[llvm-dev] The value of padding when storing an aggregate into memory
Alexander Cherepanov via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 19 13:37:30 PDT 2020
On 19/08/2020 18.55, Juneyoung Lee via llvm-dev wrote:
>> Interesting topic. Is any such optimization reachable from C?
> Yes, I think so - both PassBuilder and PassManagerBuilder add MemCpyOpt &
> IPSCCP in the default pass pipeline.
In my experience, clang (unlike gcc) preserves the naive meaning of the
source C code in dealing with padding in structs -- assignments of whole
structs copy the padding while assignments to members of structs retain
their old padding. The only exception I know is returning structs from
functions, as was pointed by Greg Parker in
I don't think this is required by any standard and it would be
interesting to find out why exactly it's done this way (and whether the
mentioned exception is a bug). This is especially wasteful in case of
OTOH it's not clear to me this has anything to do with the
transformations of LLVM IR that you mentioned. Perhaps clang just
doesn't emit any IR that could trigger those optimizations.
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