Hi! A zero or sign extension from result of some upwards_2limb operation is implemented in lower_mergeable_stmt as an extra loop which fills in the extra bits with 0s or 1s. If the delta of extended vs. unextended bit count is small, the code doesn't use a loop and emits up to a couple of stores to constant indexes, but if the delta is large, it uses cnt = (bo_bit != 0) + 1 + (rem != 0); statements. bo_bit is non-zero for bit-field loads and is done in that case as straight line, the unconditional 1 in there is for a loop which handles most of the limbs in the delta and finally (rem != 0) is for the case when the extended precision is not a multiple of limb_prec and is again done in straight line code (after the loop). The testcase ICEs because the decision what idx to use was incorrect for kind == bitint_prec_huge (i.e. when the precision delta is very large) and rem == 0 (i.e. the extended precision is multiple of limb_prec). In that case cnt is either 1 (if bo_bit == 0) or 2, and idx should be either first size_int (start) and then result of create_loop (for bo_bit != 0) or just result of create_loop, but by mistake the last case was size_int (end), which means when precision is multiple of limb_prec storing above the precision (which ICEs; but also not emitting the loop which is needed).
Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? 2023-12-05 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/112809 * gimple-lower-bitint.cc (bitint_large_huge::lower_mergeable_stmt): For separate_ext in kind == bitint_prec_huge mode if rem == 0, create for i == cnt - 1 the loop rather than using size_int (end). * gcc.dg/bitint-48.c: New test. --- gcc/gimple-lower-bitint.cc.jj 2023-12-05 09:48:14.000000000 +0100 +++ gcc/gimple-lower-bitint.cc 2023-12-05 18:55:58.996323144 +0100 @@ -2624,7 +2624,7 @@ bitint_large_huge::lower_mergeable_stmt { if (kind == bitint_prec_large || (i == 0 && bo_bit != 0)) idx = size_int (start + i); - else if (i == cnt - 1) + else if (i == cnt - 1 && (rem != 0)) idx = size_int (end); else if (i == (bo_bit != 0)) idx = create_loop (size_int (start + i), &idx_next); --- gcc/testsuite/gcc.dg/bitint-48.c.jj 2023-12-05 19:00:19.593664966 +0100 +++ gcc/testsuite/gcc.dg/bitint-48.c 2023-12-05 19:00:14.599735086 +0100 @@ -0,0 +1,23 @@ +/* PR tree-optimization/112809 */ +/* { dg-do compile { target bitint } } */ +/* { dg-options "-O2" } */ + +#if __BITINT_MAXWIDTH__ >= 512 +_BitInt (512) a; +_BitInt (256) b; +_BitInt (256) c; + +int +foo (void) +{ + return a == (b | c); +} + +void +bar (void) +{ + a /= b - 2; +} +#else +int i; +#endif Jakub