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From: "vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/97254] New: [nvptx] Define PCC_BITFIELD_TYPE_MATTERS
Date: Wed, 30 Sep 2020 11:49:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97254

            Bug ID: 97254
           Summary: [nvptx] Define PCC_BITFIELD_TYPE_MATTERS
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

While debugging gcc/testsuite/gcc.dg/pr94600-1.c, I found that nvptx doesn't
define PCC_BITFIELD_TYPE_MATTERS.

AFAIU, the theory for offloading is that settings that influence abi should=
 be
compatible with the host.

And in i386.h, we find:
...
 /* If bit field type is int, don't let it cross an int,=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
   and give entire struct the alignment of an int.  */
/* Required on the 386 since it doesn't have bit-field insns.  */
#define PCC_BITFIELD_TYPE_MATTERS 1
...

OTOH, the value may be different for other hosts.

On the llvm side, we find this comment in
llvm.git/clang/lib/Basic/Targets/ARM.cpp:
...
  // Do not respect the alignment of bit-field types when laying out=20=20=
=20=20=20=20=20=20=20=20=20=20
  // structures. This corresponds to PCC_BITFIELD_TYPE_MATTERS in gcc.=20=
=20=20=20=20=20=20=20=20=20
  UseBitFieldTypeAlignment =3D false;
...
and in llvm.git/clang/lib/Basic/Targets/NVPTX.cpp:
...
  UseBitFieldTypeAlignment =3D HostTarget->useBitFieldTypeAlignment();
...
which seems to confirm that approach.

It should be possible to get the host target by looking at the
--enable-as-accelerator, and figure out the value from there.

OTOH, it's possible that for GCC the nvptx implementation of the hook is mo=
ot,
given that part of the processing is done in the host compiler.

That does not answer the question what to do for the standalone target thou=
gh.
But probably, given that the value is set for both known offloading hosts
x86_64 and ppc, it might be a good idea to have the standalone target do the
same.=