Hi! The optimize_range_tests_to_bit_test optimization normally emits a range test first: if (entry_test_needed) { tem = build_range_check (loc, optype, unshare_expr (exp), false, lowi, high); if (tem == NULL_TREE || is_gimple_val (tem)) continue; } so during the bit test we already know that exp is in the [lowi, high] range, but skips it if we have range info which tells us this isn't necessary. Also, normally it emits shifts by exp - lowi counter, but has an optimization to use just exp counter if the mask isn't a more expensive constant in that case and lowi is > 0 and high is smaller than prec.
The following testcase is miscompiled because the two abnormal cases are triggered. The range of exp is [43, 43][48, 48][95, 95], so we on 64-bit arch decide we don't need the entry test, because 95 - 43 < 64. And we also decide to use just exp as counter, because the range test tests just for exp == 43 || exp == 48, so high is smaller than 64 too. Because 95 is in the exp range, we can't do that, we'd either need to do a range test first, i.e. if (exp - 43U <= 48U - 43U) if ((1UL << exp) & mask1)) or need to subtract lowi from the shift counter, i.e. if ((1UL << (exp - 43)) & mask2) but can't do both unless r.upper_bound () is < prec. The following patch ensures that. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? 2024-05-08 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/114965 * tree-ssa-reassoc.cc (optimize_range_tests_to_bit_test): Don't try to optimize away exp - lowi subtraction from shift count unless entry test is emitted or unless r.upper_bound () is smaller than prec. * gcc.c-torture/execute/pr114965.c: New test. --- gcc/tree-ssa-reassoc.cc.jj 2024-01-12 10:07:58.384848977 +0100 +++ gcc/tree-ssa-reassoc.cc 2024-05-07 18:18:45.558814991 +0200 @@ -3418,7 +3418,8 @@ optimize_range_tests_to_bit_test (enum t We can avoid then subtraction of the minimum value, but the mask constant could be perhaps more expensive. */ if (compare_tree_int (lowi, 0) > 0 - && compare_tree_int (high, prec) < 0) + && compare_tree_int (high, prec) < 0 + && (entry_test_needed || wi::ltu_p (r.upper_bound (), prec))) { int cost_diff; HOST_WIDE_INT m = tree_to_uhwi (lowi); --- gcc/testsuite/gcc.c-torture/execute/pr114965.c.jj 2024-05-07 18:17:16.767031821 +0200 +++ gcc/testsuite/gcc.c-torture/execute/pr114965.c 2024-05-07 18:15:52.332188943 +0200 @@ -0,0 +1,30 @@ +/* PR tree-optimization/114965 */ + +static void +foo (const char *x) +{ + + char a = '0'; + while (1) + { + switch (*x) + { + case '_': + case '+': + a = *x; + x++; + continue; + default: + break; + } + break; + } + if (a == '0' || a == '+') + __builtin_abort (); +} + +int +main () +{ + foo ("_"); +} Jakub