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.

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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) ... The same for pattern X +- C1 CMP C2 to X CMP C2 +- C1. The cases for '(X + Y) ==/!= (Z + X)' and co have the same issue or are simply not present. Sorry fold_const doesn't cover this at all. Kai