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Re: [PATCH][middle-end/88784] Middle end is missing some optimizations about unsigned

On 2019/7/1 3:30 PM, Richard Biener wrote:
On Fri, 28 Jun 2019, Andrew Pinski wrote:

On Thu, Jun 27, 2019 at 9:55 PM Li Jia He <> wrote:

On 2019/6/27 11:48 PM, Jeff Law wrote:
On 6/27/19 12:11 AM, Li Jia He wrote:

According to the optimizable case described by Qi Feng on
issue 88784, we can combine the cases into the following:

1. x >  y  &&  x != XXX_MIN  -->   x > y
2. x >  y  &&  x == XXX_MIN  -->   false
3. x <= y  &&  x == XXX_MIN  -->   x == XXX_MIN

4. x <  y  &&  x != XXX_MAX  -->   x < y
5. x <  y  &&  x == XXX_MAX  -->   false
6. x >= y  &&  x == XXX_MAX  -->   x == XXX_MAX

7. x >  y  ||  x != XXX_MIN  -->   x != XXX_MIN
8. x <= y  ||  x != XXX_MIN  -->   true
9. x <= y  ||  x == XXX_MIN  -->   x <= y

10. x <  y  ||  x != XXX_MAX  -->   x != UXXX_MAX
11. x >= y  ||  x != XXX_MAX  -->   true
12. x >= y  ||  x == XXX_MAX  -->   x >= y

Note: XXX_MIN represents the minimum value of type x.
        XXX_MAX represents the maximum value of type x.

Here we don't need to care about whether the operation is
signed or unsigned.  For example, in the below equation:

'x >  y  &&  x != XXX_MIN  -->   x > y'

If the x type is signed int and XXX_MIN is INT_MIN, we can
optimize it to 'x > y'.  However, if the type of x is unsigned
int and XXX_MIN is 0, we can still optimize it to 'x > y'.

The regression testing for the patch was done on GCC mainline on

      powerpc64le-unknown-linux-gnu (Power 9 LE)

with no regressions.  Is it OK for trunk ?

Lijia He


2019-06-27  Li Jia He  <>
          Qi Feng  <>

      PR middle-end/88784
      * gimple-fold.c (and_comparisons_contain_equal_operands): New function.
      (and_comparisons_1): Use and_comparisons_contain_equal_operands.
      (or_comparisons_contain_equal_operands): New function.
      (or_comparisons_1): Use or_comparisons_contain_equal_operands.
Would this be better done via match.pd?  ISTM this transformation would
be well suited for that framework.

Hi, Jeff

I did this because of the following test case:
_Bool comp(unsigned x, unsigned y)
    return x > y && x != 0;
The gimple file dumped on the power platform is:
comp (unsigned int x, unsigned int y)
    _Bool D.2837;
    int iftmp.0;

    if (x > y) goto <D.2841>; else goto <D.2839>;
    if (x != 0) goto <D.2842>; else goto <D.2839>;
    iftmp.0 = 1;
    goto <D.2840>;
    iftmp.0 = 0;
    D.2837 = (_Bool) iftmp.0;
    return D.2837;
However, the gimple file dumped on x86 is
comp (unsigned int x, unsigned int y)
    _Bool D.2837;

    _1 = x > y;
    _2 = x != 0;
    _3 = _1 & _2;
    _4 = (int) _3;
    D.2837 = (_Bool) _4;
    return D.2837;

The reason for the inconsistency between these two behaviors is param
logical-op-non-short-circuit.  If we add the pattern to the match.pd
file, we can only optimize the situation in which the statement is in
the same basic block (logical-op-non-short-circuit=1, x86).  But for
a cross-basic block (logical-op-non-short-circuit=0, power), match.pd
can't handle this situation.

Another reason is that I found out maybe_fold_and_comparisons and
maybe_fold_or_comparisons are not only called by ifcombine pass but
also by reassoc pass. Using this method can basically unify param
logical-op-non-short-circuit=0 or 1.

As mentioned before ifcombine pass should be using gimple-match
instead of fold_build.  Try converting ifcombine over to gimple-match
infrastructure and add these to match.pd.

Yes, I mentioned that in the PR.  The issue is that at the moment
to combine x > y with x <= y you'd have to build GENERIC trees
for both or temporary GIMPLE assign with a SSA def (and then feed
that into the GENERIC or GIMPLE match.pd path).


I did some experimentation using ‘temporary GIMPLE with a SSA def (and then feed that into the GIMPLE match.pd path’. Could we consider the code in the attachment(I did a test and the code took effect)?

Lijia He

maybe_fold_and/or_comparisons handle two exploded binary expressions
while the current match.pd entries handle at most one exploded one
(the outermost then, either AND or OR).  But it would be definitely
doable to auto-generate maybe_fold_and/or_comparisons from match.pd
patterns which is what I'd ultimatively suggest to do (in some more
generalized form maybe).  Either with a separate genmatch invocation
or as part of the --gimple processing (not sure what is more feasible

I told Li Jia He that I don't expect him to do this work.

Note I didn't review the actual patch yet.


Attachment: match.diff
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