[Bug c/36212] New: Vector alignment overides Target alignment

["hutchinsonandy at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

For avr we have multiple failures of testcases due to alignment warnings such
as:

gcc/gcc/testsuite/gcc.dg/debug/const-1.c:3: warning: alignment of 'Foo' is     
 greater than maximum object file alignment.  Using 1

Which in this example occurs due to:

    typedef float FloatVect __attribute__((__vector_size__(16)));
    FloatVect Foo = { 250000000.0, 0.0, 0.0, 0.0 };

While trying to fix this I have discovered several issues which make the
"correct" fix somewhat unclear and perhaps indicating more than one bug.

The warning occurs because alignment of the vector is 16 bytes regardless of
all target alignment controls. In particular:

#define BIGGEST_ALIGNMENT 8


By default this also determines MAX_OFILE_ALIGNMENT which is used by the
alignment check in varasm.c (if (16>1) Warning)

Problem 1) varasm.c - is the only place MAX_OFILE_ALIGNMENT is used. Here it
immediately tries to increase or decrease alignment AFTER it does check - which
suggests the check is misplaced or not useful in the first place.


To get around the test failure target MAX_OFILE_ALIGNMENT can be increased.
(and logically should be to permit user defined alignments)


Problem 2) Without hard limit imposed by MAX_OFILE_ALIGNMENT vectors are
aligned at 16 bytes - which break the supposed limit of BIGGEST_ALIGNMENT (1
byte).

Vector size is set in stor-layout.c :

        /* Always naturally align vectors.  This prevents ABI changes
           depending on whether or not native vector modes are supported.  */
        TYPE_ALIGN (type) = tree_low_cst (TYPE_SIZE (type), 0);
        break;

There is no way for target to override this alignment. It does not seem correct
to assume a specific vector alignment independent of target. Perhaps there
should be:

#ifdef  TARGET_VECTOR_ALIGN (..)
...
...
#else


Some may say that vectors are relevant only targets that support hardware
vector operations and thus alignment choice is currently moot. But..

Problem 3) gcc provide no hooks for  the target or testsuite to elegantly
disable vector extensions (that I can find anyway).


-- 
           Summary: Vector alignment overides Target alignment
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hutchinsonandy at gcc dot gnu dot org
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: avr-unknown-none


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36212