Hi, this improves the RTL generated for multi-word comparison to avoid generating useless branches. In do_jump_by_parts_greater_rtx: 1) do not generate the last cond jump, 2) do not generate the uncond branch to the drop-through label, if any, 3) generate only one comparison for the special (0 > x) case.
Tested on i586-suse-linux, OK for the mainline? 2011-03-24 Eric Botcazou <ebotca...@adacore.com> * dojump.c (do_jump_by_parts_greater_rtx): Optimize in specific cases. -- Eric Botcazou
Index: dojump.c =================================================================== --- dojump.c (revision 171345) +++ dojump.c (working copy) @@ -637,14 +637,32 @@ do_jump_by_parts_greater_rtx (enum machi { int nwords = (GET_MODE_SIZE (mode) / UNITS_PER_WORD); rtx drop_through_label = 0; + bool drop_through_if_true = false, drop_through_if_false = false; + enum rtx_code code = GT; int i; if (! if_true_label || ! if_false_label) drop_through_label = gen_label_rtx (); if (! if_true_label) - if_true_label = drop_through_label; + { + if_true_label = drop_through_label; + drop_through_if_true = true; + } if (! if_false_label) - if_false_label = drop_through_label; + { + if_false_label = drop_through_label; + drop_through_if_false = true; + } + + /* Deal with the special case 0 > x: only one comparison is necessary and + we reverse it to avoid jumping to the drop-through label. */ + if (op0 == const0_rtx && drop_through_if_true && !drop_through_if_false) + { + code = LE; + if_true_label = if_false_label; + if_false_label = drop_through_label; + drop_through_if_false = true; + } /* Compare a word at a time, high order first. */ for (i = 0; i < nwords; i++) @@ -663,17 +681,20 @@ do_jump_by_parts_greater_rtx (enum machi } /* All but high-order word must be compared as unsigned. */ - do_compare_rtx_and_jump (op0_word, op1_word, GT, - (unsignedp || i > 0), word_mode, NULL_RTX, - NULL_RTX, if_true_label, prob); + do_compare_rtx_and_jump (op0_word, op1_word, code, (unsignedp || i > 0), + word_mode, NULL_RTX, NULL_RTX, if_true_label, + prob); + + /* Emit only one comparison for 0. Do not emit the last cond jump. */ + if (op0 == const0_rtx || i == nwords - 1) + break; /* Consider lower words only if these are equal. */ do_compare_rtx_and_jump (op0_word, op1_word, NE, unsignedp, word_mode, - NULL_RTX, NULL_RTX, if_false_label, - inv (prob)); + NULL_RTX, NULL_RTX, if_false_label, inv (prob)); } - if (if_false_label) + if (!drop_through_if_false) emit_jump (if_false_label); if (drop_through_label) emit_label (drop_through_label);