[PATCH][RFC] Respect alignment of nested bitfields

[Ulrich Weigand]

Hello,

tools/clang/test/CodeGen/packed-nest-unpacked.c contains this test:

struct XBitfield {
  unsigned b1 : 10;
  unsigned b2 : 12;
  unsigned b3 : 10;
};
struct YBitfield {
  char x;
  struct XBitfield y;
} __attribute((packed));
struct YBitfield gbitfield;

unsigned test7() {
  // CHECK: @test7
  // CHECK: load i32, i32* getelementptr inbounds (%struct.YBitfield,
%struct.YBitfield* @gbitfield, i32 0, i32 1, i32 0), align 4
  return gbitfield.y.b2;
}

The "align 4" is actually wrong.  Accessing all of "gbitfield.y" as a
single
i32 is of course possible, but that still doesn't make it 4-byte aligned as
it remains packed at offset 1 in the surrounding gbitfield object.

This alignment was changed by commit r169489, which also introduced changes
to bitfield access code in CGExpr.cpp.  Code before that change used to
take
into account *both* the alignment of the field to be accessed within the
current struct, *and* the alignment of that outer struct itself; this logic
was removed by the above commit.

This can cause incorrect code to be generated (I've seen an example on
SystemZ
where code would crash due to unaligned access).

The patch below adds back logic to also consider alignment of the outer
struct
in EmitLoadOfBitfieldLValue and EmitStoreThroughBitfieldLValue.  It also
extends the test case to cover the case of storing into the nested
bitfield.

(See attached file: clang-align-bitfield)


Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

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