Hi, https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81503 identifies a problem in SLSR where an invalid replacement is made because the desired value doesn't fit in the target type. This patch addresses that issue. The signed case is simple, as we require the value not to change when restricted to the precision of the target type. For the unsigned case, a negative value is expected to be converted to the congruent positive value, so we need to check for that. We have several test cases in the test suite that exercise the negative bump for an unsigned/pointer type.
Per Jakub's suggestion I attempted to do all of this within the wide_int and widest_int framework. However, I was unable to find a way to generate maxval as a widest_int within that framework. I can use wide_int maxval = wi::max_value (prec, sign); to get a wide_int, but I can't find a way to convert this to a widest_int so that I can add it to directly to bump. I am probably missing something obvious here, so I welcome any suggestions. At any rate, using TYPE_MAX_VALUE works, it's just not as clean as I would like. BTW, I plan a follow-up patch to rewrite the complex condition for executing the main block of code, which is starting to get out of control. It will be simpler to read if the conditions are negated and used to do an immediate return. Bootstrapped and tested on powerpc64le-linux-gnu with no regressions. Is this ok for trunk? (Just noticed a tab/space problem on the comment lines, which I will fix.) Thanks, Bill [gcc] 2017-08-03 Bill Schmidt <wschm...@linux.vnet.ibm.com> PR tree-optimization/81503 * gimple-ssa-strength-reduction.c (replace_mult_candidate): Ensure folded constant fits in the target type. [gcc/testsuite] 2017-08-03 Bill Schmidt <wschm...@linux.vnet.ibm.com> PR tree-optimization/81503 * gcc.c-torture/execute/pr81503.c: New file. Index: gcc/gimple-ssa-strength-reduction.c =================================================================== --- gcc/gimple-ssa-strength-reduction.c (revision 250791) +++ gcc/gimple-ssa-strength-reduction.c (working copy) @@ -2074,6 +2074,10 @@ replace_mult_candidate (slsr_cand_t c, tree basis_ { tree target_type = TREE_TYPE (gimple_assign_lhs (c->cand_stmt)); enum tree_code cand_code = gimple_assign_rhs_code (c->cand_stmt); + unsigned int prec = TYPE_PRECISION (target_type); + tree maxval = (POINTER_TYPE_P (target_type) + ? TYPE_MAX_VALUE (sizetype) + : TYPE_MAX_VALUE (target_type)); /* It is highly unlikely, but possible, that the resulting bump doesn't fit in a HWI. Abandon the replacement @@ -2082,6 +2086,17 @@ replace_mult_candidate (slsr_cand_t c, tree basis_ types but allows for safe negation without twisted logic. */ if (wi::fits_shwi_p (bump) && bump.to_shwi () != HOST_WIDE_INT_MIN + /* It is more likely that the bump doesn't fit in the target + type, so check whether constraining it to that type changes + the value. For a signed type, the value mustn't change. + For an unsigned type, the value may only change to a + congruent value (for negative bumps). */ + && (TYPE_UNSIGNED (target_type) + || wi::eq_p (bump, wi::ext (bump, prec, SIGNED))) + && (!TYPE_UNSIGNED (target_type) + || wi::eq_p (bump, wi::ext (bump, prec, UNSIGNED)) + || wi::eq_p (bump + wi::to_widest (maxval) + 1, + wi::ext (bump, prec, UNSIGNED))) /* It is not useful to replace casts, copies, negates, or adds of an SSA name and a constant. */ && cand_code != SSA_NAME Index: gcc/testsuite/gcc.c-torture/execute/pr81503.c =================================================================== --- gcc/testsuite/gcc.c-torture/execute/pr81503.c (nonexistent) +++ gcc/testsuite/gcc.c-torture/execute/pr81503.c (working copy) @@ -0,0 +1,15 @@ +unsigned short a = 41461; +unsigned short b = 3419; +int c = 0; + +void foo() { + if (a + b * ~(0 != 5)) + c = -~(b * ~(0 != 5)) + 2147483647; +} + +int main() { + foo(); + if (c != 2147476810) + return -1; + return 0; +}