Bug 12085 – PIC code on X86 should use thunk to get PC

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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