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Date: Thu, 26 Feb 2009 16:57:00 -0000
Subject: [Bug middle-end/39315]  New: Unaligned move used on aligned stack variable
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From: "hjl dot tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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[hjl@gnu-6 pr]$ cat x.c
typedef float __m128 __attribute__ ((__vector_size__ (16), __may_alias__));
extern void bar (__m128 *);
void
foo (__m128 *x)
{
  __m128 b = *x;
  bar (&b);
}
[hjl@gnu-6 pr]$ make x.s
/export/build/gnu/gcc-work/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-work/build-x86_64-linux/gcc/ -S -o x.s -O2
-fno-asynchronous-unwind-tables x.c
[hjl@gnu-6 pr]$ cat x.s
        .file   "x.c"
        .text
        .p2align 4,,15
.globl foo
        .type   foo, @function
foo:
        subq    $24, %rsp
        movaps  (%rdi), %xmm0
        movq    %rsp, %rdi
        movlps  %xmm0, (%rsp)
        movhps  %xmm0, 8(%rsp)
        call    bar
        addq    $24, %rsp
        ret

expand_one_stack_var_at has

  /* Set alignment we actually gave this decl.  */
  offset -= frame_phase;
  align = offset & -offset;
  align *= BITS_PER_UNIT;
  if (align > STACK_BOUNDARY || align == 0)
    align = STACK_BOUNDARY;
  DECL_ALIGN (decl) = align;
  DECL_USER_ALIGN (decl) = 0;

  set_mem_attributes (x, decl, true);

That is we always set alignment attribute of a stack variable
to <= STACK_BOUNDARY even if we can align it properly.  One
consequence is unaligned move is used on aligned memory.


-- 
           Summary: Unaligned move used on aligned stack variable
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hjl dot tools at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39315