> The problem here seems to be that the FE (or the GIMPLE generation)
> transforms
>
> addr + ((on_off-1)*2)
>
> into
>
> addr + ((1-on_off)*-2)
>
> and nothing in -Os has the wit to recover. This badly affects targets
> with no hardware multiplier, which end up calling libgcc to do the
> multiply.
>
> Is this a well-known problem?
We very recently ran into it, although it's quite old. It comes from:
2005-11-19 Richard Guenther <rguent...@suse.de>
PR middle-end/23294
* fold-const.c (fold_plusminus_mult_expr): New function.
(fold_binary): Use to canonicalize PLUS_EXPR and MINUS_EXPR
cases, remove now unnecessary code.
which has changed the canonicalization of addresses as well, without anyone
apparently noticing. In particular,
[...]
We also canonicalize (X + 7) * 4 into X * 4 + 28 in the hope that either
the machine has a multiply-accumulate insn or that this is part of an
addressing calculation.
[...]
static tree
extract_muldiv (tree t, tree c, enum tree_code code, tree wide_type,
bool *strict_overflow_p)
is now wrong since it's undone by fold_plusminus_mult_expr.
I've attached the patch (against the 4.3 branch) we use locally.
* fold-const.c (extract_muldiv): Remove obsolete comment.
(fold_plusminus_mult_expr): Use only positive power-of-two factors.
* expr.c (get_inner_reference): Canonicalize offset.
--
Eric Botcazou
*** gcc/fold-const.c.0 2008-12-11 10:47:40.000000000 +0100
--- gcc/fold-const.c 2008-12-11 20:08:18.000000000 +0100
*************** optimize_minmax_comparison (enum tree_co
*** 6010,6019 ****
expression would not overflow or that overflow is undefined for the type
in the language in question.
- We also canonicalize (X + 7) * 4 into X * 4 + 28 in the hope that either
- the machine has a multiply-accumulate insn or that this is part of an
- addressing calculation.
-
If we return a non-null expression, it is an equivalent form of the
original computation, but need not be in the original type.
--- 6010,6015 ----
*************** fold_plusminus_mult_expr (enum tree_code
*** 7459,7497 ****
else if (operand_equal_p (arg01, arg10, 0))
same = arg01, alt0 = arg00, alt1 = arg11;
! /* No identical multiplicands; see if we can find a common
! power-of-two factor in non-power-of-two multiplies. This
! can help in multi-dimensional array access. */
! else if (host_integerp (arg01, 0)
! && host_integerp (arg11, 0))
! {
! HOST_WIDE_INT int01, int11, tmp;
! bool swap = false;
! tree maybe_same;
! int01 = TREE_INT_CST_LOW (arg01);
! int11 = TREE_INT_CST_LOW (arg11);
/* Move min of absolute values to int11. */
! if ((int01 >= 0 ? int01 : -int01)
! < (int11 >= 0 ? int11 : -int11))
! {
tmp = int01, int01 = int11, int11 = tmp;
! alt0 = arg00, arg00 = arg10, arg10 = alt0;
! maybe_same = arg01;
swap = true;
}
else
! maybe_same = arg11;
! if (exact_log2 (abs (int11)) > 0 && int01 % int11 == 0)
! {
alt0 = fold_build2 (MULT_EXPR, TREE_TYPE (arg00), arg00,
build_int_cst (TREE_TYPE (arg00),
! int01 / int11));
! alt1 = arg10;
! same = maybe_same;
if (swap)
! maybe_same = alt0, alt0 = alt1, alt1 = maybe_same;
}
}
--- 7455,7497 ----
else if (operand_equal_p (arg01, arg10, 0))
same = arg01, alt0 = arg00, alt1 = arg11;
! /* No identical multiplicands; see if we can find a common positive
! power-of-two factor. This can help in multi-dim array accesses. */
! else if (host_integerp (arg01, 0) && host_integerp (arg11, 0))
! {
! HOST_WIDE_INT int01 = TREE_INT_CST_LOW (arg01);
! HOST_WIDE_INT int11 = TREE_INT_CST_LOW (arg11);
! HOST_WIDE_INT factor, tmp;
! tree tmp_tree;
! bool swap;
/* Move min of absolute values to int11. */
! if ((int01 >= 0 ? int01 : -int01) < (int11 >= 0 ? int11 : -int11))
! {
tmp = int01, int01 = int11, int11 = tmp;
! tmp_tree = arg00, arg00 = arg10, arg10 = tmp_tree;
swap = true;
}
else
! swap = false;
! if (int11 < 0)
! factor = -int11;
! else
! factor = int11;
!
! if (exact_log2 (factor) > 0 && (int01 % factor) == 0)
! {
alt0 = fold_build2 (MULT_EXPR, TREE_TYPE (arg00), arg00,
build_int_cst (TREE_TYPE (arg00),
! int01 / factor));
! if (int11 < 0)
! alt1 = fold_build1 (NEGATE_EXPR, TREE_TYPE (arg10), arg10);
! else
! alt1 = arg10;
if (swap)
! tmp_tree = alt0, alt0 = alt1, alt1 = tmp_tree;
! same = build_int_cst (TREE_TYPE (arg10), factor);
}
}
*** gcc/expr.c.0 2008-12-11 10:54:49.000000000 +0100
--- gcc/expr.c 2008-12-11 20:29:28.000000000 +0100
*************** get_inner_reference (tree exp, HOST_WIDE
*** 6042,6047 ****
--- 6042,6066 ----
{
*pbitpos = tree_low_cst (bit_offset, 0);
*poffset = offset;
+
+ /* The middle-end used to canonicalize (X + 7) * 4 into X * 4 + 28 at
+ the tree level, but that is no longer the case. However the latter
+ form is still the preferred one in addressing calculations. */
+ if (TREE_CODE (offset) == MULT_EXPR
+ && TREE_CODE (TREE_OPERAND (offset, 1)) == INTEGER_CST)
+ {
+ tree op0 = TREE_OPERAND (offset, 0), disp;
+ enum tree_code code = TREE_CODE (op0);
+ if ((code == PLUS_EXPR || code == MINUS_EXPR)
+ && TREE_CODE (TREE_OPERAND (op0, 1)) == INTEGER_CST
+ && (disp = int_const_binop (MULT_EXPR, TREE_OPERAND (op0, 1),
+ TREE_OPERAND (offset, 1), 0))
+ && !TREE_OVERFLOW (disp))
+ *poffset = build2 (code, TREE_TYPE (offset),
+ size_binop (MULT_EXPR, TREE_OPERAND (op0, 0),
+ TREE_OPERAND (offset, 1)),
+ disp);
+ }
}
/* We can use BLKmode for a byte-aligned BLKmode bitfield. */
