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*From*: Richard Sandiford <richard dot sandiford at linaro dot org>*To*: Bin Cheng <Bin dot Cheng at arm dot com>*Cc*: "gcc-patches\@gcc.gnu.org" <gcc-patches at gcc dot gnu dot org>, nd <nd at arm dot com>*Date*: Sat, 10 Jun 2017 10:40:58 +0100*Subject*: Re: [PATCH PR78005]Fix miscompare issue by computing correct guard condition for vectorized loop*Authentication-results*: sourceware.org; auth=none*References*: <VI1PR0802MB2176844C26FACE87D72DF57FE7D20@VI1PR0802MB2176.eurprd08.prod.outlook.com>

Sorry to return this old patch, but: Bin Cheng <Bin.Cheng@arm.com> writes: > -/* Calculate the number of iterations under which scalar loop will be > - preferred than vectorized loop. NITERS_PROLOG is the number of > - iterations of prolog loop. If it's integer const, the integer > - number is also passed by INT_NITERS_PROLOG. VF is vector factor; > - TH is the threshold for vectorized loop if CHECK_PROFITABILITY is > - true. This function also store upper bound of the result in BOUND. */ > +/* Calculate the number of iterations above which vectorized loop will be > + preferred than scalar loop. NITERS_PROLOG is the number of iterations > + of prolog loop. If it's integer const, the integer number is also passed > + in INT_NITERS_PROLOG. BOUND_PROLOG is the upper bound (included) of > + number of iterations of prolog loop. VFM1 is vector factor minus one. > + If CHECK_PROFITABILITY is true, TH is the threshold below which scalar > + (rather than vectorized) loop will be executed. This function stores > + upper bound (included) of the result in BOUND_SCALAR. */ > > static tree > vect_gen_scalar_loop_niters (tree niters_prolog, int int_niters_prolog, > - int bound_prolog, int vf, int th, int *bound, > - bool check_profitability) > + int bound_prolog, int vfm1, int th, > + int *bound_scalar, bool check_profitability) > { > tree type = TREE_TYPE (niters_prolog); > tree niters = fold_build2 (PLUS_EXPR, type, niters_prolog, > - build_int_cst (type, vf)); > + build_int_cst (type, vfm1)); > > - *bound = vf + bound_prolog; > + *bound_scalar = vfm1 + bound_prolog; > if (check_profitability) > { > - th++; > + /* TH indicates the minimum niters of vectorized loop, while we > + compute the maximum niters of scalar loop. */ > + th--; Are you sure about this last change? It looks like it should be dropping the increment rather than replacing it with a decrement. It looks like the threshold is already the maximum niters for the scalar loop. It's set by: min_scalar_loop_bound = ((PARAM_VALUE (PARAM_MIN_VECT_LOOP_BOUND) * vectorization_factor) - 1); /* Use the cost model only if it is more conservative than user specified threshold. */ th = (unsigned) min_scalar_loop_bound; if (min_profitable_iters && (!min_scalar_loop_bound || min_profitable_iters > min_scalar_loop_bound)) th = (unsigned) min_profitable_iters; LOOP_VINFO_COST_MODEL_THRESHOLD (loop_vinfo) = th; (Note the "- 1" in the min_scalar_loop_bound. The multiplication result is the minimum niters for the vector loop.) min_profitable_iters sounds like it _ought_ to be the minimum niters for which the vector loop is used, but vect_estimate_min_profitable_iters instead returns the largest niters for which the scalar loop should be preferred: /* Cost model disabled. */ if (unlimited_cost_model (LOOP_VINFO_LOOP (loop_vinfo))) { dump_printf_loc (MSG_NOTE, vect_location, "cost model disabled.\n"); *ret_min_profitable_niters = 0; *ret_min_profitable_estimate = 0; return; } [...] /* Because the condition we create is: if (niters <= min_profitable_iters) then skip the vectorized loop. */ min_profitable_iters--; [...] min_profitable_estimate --; Also, when deciding whether the threshold needs to be used, we have: th = LOOP_VINFO_COST_MODEL_THRESHOLD (loop_vinfo); if (th >= LOOP_VINFO_VECT_FACTOR (loop_vinfo) - 1 && !LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)) { if (dump_enabled_p ()) dump_printf_loc (MSG_NOTE, vect_location, "Profitability threshold is %d loop iterations.\n", th); check_profitability = true; } where again the "- 1" seems to assume that the threshold is already the maximum niters for the scalar loop. (Although checking for == seems a bit pointless too.) The corresponding profitability check for loop versioning still assumes that th is the maximum niters for the scalar loop: if (check_profitability) cond_expr = fold_build2 (GT_EXPR, boolean_type_node, scalar_loop_iters, build_int_cst (TREE_TYPE (scalar_loop_iters), th)); since we use GT_EXPR instead of GE_EXPR. Thanks, Richard

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