efriedma added a comment.
The -ffine-grained-bitfield-accesses seems to generate weird results in certain
cases. For example, on x86 we generate an unaligned load in the following
example.
struct S { long c : 8; long z: 24; long : 0; };
struct S s;
int f() { return s.c+s.z; }
I guess that's not really an issue with this patch specifically, though; the
general algorithm is just sort of weird.
================
Comment at: clang/lib/CodeGen/CGRecordLayoutBuilder.cpp:414
auto IsBetterAsSingleFieldRun = [&](uint64_t OffsetInRecord,
uint64_t StartBitOffset) {
if (!Types.getCodeGenOpts().FineGrainedBitfieldAccesses)
----------------
StartBitOffset is the offset in bits of the beginning of the current run, the
comment above explains that. It's not obvious what OffsetInRecord is; I guess
it's the size in bits of the current run? The name isn't really intuitive.
This patch seems reasonable; you want to allow any legal integer, and any
smaller type that can probably be naturally loaded and stored.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D79155/new/
https://reviews.llvm.org/D79155
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