Re: [PATCH v3 2/2] aarch64: Fix bit-field alignment in param passing [PR105549]

[Richard Sandiford <richard.sandiford@arm.com>]

Christophe Lyon <christophe.lyon@arm.com> writes:
> While working on enabling DFP for AArch64, I noticed new failures in
> gcc.dg/compat/struct-layout-1.exp (t028) which were not actually
> caused by DFP types handling. These tests are generated during 'make
> check' and enabling DFP made generation different (not sure if new
> non-DFP tests are generated, or if existing ones are generated
> differently, the tests in question are huge and difficult to compare).
>
> Anyway, I reduced the problem to what I attach at the end of the new
> gcc.target/aarch64/aapcs64/va_arg-17.c test and rewrote it in the same
> scheme as other va_arg* AArch64 tests.  Richard Sandiford further
> reduced this to a non-vararg function, added as a second testcase.
>
> This is a tough case mixing bit-fields and alignment, where
> aarch64_function_arg_alignment did not follow what its descriptive
> comment says: we want to use the natural alignment of the bit-field
> type only if the user didn't reduce the alignment for the bit-field
> itself.
>
> The patch also adds a comment and assert that would help someone who
> has to look at this area again.
>
> The fix would be very small, except that this introduces a new ABI
> break, and we have to warn about that.  Since this actually fixes a
> problem introduced in GCC 9.1, we keep the old computation to detect
> when we now behave differently.
>
> This patch adds two new tests (va_arg-17.c and
> pr105549.c). va_arg-17.c contains the reduced offending testcase from
> struct-layout-1.exp for reference.  We update some tests introduced by
> the previous patch, where parameters with bit-fields and packed
> attribute now emit a different warning.
>
> v2->v3: testcase update
>
> 2022-11-28  Christophe Lyon  <christophe.lyon@arm.com>
> 	    Richard Sandiford  <richard.sandiford@arm.com>
>
> 	gcc/
> 	PR target/105549
> 	* config/aarch64/aarch64.cc (aarch64_function_arg_alignment):
> 	Check DECL_PACKED for bitfield.
> 	(aarch64_layout_arg): Warn when parameter passing ABI changes.
> 	(aarch64_function_arg_boundary): Do not warn here.
> 	(aarch64_gimplify_va_arg_expr): Warn when parameter passing ABI
> 	changes.
>
> 	gcc/testsuite/
> 	PR target/105549
> 	* gcc.target/aarch64/bitfield-abi-warning-align16-O2.c: Update.
> 	* gcc.target/aarch64/bitfield-abi-warning-align16-O2-extra.c: Update.
> 	* gcc.target/aarch64/bitfield-abi-warning-align32-O2.c: Update.
> 	* gcc.target/aarch64/bitfield-abi-warning-align32-O2-extra.c: Update.
> 	* gcc.target/aarch64/aapcs64/va_arg-17.c: New test.
> 	* gcc.target/aarch64/pr105549.c: New test.
> 	* g++.target/aarch64/bitfield-abi-warning-align16-O2.C: Update.
> 	* g++.target/aarch64/bitfield-abi-warning-align16-O2-extra.C: Update.
> 	* g++.target/aarch64/bitfield-abi-warning-align32-O2.C: Update.
> 	* g++.target/aarch64/bitfield-abi-warning-align32-O2-extra.C: Update.
> ---
>  gcc/config/aarch64/aarch64.cc                 | 148 ++++++++++++++----
>  .../bitfield-abi-warning-align16-O2-extra.C   |  64 ++++----
>  .../aarch64/bitfield-abi-warning-align16-O2.C |  48 +++---
>  .../bitfield-abi-warning-align32-O2-extra.C   | 131 +++++++---------
>  .../aarch64/bitfield-abi-warning-align32-O2.C | 132 ++++++++--------
>  .../gcc.target/aarch64/aapcs64/va_arg-17.c    | 105 +++++++++++++
>  .../bitfield-abi-warning-align16-O2-extra.c   |  64 ++++----
>  .../aarch64/bitfield-abi-warning-align16-O2.c |  48 +++---
>  .../bitfield-abi-warning-align32-O2-extra.c   | 131 +++++++---------
>  .../aarch64/bitfield-abi-warning-align32-O2.c | 132 ++++++++--------
>  gcc/testsuite/gcc.target/aarch64/pr105549.c   |  12 ++
>  11 files changed, 587 insertions(+), 428 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/aarch64/aapcs64/va_arg-17.c
>  create mode 100644 gcc/testsuite/gcc.target/aarch64/pr105549.c
> [...]
> @@ -68,14 +62,14 @@
>  /* { dg-note {parameter passing for argument of type 'S4' changed in GCC 9.1} "" { target *-*-* } 103 } f4_stdarg */
>  /* { dg-note {parameter passing for argument of type 'S8' changed in GCC 9.1} "" { target *-*-* } 104 } f8_stdarg */
>  
> -/* Parameter passing for these should not have changed in GCC 9.1 (PR 105549).
> +/* FIXME Parameter passing for these should not have changed in GCC 9.1 (PR 105549).
>     Fortunately we warn. Note the discrepancy with lines 120-124 below: we warn
>     in the callee, but not in the caller.  */

Looks like a stray change.  Same for the C test.

OK otherwise, thanks.

Richard