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9baaae9344
Fixes PR27315. The post-inc version of an add recurrence needs to "follow the same rules" as a normal add or subtract expression. Otherwise we miscompile programs like ``` int main() { int a = 0; unsigned a_u = 0; volatile long last_value; do { a_u += 3; last_value = (long) ((int) a_u); if (will_add_overflow(a, 3)) { // Leave, and don't actually do the increment, so no UB. printf("last_value = %ld\n", last_value); exit(0); } a += 3; } while (a != 46); return 0; } ``` This patch changes SCEV to put no-wrap flags on post-inc add recurrences only when the poison from a potential overflow will go ahead to cause undefined behavior. To avoid regressing performance too much, I've assumed infinite loops without side effects is undefined behavior to prove poison<->UB equivalence in more cases. This isn't ideal, but is not new to LLVM as a whole, and far better than the situation I'm trying to fix. llvm-svn: 271151 |
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