[PATCH] D64288: [LLD] [COFF] Share the tail in delayimport symbol thunks

[Rui Ueyama via Phabricator via llvm-commits]

ruiu added inline comments.


================
Comment at: COFF/DLL.cpp:195
+    0x48, 0x8D, 0x05, 0, 0, 0, 0,       // lea     rax, [__imp_<FUNCNAME>]
+    0xE9, 0, 0, 0, 0,                   // jmp     __tailMerge_<lib>
+};
----------------
rnk wrote:
> mstorsjo wrote:
> > ruiu wrote:
> > > Intel recommends jump targets being aligned to 16 bytes. Does this jump target aligned to the boundaries? Last time I tried, I observed a non-negligible difference between unaligned and aligned jump targets.
> > I guess that can be done by just marking the tail chunk as 16 byte aligned? Aligning the thunks themselves feels wasteful though.
> As far as performance is concerned, each delay load thunk runs once, so I would leave them unaligned to optimize for size.
> 
> It might end up mattering for CFI, actually. There's some assumption there that indirect branch targets (which these thunks are) are aligned. I forget the details, though.
> 
> I'm also wondering if we shouldn't mark the delay load thunks as hotpatchable so they are affected by `/functionpadmin:`. It seems separable though.
Ah, good point. If they are executed only once, there's no point to align them to trade speed with binary size.


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  https://reviews.llvm.org/D64288/new/

https://reviews.llvm.org/D64288