2012/3/21 Richard Guenther <richard.guent...@gmail.com>: > 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

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Sure, the test code shown in this patch isn't that unusual. Especially in gimple (by using different statements) such construct are happening. Eg.: int f1 (int a, int b, int c) { if ((a + b) == (c + a)) return 1; return 0; } int f2 (int a, int b, int c) { if ((a ^ b) == (a ^ c)) return 1; return 0; } int f2 (int a, int b) { if (-a == (b - a)) return 1; return 0; } In all those cases the use of variable should be optimized out. Instead we are producing pretty weak code for those cases. Kai