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When debugging on windows-msvc, closure environments cannot be inspected #83709

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wesleywiser opened this issue Mar 31, 2021 · 1 comment · Fixed by #83941
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When debugging on windows-msvc, closure environments cannot be inspected #83709

wesleywiser opened this issue Mar 31, 2021 · 1 comment · Fixed by #83941
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A-closures Area: Closures (`|…| { … }`) A-debuginfo Area: Debugging information in compiled programs (DWARF, PDB, etc.) C-bug Category: This is a bug. O-windows-msvc Toolchain: MSVC, Operating system: Windows T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@wesleywiser
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I compiled the following program and started debugging in Windbg:

fn main() {
    let x: i32 = 123;
    let y: i32 = x * 2;
    let closure = || {
        x   y
    };

    std::process::exit(closure());
}

but after stepping inside closure, it's not possible to inspect the x or y variables.

image

Meta

rustc --version --verbose:

rustc 1.52.0-nightly (4a8b6f708 2021-03-11)
binary: rustc
commit-hash: 4a8b6f708c38342a6c74aa00cf4323774c7381a6
commit-date: 2021-03-11
host: x86_64-pc-windows-msvc
release: 1.52.0-nightly
LLVM version: 12.0.0

Windbg 1.2103.01004.0
@wesleywiser wesleywiser added A-debuginfo Area: Debugging information in compiled programs (DWARF, PDB, etc.) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. O-windows-msvc Toolchain: MSVC, Operating system: Windows C-bug Category: This is a bug. labels Mar 31, 2021
@jonas-schievink jonas-schievink added the A-closures Area: Closures (`|…| { … }`) label Mar 31, 2021
@wesleywiser
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Running cvdump.exe on the pdb shows the function is missing local variable information entirely:

(000178) S_LPROC32: [0001:00000100], Cb: 00000042, Type:             0x1021, async_stack::main::{{closure}}
         Parent: 00000000, End: 000001E0, Next: 00000000
         Debug start: 00000000, Debug end: 00000000

(0001C0)  S_FRAMEPROC:
          Frame size = 0x00000038 bytes
          Pad size = 0x00000000 bytes
          Offset of pad in frame = 0x00000000
          Size of callee save registers = 0x00000000
          Address of exception handler = 0000:00000000
          Function info: inl_specified invalid_pgo_counts Local=rsp Param=rsp (0x00014020)

(0001E0) S_END

where as the DWARF info is generated correctly:

0x00000323:     DW_TAG_namespace
                  DW_AT_name    ("main")

0x00000328:       DW_TAG_subprogram
                    DW_AT_low_pc        (0x00000000000052c0)
                    DW_AT_high_pc       (0x0000000000005307)
                    DW_AT_frame_base    (DW_OP_reg7 RSP)
                    DW_AT_linkage_name  ("_ZN11async_stack4main28_$u7b$$u7b$closure$u7d$$u7d$17h80799f1c76fc13d8E")
                    DW_AT_name  ("{{closure}}")
                    DW_AT_decl_file     ("/mnt/d/code/temp/async_stack/src/main.rs")
                    DW_AT_decl_line     (6)
                    DW_AT_type  (0x00000388 "i32")

0x00000345:         DW_TAG_variable
                      DW_AT_location    (DW_OP_fbreg  16, DW_OP_deref, DW_OP_deref)
                      DW_AT_name        ("x")
                      DW_AT_decl_file   ("/mnt/d/code/temp/async_stack/src/main.rs")
                      DW_AT_decl_line   (4)
                      DW_AT_type        (0x00000388 "i32")

0x00000355:         DW_TAG_variable
                      DW_AT_location    (DW_OP_fbreg  16, DW_OP_deref, DW_OP_plus_uconst 0x8, DW_OP_deref)
                      DW_AT_name        ("y")
                      DW_AT_decl_file   ("/mnt/d/code/temp/async_stack/src/main.rs")
                      DW_AT_decl_line   (5)
                      DW_AT_type        (0x00000388 "i32")

0x00000367:         NULL

So this is an issue with the debug info generation itself and not a bug in Windbg.

@wesleywiser wesleywiser mentioned this issue Apr 6, 2021
Merged
@bors bors closed this as completed in 619d19c Apr 9, 2021
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Labels
A-closures Area: Closures (`|…| { … }`) A-debuginfo Area: Debugging information in compiled programs (DWARF, PDB, etc.) C-bug Category: This is a bug. O-windows-msvc Toolchain: MSVC, Operating system: Windows T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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Improve debuginfo for closures and async functions on Windows MSVC wesleywiser/rust
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@wesleywiser @jonas-schievink