[Bug tree-optimization/19131] New: alloca returning unnecessarily aligned pointer and uses too much memory

[rguenth at tat dot physik dot uni-tuebingen dot de]

The testcase

int foo(int bar)
{
        int i, res = 0;
        for (i=0; i<bar; ++i) {
                int *x = __builtin_alloca(4);
                res += *x;
        }
        return res;
}

uses a lot more memory than necessary because of __builtin_alloca
apparently returning 16-byte aligned stack space (-O2 -fomit-frame-pointer):

.L5:
        subl    $32, %esp       #,
        incl    %edx    # i
        leal    15(%esp), %eax  #, tmp64
        andl    $-16, %eax      #, tmp64
        addl    (%eax), %ecx    #, res
        cmpl    %edx, %ebx      # i, bar
        jg      .L5     #,

I cannot find anything in the C standard about alloca, but alignment
bigger than the requested storage size should be not needed (though
this may be architecture dependent).

At least playing with alignment should be moved outside of the loop,
so we could save half of the wasted memory (I know the testcase is
dumb, but it's at least simple).

Btw. the same problem applies to 3.4, 3.3 doesn't play alignment
games but still allocates 16 bytes each time (that won't be aligned
to 16 bytes this way anyway?).  2.95 beats all of them again in
simplicity:

.L6:
        addl $-16,%esp
        addl (%esp),%eax
        decl %edx
        jnz .L6

-- 
           Summary: alloca returning unnecessarily aligned pointer and uses
                    too much memory
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at tat dot physik dot uni-tuebingen dot de
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131