[PATCH] D59797: [COFF] Reorder fields in Chunk and SectionChunk to reduce their size

[Shoaib Meenai via Phabricator via cfe-commits]

smeenai added inline comments.


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Comment at: lld/COFF/Chunks.cpp:47
 
+namespace {
+// This class exists just for the purpose of calculating the expected size of
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pcc wrote:
> rnk wrote:
> > ruiu wrote:
> > > rnk wrote:
> > > > ruiu wrote:
> > > > > rnk wrote:
> > > > > > ruiu wrote:
> > > > > > > This might be useful but at the same time it looks a bit overly cautious? Perhaps the symbol size is more important but we don't have something like this for them, for example.
> > > > > > Well, we don't have checks for symbol size yet. :)
> > > > > > 
> > > > > > Given that we don't have performance monitoring, I really want people to think hard before they casually add another field to SectionChunk. I wouldn't insist on it if we did, but these types of static_asserts have proven useful in LLVM for preventing size creep.
> > > > > Then maybe reiterating everything again, how about checking directly with a number like `static_assert(sizeof(Chunk) == 48)`? This should suffice to prevent making the struct larger by accident.
> > > > It would be wrong for 32-bit. Rather than trying to express it as math on sizeof(void*), I think the struct makes it clearer. And it helps document hidden members like vptr.
> > > Yeah, but it still feels weird to me to repeat all the members again in a different file just for the purpose of assertion. For 32-bit, we can just set the upper bound like `static_assert(sizeof(Chunk) <= 48)` where 48 is the size of the struct in 64-bit.
> > Fair enough. Honestly, writing down all the fields in one place helped me identify the profitable reorderings, but we don't need to commit it.
> I found D59044 (and, to a lesser extent, D59269) by staring at the output of clang with `-Xclang -fdump-record-layouts`. Maybe it would be worth adding this trick to the documentation somewhere?
The clang static analyzer has a padding checker (https://clang.llvm.org/docs/analyzer/checkers.html#optin-performance-padding), and clang-tools-extra has clang-reorder-fields to automatically reorder fields according to the padding checker's findings. I haven't used those myself, but a coworker had success with them in the past.


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https://reviews.llvm.org/D59797