Re: Unit Type for Rust

[Philip Herron <philip.herron@embecosm.com>]

[-- Attachment #1.1: Type: text/plain, Size: 4318 bytes --]

On 28/06/2021 14:49, Philip Herron wrote:
> Hi everyone,
>
> In Rust the language has the notion of the unit type '()', so for example:
>
>  fn foo ->i32 { ... }
>  fn bar() { ... }
>
> Foo has the return type i32, and bar has no return type, which means it
> is unit-type so that it can be a value assignable just like any other
> function call. You can also declare unit-structs or unit as a type on
> variables:
>
>  struct foobar; // empty unit struct
>  let a:() = (); // unit type
>
> I thought I could use GCC's void_type_node to represent the unit type
> and void_node for the value when assigning or using it, but this causes
> the ICE:
>
> ```
> In function ‘test’:
> rust1: internal compiler error: in create_tmp_var, at gimple-expr.c:482
> 0x13fd3bf create_tmp_var(tree_node*, char const*)
>         ../../gccrs/gcc/gimple-expr.c:482
> 0xe5d195 Gcc_backend::temporary_variable(Bfunction*, Bblock*, Btype*,
> Bexpression*, bool, Location, Bstatement**)
>         ../../gccrs/gcc/rust/rust-gcc.cc:2889
> 0xfe3479 Rust::HIR::Function::accept_vis(Rust::HIR::HIRVisitor&)
>         ../../gccrs/gcc/rust/hir/tree/rust-hir-full-test.cc:4414
> 0xf95cdb Rust::Compile::CompileCrate::go()
>         ../../gccrs/gcc/rust/backend/rust-compile.cc:49
> 0xf95b8b Rust::Compile::CompileCrate::Compile(Rust::HIR::Crate&,
> Rust::Compile::Context*)
>         ../../gccrs/gcc/rust/backend/rust-compile.cc:39
> 0xee92e7 Rust::Session::parse_file(char const*)
>         ../../gccrs/gcc/rust/rust-session-manager.cc:596
> 0xee8d76 Rust::Session::parse_files(int, char const**)
>         ../../gccrs/gcc/rust/rust-session-manager.cc:459
> 0xe45264 grs_langhook_parse_file
>         ../../gccrs/gcc/rust/rust-lang.cc:171
>
> ```
>
> I think because void_node is likely not COMPLETE_TYPE_P which means it
> hits the assertion when you need temporary's.
>
>
> Then Tom Tromey suggested I try a zero precision integer so I called:
> make_unsigned_type (0) for the type and then use integer_zero_node for
> the value, and this solves the problem; however, if I use this zero
> precision integer type for the return type on functions and turn
> optimizations on I get the ICE:
>
>    ```
>    test.rs: In function ‘main’:
>    test.rs:16:1: internal compiler error: in min_value, at wide-int.cc:346
>       16 | fn main() {
>          | ^
>    0x1d551d5 wi::min_value(unsigned int, signop)
>            ../../gccrs/gcc/wide-int.cc:346
>    0x1146ca5 irange::set_varying(tree_node*)
>            ../../gccrs/gcc/value-range.h:476
>    0x1ce5970 value_range_equiv::set_varying(tree_node*)
>            ../../gccrs/gcc/value-range-equiv.cc:71
>    0x1d3da07 vr_values::set_def_to_varying(tree_node const*)
>            ../../gccrs/gcc/vr-values.c:230
>    0x1d3da70 vr_values::set_defs_to_varying(gimple*)
>            ../../gccrs/gcc/vr-values.c:241
>    0x1c78b2f vrp_prop::visit_stmt(gimple*, edge_def**, tree_node**)
>            ../../gccrs/gcc/tree-vrp.c:4001
>    0x1ad8519 ssa_propagation_engine::simulate_stmt(gimple*)
>            ../../gccrs/gcc/tree-ssa-propagate.c:230
>    0x1ad8a0e ssa_propagation_engine::simulate_block(basic_block_def*)
>            ../../gccrs/gcc/tree-ssa-propagate.c:337
>    0x1ad9f2e ssa_propagation_engine::ssa_propagate()
>            ../../gccrs/gcc/tree-ssa-propagate.c:800
>    0x1c7a0b0 execute_vrp
>            ../../gccrs/gcc/tree-vrp.c:4512
>    0x1c7a3e4 execute
>            ../../gccrs/gcc/tree-vrp.c:4620
>    Please submit a full bug report,
>    ```
>
> The backtrace looks as though the optimizer is looking for min value for
> a default for the return value, but it's a zero precision integer that
> hits the assertion. Note running with -O0, the assertion does not get hit.
>
>
> At the moment, I have left functions with return type unit to keep using
> void_type_node and everywhere else, use this new zero precision integer.
> I am not sure what the best approach is here; I was hoping to solicit
> feedback on what I am doing with the folks here on the mailing list.
>
> Thanks
>
> --Phil
>

Adding in the GCC mailing list for feedback.

Thanks

--Phil


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 665 bytes --]