Bug 22170 – interface thunk doesn't set EBX to GOT

Issue 9729 has regressed during the fix for issue 10462 (https://github.com/dlang/dmd/pull/2278) We still need to load the GOT within the interface thunk because only the thunk can load the correct GOT.

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.

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?

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.

(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.

@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

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

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