal/sizr/sizr-format-zig/src/code.zig:8:18
~ #96 | SrcBr                 | (unknown)   | - | goto $EndIf_76 // comptime = #73
append new bb Else_86

AIR which is equivalent to the outputted LLVM IR (has the same undominated instructions on one branch)

Then_85:
    #115| GenOptionalUnwrapPtr  | *u32        | - | &#1.*.? // no safety
    #117| GenStorePtr           | void        | - | *#115 = #116
    :116| GenConst              | u32         | 1 | 4
    #120| GenStorePtr           | void        | - | *#115 = #119
    :119| GenConst              | u32         | 2 | 4
    #122| GenBr                 | noreturn    | - | goto $EndIf_100
Else_86:
// it looks like this corresponds exactly to the llvm labels
// #115 is the same as %19, being reused even though it isn't
// dominated
    #125| GenStorePtr           | void        | - | *#115 = #124
    :124| GenConst              | u32         | 1 | 5
    #128| GenStorePtr           | void        | - | *#115 = #127
    :127| GenConst              | u32         | 2 | 5
    #130| GenBr                 | noreturn    | - | goto $EndIf_100

produced LLVM IR

Then1:                                            ; preds = %Else
  %18 = getelementptr inbounds %"?u32", %"?u32"* %0, i32 0, i32 1
  store i1 true, i1* %18, align 1
  %19 = getelementptr inbounds %"?u32", %"?u32"* %0, i32 0, i32 0
  store i32 4, i32* %19, align 4
  store i32 4, i32* %19, align 4
  br label %EndIf

Else2:                                            ; preds = %Else
  store i32 5, i32* %19, align 4
  store i32 5, i32* %19, align 4
  br label %EndIf

I was able to compile the output llvm ir with llc by adding a separate getelementptr instruction to the Else2 branch

[MichaelBelousov]

If someone who can fix this faster than me isn't available, I can work on a pull request slowly.

[MichaelBelousov]

here's an additional smaller reproduction:

fn f(param: u32) ?u32 {
    return if (param == 1) @as(u32, 2)
    else if (param == 2) @as(u32, 4)
    else if (param == 3) @as(u32, 5)
    else @as(u32, 6);
}

pub fn main() void { _ = f(7); }

I much better understand what's going on in the compiler that triggers this, but not enough yet to produce a fix. still slowly reading through the analysis code to figure it out

[Vexu]

Can be worked around by casting to ?u32, duplicate of #3750, fixed in stage2.