MinGW: improve compatibility with LLD

[mati865]

LLD unlike LD doesn't implement linker script support for MinGW and this has so low priority it may be never implemented. However they both support MSVC like .def files.
Once this and rust-lang/cargo#6875 reach beta and bootstrap compiler is updated using LLD as the linker will work fine.

[rust-highfive]

r? @petrochenkov

(rust_highfive has picked a reviewer for you, use r? to override)

[mati865]

Updated description, it was PEBCAK and this PR doesn't help with #50176

[petrochenkov]

@bors r+

[bors]

📌 Commit 809aa16c40d962c7a5090223a5ce98cd57ee233e has been approved by petrochenkov

[mati865]

Just noticed that the commit message was malformed, sorry for the trouble.

[mati865]

@petrochenkov could you r+ it again? ^

[petrochenkov]

@bors r+

[bors]

📌 Commit 44e602f has been approved by petrochenkov

[bors]

⌛ Testing commit 44e602f with merge 54fd296879f8ec8875d2aaf5735bc395c701ad83...

[bors]

💔 Test failed - checks-azure

[mati865]

Some symbols are not exported in the import libs. This was not visible before #70937 because LD could not find import lib (wrong extension). Good thing #70937 has landed before this PR.

This gonna be fun...

[mati865]

It's much worse than I thought. I'll see if I can implement version-script support for LLD.

[petrochenkov]

@mati865

It's much worse than I thought.

😆
What exactly? ld doesn't really support .def?

[mati865]

It does support it at least to some extent. Either the .def file needs some more work on it or it's just bugged.

Regressions I have found:

• DLLs have additional dependencies:
  • symbols exported via .def (bad):

    $ ntldd cci_const_block.dll
    std-979e3b2f81b3e5a5.dll => D:\Projekty\rust\build\x86_64-pc-windows-gnu\stage2\bin\std-979e3b2f81b3e5a5.dll (0x0000000000ef0000)
    KERNEL32.dll => C:\WINDOWS\SYSTEM32\KERNEL32.dll (0x00000000016f0000)
    msvcrt.dll => C:\WINDOWS\SYSTEM32\msvcrt.dll (0x00000000006e0000)
    

  • symbols exported via version-script (good):

    $ ntldd cci_const_block.dll
    KERNEL32.dll => C:\WINDOWS\SYSTEM32\KERNEL32.dll (0x0000000000eb0000)
    msvcrt.dll => C:\WINDOWS\SYSTEM32\msvcrt.dll (0x0000000000eb0000)
    

• linking to DLLs via implibs causes segfaults (direct linking to DLL):
  • .def (bad):

    $ nm libcci_const_block.dll.a | grep BLOCK_FN_DEF
    0000000000000000 I __imp__ZN15cci_const_block12BLOCK_FN_DEF17h2bbe18e6734ce46cE
    0000000000000000 T _ZN15cci_const_block12BLOCK_FN_DEF17h2bbe18e6734ce46cE
    

  • version-script (good):

    $ nm const-block-cross-crate-fn/auxiliary/libcci_const_block.dll.a | grep BLOCK_FN_DEF
    0000000000000000 I __imp__ZN15cci_const_block12BLOCK_FN_DEF17h2bbe18e6734ce46cE
    0000000000000000 I __nm__ZN15cci_const_block12BLOCK_FN_DEF17h2bbe18e6734ce46cE
    

  • the DLL:

    $ nm cci_const_block.dll | grep BLOCK_FN_DEF
    0000000067cc5000 R _ZN15cci_const_block12BLOCK_FN_DEF17h2bbe18e6734ce46cE
    0000000067cc13b0 t _ZN15cci_const_block12BLOCK_FN_DEF3foo17h3d71b48312ebbcc6E
    

[mati865]

Another attempt at breaking improving MinGW: #72049.