https://issues.dlang.org/show_bug.cgi?id=22170

Iain Buclaw <ibuclaw@gdcproject.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |backend, wrong-code
                 CC|                            |ibuclaw@gdcproject.org

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https://issues.dlang.org/show_bug.cgi?id=22170

--- Comment #1 from Iain Buclaw <ibuclaw@gdcproject.org> ---
It looks like what DMD is doing is loading the GOT into EBX before every function call.

i.e: Abridged version of objdump
---
push   %ebp
mov    %esp,%ebp
sub    $0x28,%esp
mov    %ebx,-0x28(%ebp)
mov    %esi,-0x24(%ebp)

mov    -0x1c(%ebp),%ebx
call   147c8 <_D5mydll10multiply10FiZi@plt>
mov    -0x1c(%ebp),%ebx
call   *%esi
mov    -0x1c(%ebp),%ebx
call   145b0 <_D5mydll1S3addMFiZi@plt>
mov    -0x1c(%ebp),%ebx
call   145b0 <_D5mydll1S3addMFiZi@plt>
mov    -0x1c(%ebp),%ebx
call   14560 <_D5mydll1I6createFZCQs1C@plt>
mov    -0x1c(%ebp),%ebx
mov    (%eax),%ecx
call   *0x4(%ecx)

xor    %eax,%eax
mov    -0x28(%ebp),%ebx
mov    -0x24(%ebp),%esi
leave
ret
---

Surely it'd be more efficient to load GOT in the prologue, then restore the previous in the epilogue.

i.e: The above rewritten:
---
push   %ebp
mov    %esp,%ebp
sub    $0x28,%esp
mov    %ebx,-0x28(%ebp) # <- looks like a save (better push %ebx?)
mov    %esi,-0x24(%ebp)
mov    -0x1c(%ebp),%ebx # <- Added load GOT here

call   147c8 <_D5mydll10multiply10FiZi@plt>
call   *%esi
call   145b0 <_D5mydll1S3addMFiZi@plt>
call   145b0 <_D5mydll1S3addMFiZi@plt>
call   14560 <_D5mydll1I6createFZCQs1C@plt>
mov    (%eax),%ecx
call   *0x4(%ecx)

xor    %eax,%eax
mov    -0x28(%ebp),%ebx # <- looks like a restore (better pop %ebx?)
mov    -0x24(%ebp),%esi
leave
ret
---

So it seems that the save/restore is already being done for normal functions, but this isn't being done for thunks.

--

https://issues.dlang.org/show_bug.cgi?id=22170

--- Comment #2 from Iain Buclaw <ibuclaw@gdcproject.org> ---
GDC doesn't run into this by calling the aliased symbol directly in the thunk, e.g:

GDC
---
subl   $0x8,0x4(%esp)
jmp    0xf7f34ffb <_D5mydll1C3fooMFCQp1IZCQvQr>
---

DMD
---
sub    $0x8,%eax
jmp    0xf7f06970 <_D5mydll1C3fooMFCQp1IZCQvQr@plt>
---


GDC only generates thunks for symbols that are being emitted in this compilation, AFAIK it's not possible to have a thunk for an external symbol in DMD as well?

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https://issues.dlang.org/show_bug.cgi?id=22170

--- Comment #3 from Iain Buclaw <ibuclaw@gdcproject.org> ---
The fact that GOT is loaded into EBX before every function call is at best a performance bug and not related to this issue, so moved it to issue 22172.

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https://issues.dlang.org/show_bug.cgi?id=22170

--- Comment #4 from Iain Buclaw <ibuclaw@gdcproject.org> ---
(In reply to Iain Buclaw from comment #2)
> DMD
> ---
> sub    $0x8,%eax
> jmp    0xf7f06970 <_D5mydll1C3fooMFCQp1IZCQvQr@plt>
> ---
Removing the PLT (same as PIE https://github.com/dlang/dmd/blob/master/src/dmd/backend/elfobj.d#L3125), DMD only ever seems to generate a jmp to an offset of the target function.

---
sub    $0x8,%eax
jmp    0xf7f06970 <_D5mydll1C3fooMFCQp1IZCQvQr+4>
---

Which also looks wrong in comparison to GDC and triggers a segfault.

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https://issues.dlang.org/show_bug.cgi?id=22170

Dlang Bot <dlang-bot@dlang.rocks> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pull

--- Comment #5 from Dlang Bot <dlang-bot@dlang.rocks> ---
@ibuclaw created dlang/dmd pull request #12950 "fix Issue 22170 - interface thunk doesn't set EBX to GOT" fixing this issue:

- fix Issue 22170 - interface thunk doesn't set EBX to GOT

https://github.com/dlang/dmd/pull/12950

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https://issues.dlang.org/show_bug.cgi?id=22170

Dlang Bot <dlang-bot@dlang.rocks> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from Dlang Bot <dlang-bot@dlang.rocks> ---
dlang/dmd pull request #12950 "fix Issue 22170 - interface thunk doesn't set EBX to GOT" was merged into stable:

- 1f723f0f17bc9fc9557e22cbfefc913bdfab6037 by Iain Buclaw:
  fix Issue 22170 - interface thunk doesn't set EBX to GOT

https://github.com/dlang/dmd/pull/12950

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https://issues.dlang.org/show_bug.cgi?id=22170

--- Comment #7 from Dlang Bot <dlang-bot@dlang.rocks> ---
dlang/dmd pull request #12953 "merge stable" was merged into master:

- d0406f3afea2c8365eef162cf6c6636aaef5a105 by Iain Buclaw:
  fix Issue 22170 - interface thunk doesn't set EBX to GOT (#12950)

  * fix Issue 22170 - interface thunk doesn't set EBX to GOT

  * dshell: Add dll tests for issue 10462

https://github.com/dlang/dmd/pull/12953

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