[Issue 12085] New: PIC code on X86 should use thunk to get PC

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12085

           Summary: PIC code on X86 should use thunk to get PC
           Product: D
           Version: unspecified
          Platform: x86
        OS/Version: Linux
            Status: NEW
          Keywords: dll
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: code at dawg.eu


--- Comment #0 from Martin Nowak <code at dawg.eu> 2014-02-05 11:49:00 PST ---
If one looks at GCC output of PIC code they use a small function to obtain the
PC, like so.

__x86.get_pc_thunk.bx LABEL NEAR
        mov     ebx, dword ptr [esp]                    ; 0430 _ 8B. 1C 24
        ret                                             ; 0433 _ C3

Then the thunk is called to get the PC into EBX.

        call    __x86.get_pc_thunk.bx                   ; 0444 _ E8, FFFFFFE7

What we currently do looks like this.

        push    ebx                                     ; 0564 _ 53
        call    ?_014                                   ; 0565 _ E8, 00000000

?_014   LABEL NEAR
        pop     ebx                                     ; 056A _ 5B

This is suboptimal for two reasons, first we're wasting 2 bytes per call for
the push and pop of ebx, second we invalidate the return stack buffer for each
of these calls, because there is no matching return.
I suggest to switch to the same technique that GCC is using.
We could even use the exact same naming so that the thunks produced by GCC for
C code and dmd for D code could be merged at link time.

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