It looks like DMD is loading the GOT into EBX before every function call.
i.e: Abridged version of objdump of a program (from issue 22170)
---
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 # store GOT
call 147c8 <_D5mydll10multiply10FiZi@plt>
mov -0x1c(%ebp),%ebx # store GOT
call *%esi
mov -0x1c(%ebp),%ebx # store GOT
call 145b0 <_D5mydll1S3addMFiZi@plt>
mov -0x1c(%ebp),%ebx # store GOT
call 145b0 <_D5mydll1S3addMFiZi@plt>
mov -0x1c(%ebp),%ebx # store GOT
call 14560 <_D5mydll1I6createFZCQs1C@plt>
mov -0x1c(%ebp),%ebx # store GOT
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
---
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) # <- save previous GOT
mov %esi,-0x24(%ebp)
mov -0x1c(%ebp),%ebx # <- store GOT
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 # <- restore previous GOT
mov -0x24(%ebp),%esi
leave
ret
---
Comment #1 by robert.schadek — 2024-12-13T19:17:50Z