On Mon, 1 Dec 2014, Joseph Myers wrote:
> On Mon, 1 Dec 2014, Richard Biener wrote:
>
> > +/* Combine two successive divisions. */
> > +(for div (trunc_div ceil_div floor_div round_div exact_div)
>
> This doesn't seem correct for all kinds of division and signedness of
> arguments.
>
> TRUNC_DIV_EXPR (C division) and EXACT_DIV_EXPR should be OK regardless.
> But for CEIL_DIV_EXPR and FLOOR_DIV_EXPR, in ((a / b) / c) you need c to
> be positive so that both roundings are in the same direction (consider
> e.g. 3 FLOOR_DIV -2 FLOOR_DIV -2 == 1, but 3 FLOOR_DIV 4 == 0). And for
> ROUND_DIV_EXPR, it can be incorrect whatever the signs (e.g. 2 ROUND_DIV 3
> ROUND_DIV 2 == 1, but 2 ROUND_DIV 6 == 0).
>
> I'm not sure if these forms of division actually occur in places where
> this could cause a problem, but it does look like Ada may enable you to
> generate ROUND_DIV_EXPR.
Hmm. I thought I was following what extract_muldiv_1 does (but of course
that function is quite hard to follow).
case TRUNC_DIV_EXPR: case CEIL_DIV_EXPR: case FLOOR_DIV_EXPR:
case ROUND_DIV_EXPR: case EXACT_DIV_EXPR:
...
/* If these are the same operation types, we can associate them
assuming no overflow. */
if (tcode == code)
{
bool overflow_p = false;
bool overflow_mul_p;
signop sign = TYPE_SIGN (ctype);
wide_int mul = wi::mul (op1, c, sign, &overflow_mul_p);
overflow_p = TREE_OVERFLOW (c) | TREE_OVERFLOW (op1);
if (overflow_mul_p
&& ((sign == UNSIGNED && tcode != MULT_EXPR) || sign ==
SIGNED))
overflow_p = true;
if (!overflow_p)
return fold_build2 (tcode, ctype, fold_convert (ctype, op0),
wide_int_to_tree (ctype, mul));
but yeah, I see you are correct. I'll restrict the pattern to
TRUNC_DIV_EXPR and EXACT_DIV_EXPR which are the only ones I've
ever seen generated from C.
I don't know how to generate testcases for the other division
opcodes - eventually we should have builtins just for the sake
of being able to generate testcases...
Committed as obvoious.
Thanks,
Richard.
2014-12-02 Richard Biener <rguent...@suse.de>
* match.pd: Restrict division combining to trunc_div and
exact_div.
Index: gcc/match.pd
===================================================================
--- gcc/match.pd (revision 218258)
+++ gcc/match.pd (working copy)
@@ -129,8 +129,9 @@ (define_operator_list inverted_tcc_compa
&& TYPE_UNSIGNED (type))
(trunc_div @0 @1)))
-/* Combine two successive divisions. */
-(for div (trunc_div ceil_div floor_div round_div exact_div)
+/* Combine two successive divisions. Note that combining ceil_div
+ and floor_div is trickier and combining round_div even more so. */
+(for div (trunc_div exact_div)
(simplify
(div (div @0 INTEGER_CST@1) INTEGER_CST@2)
(with {
