On Thu, Mar 15, 2012 at 3:45 PM, Kai Tietz <ktiet...@googlemail.com> wrote: > 2012/3/15 Richard Guenther <richard.guent...@gmail.com>: >> On Thu, Mar 15, 2012 at 2:46 PM, Kai Tietz <ktiet...@googlemail.com> wrote: >>> 2012/3/15 Richard Guenther <richard.guent...@gmail.com>: >>>> On Thu, Mar 15, 2012 at 2:09 PM, Kai Tietz <ktiet...@googlemail.com> wrote: >>>>> Hi, >>>>> >>>>> this is the second part of the patch for this problem. It adds some >>>>> basic simplifications for ==/!= >>>>> comparisons for eliminating redudant operands. >>>>> >>>>> It adds the following patterns: >>>>> -X ==/!= Z - X -> Z ==/!= 0. >>>>> ~X ==/!= Z ^ X -> Z ==/!= ~0 >>>>> X ==/!= X - Y -> Y == 0 >>>>> X ==/!= X + Y -> Y == 0 >>>>> X ==/!= X ^ Y -> Y == 0 >>>>> (X - Y) ==/!= (Z - Y) -> X ==/!= Z >>>>> (Y - X) ==/!= (Y - Z) -> X ==/!= Z >>>>> (X + Y) ==/!= (X + Z) -> Y ==/!= Z >>>>> (X + Y) ==/!= (Z + X) -> Y ==/!= Z >>>>> (X ^ Y) ==/!= (Z ^ X) -> Y ==/!= Z >>>> >>>> Can you re-base this patch to work without the previous one? Also >>>> please coordinate with Andrew. Note that all of these(?) simplifications >>>> are already done by fold_comparison which we could share if you'd split >>>> out the EXPR_P op0/op1 cases with separated operands/code. >>>> >>>> Richard. >>> >>> Hmm, fold_comparison doesn't do the same thing as it checks for >>> possible overflow. This is true for comparisons not being ==/!= or >>> having operands of none-integral-type. But for ==/!= with integral >>> typed arguments the overflow doesn't matter at all. And exactly this >>> is what patch implements here. >> >> fold_comparison does not check for overflow for ==/!=. >> >>> This optimization of course is just desired in non-AST form, as we >>> otherwise loose information in FE. Therefore I didn't added it to >>> fold_const. >> >> Which pieces are not already in fold-const btw? forwprop already >> re-constructs trees for the defs of the lhs/rhs of a comparison. >> >> Richard. > > I have tried to use here instead a call to fold_build2 instead, and I > had to notice that it didn't optimized a single case (beside the - and > ~ case on both sides). > > I see in fold const for example in the pattern 'X +- C1 CMP Y +- C2' > to 'X CMP Y +- C2 +- C1' explicit the check for it. > > ... > /* Transform comparisons of the form X +- C1 CMP Y +- C2 to > X CMP Y +- C2 +- C1 for signed X, Y. This is valid if > the resulting offset is smaller in absolute value than the > original one. */ > if (TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0)) > && (TREE_CODE (arg0) == PLUS_EXPR || TREE_CODE (arg0) == MINUS_EXPR) > ...
Because the transform is not valid if Y +- C2 +- C1 overflows. It is not valid because overflow is undefined, not because the comparison would do the wrong thing. You'd have to change the addition to unsigned. > The same for pattern X +- C1 CMP C2 to X CMP C2 +- C1. Well, this is obviously just a missed optimization in fold-const.c then. Mind conditionalizing the overflow check to codes not NE_EXPR or EQ_EXPR? > The cases for '(X + Y) ==/!= (Z + X)' and co have the same issue or > are simply not present. That's true. I suppose they were considered too special to worry about. Did you see these cases in real code? > Sorry fold_const doesn't cover this at all. It covers part of it. > Kai