On Mon, Jul 14, 2014 at 3:05 PM, Richard Biener
<[email protected]> wrote:
> On Mon, Jul 14, 2014 at 12:07 PM, Prathamesh Kulkarni
> <[email protected]> wrote:
>> I was wondering if it was a good idea to implement
>> predicate on expressions ?
>>
>> Sth like:
>> (match_and_simplify
>> (op (op2:predicate @0))
>> transform)
>>
>> instead of:
>> (match_and_simplify
>> (op (op2@1 @0))
>> if (predicate (@1))
>> transform)
>>
>> When predicate is simple as just being a macro/function,
>> we could use this style and when the predicate is more complex
>> resort to using if-expr (or write the predicate as macro in
>> gimple-match-head.c
>> and use the macro in pattern instead ...)
>>
>> Example:
>> we could rewrite the pattern
>> (match_and_simplify
>> (plus:c @0 (negate @1))
>> if (!TYPE_SATURATING (type))
>> (minus @0 @1))
>>
>> to
>>
>> (match_and_simplify
>> (plus:c:NOT_TYPE_SATURATING_P @0 (negate @1))
>> (minus @0 @1))
>>
>> with NOT_TYPE_SATURATING_P predicate defined
>> appropriately in gimple-match-head.c
>>
>> However I am not entirely sure if adding predicates on expressions
>> would be very useful....
>
> Well. I think there are two aspects to this. First is pattern
> readability where I think that the if-expr form is more readable.
> Second is the ability to do less work in the code generated
> from the decision tree.
>
> For example most of the patterns from associate_plusminus
> still miss the !TYPE_SATURATING && !FLOAT_TYPE_P &&
> !FIXED_POINT_TYPE_P if-expr. That is, we'd have
>
> /* (A +- B) - A -> +-B. */
> (match_and_simplify
> (minus (plus @0 @1) @0)
> if (!TYPE_SATURATING (type)
> && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
> @1)
> (match_and_simplify
> (minus (minus @0 @1) @0)
> if (!TYPE_SATURATING (type)
> && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
> (negate @1))
> /* (A +- B) -+ B -> A. */
> (match_and_simplify
> (minus (plus @0 @1) @1)
> if (!TYPE_SATURATING (type)
> && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
> @0)
> (match_and_simplify
> (plus:c (minus @0 @1) @1)
> if (!TYPE_SATURATING (type)
> && !FLOAT_TYPE_P (type) && !FIXED_POINT_TYPE_P (type))
> @0)
>
> with code-generation checking the if-expr after matching. And
> with using expression predicates we'd be able to check the
> predicate when matching the outermost 'minus' and "CSE"
> the predicate check for the first three patterns. Runtime-wise
> it depends on whether there is a point to back-track to.
Actually now that I look at the current state of the testsuite on the
branch and notice
FAIL: gcc.c-torture/execute/20081112-1.c execution, -O1
which points at
(match_and_simplify
(plus (plus @0 INTEGER_CST_P@1) INTEGER_CST_P@2)
(plus @0 (plus @1 @2))
which we may not apply to (a - 1) + INT_MIN as -1 + INT_MIN
overflows and a + (-1 + INT_MIN) then introduces undefined
signed integer overflow. tree-ssa-forwprop.c checks TREE_OVERFLOW
on the result of (plus @1 @2) and disables the simplification
properly. We can do the same with re-writing the pattern to
(match_and_simplify
(plus (plus @0 INTEGER_CST_P@1) INTEGER_CST_P@2)
/* If the constant operation overflows we cannot do the transform
as we would introduce undefined overflow, for example
with (a - 1) + INT_MIN. */
if (!TREE_OVERFLOW (@1 = int_const_binop (PLUS_EXPR, @1, @2)))
(plus @0 @1))
also using something I'd like to more formally allow (re-using sth
computed in the if-expr in the replacement). But of course writing
it this way is ugly and the following would be nicer?
(match_and_simplify
(plus (plus @0 INTEGER_CST_P@1) INTEGER_CST_P@2)
(plus @0 (plus:!TREE_OVERFLOW @1 @2)))
? That would be predicates on replacement expressions ...
(also negated predicates).
Now it doesn't look all-that-pretty :/
Another possibility is to always fail if TREE_OVERFLOW constants
leak into the replacement IL. (but I'd like to avoid those behind-the-backs
things at the moment)
Richard.
