[cfe-dev] the "computed goto" problem

[James Y Knight via cfe-dev]

Definitely a bug.

Looks like a bug in the combination of block addresses, constexpr, and
lambda functions. Note that the "labels" global value gets duplicated --
one for the function it's defined in, and then a copy for the inner lambda
function. The copy for the lambda got the block references broken, by
attempting to change the function the block is defined in, to the lambda.
Obviously, those blocks don't actually exist within the lambda function, so
that's busted!

@__const._Z22run_with_computed_gotoPK8bytecode.labels = private unnamed_addr
constant [3 x i8*] [i8* blockaddress(@_Z22run_with_computed_gotoPK8bytecode,
%9), i8* blockaddress(@_Z22run_with_computed_gotoPK8bytecode, %13), i8*
blockaddress(@_Z22run_with_computed_gotoPK8bytecode, %17)], align 16
@__const._Z22run_with_computed_gotoPK8bytecode.labels.1 = private
unnamed_addr constant [3 x i8*] [i8* blockaddress(@
"_ZZ22run_with_computed_gotoPK8bytecodeENK3$_0clEv", <badref>), i8*
blockaddress(@"_ZZ22run_with_computed_gotoPK8bytecodeENK3$_0clEv", <badref>),
i8* blockaddress(@"_ZZ22run_with_computed_gotoPK8bytecodeENK3$_0clEv", <
badref>)], align 16

If you want a workaround, I suggest not using the lambda. Changing it to
e.g.
  #define next() labels[*instructions++]
should be fine.

On Thu, Oct 3, 2019 at 8:48 AM De Azevedo Piovezan, Felipe via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hello cfe,
>
> This year at CppCon (https://www.youtube.com/watch?v=9cPU1NdsgDQ), there
> was a funny talk about the use of what they called a "computed goto".
> I was playing around with the idea, and there might be something incorrect
> about the code generated by clang when constexpr is involved (starting with
> 9.0).
>
> The idea is this: the address of a bunch of labels are taken and stored
> into a table:
>
>   enum bytecode : int8_t { add1, add2, halt };
>
>   constexpr void* labels[] = {
>     [bytecode::add1] = &&add1_label,
>     [bytecode::add2] = &&add2_label,
>     [bytecode::halt] = &&halt_label,
>   };
>
>   //labels defined here...
>
> However, this is the IR generated with -O0 -emit-llvm:
>
>   @labels = private unnamed_addr constant [3 x i8*] [i8*
> blockaddress(@"label1", <badref>), i8* blockaddress(@"label2", <badref>),
> i8* blockaddress(@"label3", <badref>)], align 16
>
> Note the badrefs! Also, we generate a basic block with no predecessors:
>
>   12: ; No predecessors!
>   indirectbr i8* undef, [label <badref>, label <badref>, label <badref>]
>
> As such, as soon as we start optimizing, the whole function is optimized
> away.
> If we remove constexpr, however, everything seems sane. Godbolt link:
> https://godbolt.org/z/Anc9EI
>
> Any insights on what might be happening here? I suspect a lot of people
> will play around with this construction just "for fun" and will encounter
> this.
>
> --
> Felipe
>
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> cfe-dev at lists.llvm.org
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>
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