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*From*: Jakub Jelinek <jakub at redhat dot com>*To*: Richard Biener <rguenther at suse dot de>*Cc*: gcc-patches at gcc dot gnu dot org*Date*: Mon, 30 Sep 2013 15:29:08 +0200*Subject*: [PATCH] Fix A < 0 ? <sign bit of A> : 0 optimization (PR middle-end/58564)*Authentication-results*: sourceware.org; auth=none*Reply-to*: Jakub Jelinek <jakub at redhat dot com>

Hi! Apparently sign_bit_p looks through both sign and zero extensions. That is just fine for the single bit test optimization, where we know the constant is a power of two, and it is (A & power_of_two) == 0 (or != 0) test. sign_bit_p is also used to check for minimum value of some integral type, because it is called with the same argument twice, it must be INTEGER_CST if non-NULL is returned and thus there is no zero extensions. But in the last spot where sign_bit_p is used, sign extensions would be fine, ((int) x) < 0 for say signed char or signed short x iff x < 0, but if there is a zero extension, it is just a possible missed optimization on the comparison. Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk/4.8? 2013-09-30 Jakub Jelinek <jakub@redhat.com> PR middle-end/58564 * fold-const.c (fold_ternary_loc): For A < 0 : <sign bit of A> : 0 optimization, punt if sign_bit_p looked through any zero extension. * gcc.c-torture/execute/pr58564.c: New test. --- gcc/fold-const.c.jj 2013-09-27 15:42:37.000000000 +0200 +++ gcc/fold-const.c 2013-09-30 11:19:06.333978484 +0200 @@ -14196,14 +14196,29 @@ fold_ternary_loc (location_t loc, enum t && integer_zerop (op2) && (tem = sign_bit_p (TREE_OPERAND (arg0, 0), arg1))) { + /* sign_bit_p looks through both zero and sign extensions, + but for this optimization only sign extensions are + usable. */ + tree tem2 = TREE_OPERAND (arg0, 0); + while (tem != tem2) + { + if (TREE_CODE (tem2) != NOP_EXPR + || TYPE_UNSIGNED (TREE_TYPE (TREE_OPERAND (tem2, 0)))) + { + tem = NULL_TREE; + break; + } + tem2 = TREE_OPERAND (tem2, 0); + } /* sign_bit_p only checks ARG1 bits within A's precision. If <sign bit of A> has wider type than A, bits outside of A's precision in <sign bit of A> need to be checked. If they are all 0, this optimization needs to be done in unsigned A's type, if they are all 1 in signed A's type, otherwise this can't be done. */ - if (TYPE_PRECISION (TREE_TYPE (tem)) - < TYPE_PRECISION (TREE_TYPE (arg1)) + if (tem + && TYPE_PRECISION (TREE_TYPE (tem)) + < TYPE_PRECISION (TREE_TYPE (arg1)) && TYPE_PRECISION (TREE_TYPE (tem)) < TYPE_PRECISION (type)) { --- gcc/testsuite/gcc.c-torture/execute/pr58564.c.jj 2013-09-30 11:09:38.691122488 +0200 +++ gcc/testsuite/gcc.c-torture/execute/pr58564.c 2013-09-30 11:09:14.000000000 +0200 @@ -0,0 +1,14 @@ +/* PR middle-end/58564 */ + +extern void abort (void); +int a, b; +short *c, **d = &c; + +int +main () +{ + b = (0, 0 > ((&c == d) & (1 && (a ^ 1)))) | 0U; + if (b != 0) + abort (); + return 0; +} Jakub

**Follow-Ups**:**Re: [PATCH] Fix A < 0 ? <sign bit of A> : 0 optimization (PR middle-end/58564)***From:*Richard Biener

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