[Bug rtl-optimization/40838] New: gcc shouldn't assume that the stack is aligned

[mikulas at artax dot karlin dot mff dot cuni dot cz]

typedef int v4si __attribute__ ((vector_size (16)));

v4si y(v4si *s3)
{
        return *s3;
}

extern v4si s1, s2;

v4si x(void)
{
        v4si s3 = s1 + s2;
        return y(&s3);
}

And compile it with -O2 -fno-inline -msse2 -fomit-frame-pointer

The variable s3 is stored using unaligned store (movdqu) and loaded using
aligned load (movdqa). -mpreferred-stack-boundary=4 doesn't guarantee stack
alignment, it only advises that there is stack alignment (the function may be
called from OS callback, signal, another library compiled with lesser
alignment, etc... --- and i386 mandates only 4-byte stack alignment), so use of
movdqa is incorrect. (does GCC ABI mandate that all vector types must be
aligned? If so, then movdqa is correct, but storing it on the stack, relying on
alignment -mpreferred-stack-boundary=4 is not correct).

Now, if you compile it with -mpreferred-stack-boundary=2, function "x" aligns
the stack but uses movdqu to store on the aligned stack, so it generates
suboptimal code.


-- 
           Summary: gcc shouldn't assume that the stack is aligned
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mikulas at artax dot karlin dot mff dot cuni dot cz
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838