Enable function sections with windows-gnu

[mati865]

Passed tests locally on x86_64 with LLVM assertions enabled.

[rust-highfive]

r? @ecstatic-morse

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[nagisa]

IIRC the reason it was disabled until now was because on Windows there's an egregiously small limit on sections in object files, which then leads to compilation failures. Is windows-gnu not affected by this?

[mati865]

Apparently it was disabled because of the LLVM assertions: #13846
LLVM supports doing it for both MSVC and GNU Windows targets. Short topic that made me try it again: mstorsjo/llvm-mingw#148

I have tried enabling it some time ago but got ui tests crashes.
Just recently I have noticed enabling ASLR or functions_sections had very similar (or even identical) crashes as one of my mistakes when working on #72049. To be exact the crashes happened when statics were imported from other DLLs without dllimport.
If you want I can verify tomorrow if this is unexpected "fallout" from #72049 or something else has fixed it.

[mstorsjo]

IIRC the reason it was disabled until now was because on Windows there's an egregiously small limit on sections in object files, which then leads to compilation failures. Is windows-gnu not affected by this?

FWIW, the limit on sections in an object file is higher if compiling in "big object" mode. IIRC GCC and/or MSVC require you to pass specific flags for enabling this, while LLVM enables this format automatically as needed.

[ecstatic-morse]

r? @nagisa

[nagisa]

@bors r+

[bors]

📌 Commit b655027 has been approved by nagisa

[bors]

⌛ Testing commit b655027 with merge 18dd0795f6aecafa408c6b35e9b28d2798a6ea8a...

[bors]

💔 Test failed - checks-actions

[mati865]

Appears to be bug fixed in binutils 2.32 I'll look for a workaround.

[mati865]

I can reproduce it locally with the same Binutils version as on CI, Binutils 2.34 are not affected.
I doubt we can raise minimal Binutils version to 2.32 (released 2019-02-02) soon, this will have to wait until the requirement can be bumped or we migrate to LLD for MinGW.

@mstorsjo do you remember if this is easily workaroundable in LLVM?

[mstorsjo]

I can reproduce it locally with the same Binutils version as on CI, Binutils 2.34 are not affected.
I doubt we can raise minimal Binutils version to 2.32 (released 2019-02-02) soon, this will have to wait until the requirement can be bumped or we migrate to LLD for MinGW.

@mstorsjo do you remember if this is easily workaroundable in LLVM?

What issue in particular are you referring to here?

[mati865]

@mstorsjo I apologize I thought I had included the link. Here is ld.bfd bug report: https://sourceware.org/bugzilla/show_bug.cgi?id=23872
The error:

  Mingw-w64 runtime failure:
    Unknown pseudo relocation protocol version 16777216.

[mstorsjo]

@mstorsjo I apologize I thought I had included the link. Here is ld.bfd bug report: https://sourceware.org/bugzilla/show_bug.cgi?id=23872
The error:

  Mingw-w64 runtime failure:
    Unknown pseudo relocation protocol version 16777216.

Ah, that one.

I have one potential fix in mind - appending a final object file that contains .rdata contents that is aligned to 4 bytes, included as the last object file in the link command. The object file can be assembled from e.g. this:

        .section .rdata$zzzz
        .align 4
        .long 0

That should take care to make sure the end of the .rdata section is aligned to 4 bytes, avoiding that particular ld bug.

[mati865]

Unfortunately that didn't work. I'll just close it for now.