[Issue 22172] New: GOT address is stored to EBX before every function call

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22172

          Issue ID: 22172
           Summary: GOT address is stored to EBX before every function
                    call
           Product: D
           Version: D2
          Hardware: x86
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ibuclaw at gdcproject.org

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 at plt>
mov    -0x1c(%ebp),%ebx # store GOT
call   *%esi
mov    -0x1c(%ebp),%ebx # store GOT
call   145b0 <_D5mydll1S3addMFiZi at plt>
mov    -0x1c(%ebp),%ebx # store GOT
call   145b0 <_D5mydll1S3addMFiZi at plt>
mov    -0x1c(%ebp),%ebx # store GOT
call   14560 <_D5mydll1I6createFZCQs1C at 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 at plt>
call   *%esi
call   145b0 <_D5mydll1S3addMFiZi at plt>
call   145b0 <_D5mydll1S3addMFiZi at plt>
call   14560 <_D5mydll1I6createFZCQs1C at plt>
mov    (%eax),%ecx
call   *0x4(%ecx)

xor    %eax,%eax
mov    -0x28(%ebp),%ebx # <- restore previous GOT 
mov    -0x24(%ebp),%esi
leave
ret
---

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