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While looking into the std::vector _M_realloc_insert codegen I noticed that call of __throw_bad_alloc is predicted with 10% probability. This is because the conditional guarding it has __builtin_expect (cond, 0) on it. This incorrectly takes precedence over more reliable heuristics predicting that call to cold noreturn is likely not going to happen. So I reordered the predictors so __builtin_expect_with_probability comes first after predictors that never makes a mistake (so user can use it to always specify the outcome by hand). I also downgraded malloc predictor since I do think user-defined malloc functions & new operators may behave funny ways and moved usual __builtin_expect after the noreturn cold predictor. This triggered latent bug in expr_expected_value_1 where if (*predictor < predictor2) *predictor = predictor2; should be: if (predictor2 < *predictor) *predictor = predictor2; which eventually triggered an ICE on combining heuristics. This made me notice that we can do slightly better while combining expected values in case only one of the parameters (such as in a*b when we expect a==0) can determine overall result. Note that the new code may pick weaker heuristics in case that both values are predicted. Not sure if this scenario is worth the extra CPU time: there is not correct way to combine the probabilities anyway since we do not know if the predictions are independent, so I think users should not rely on it. Fixing this issue uncovered another problem. In 2018 Martin Liska added code predicting that MALLOC returns non-NULL but instead of that he predicts that it returns true (boolean 1). This sort of works for testcase testing malloc (10) != NULL but, for example, we will predict malloc (10) == malloc (10) as true, which is not right and such comparsion may happen in real code I think proper way is to update expr_expected_value_1 to work with value ranges, but that needs greater surgery so I decided to postpone this and only add FIXME and fill PR110499. gcc/ChangeLog: PR middle-end/109849 * predict.cc (estimate_bb_frequencies): Turn to static function. (expr_expected_value_1): Fix handling of binary expressions with predicted values. * predict.def (PRED_MALLOC_NONNULL): Move later in the priority queue. (PRED_BUILTIN_EXPECT_WITH_PROBABILITY): Move to almost top of the priority queue. * predict.h (estimate_bb_frequencies): No longer declare it. gcc/testsuite/ChangeLog: PR middle-end/109849 * gcc.dg/predict-18.c: Improve testcase. |
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