Hi, Since SCCVN operates on SSA graph instead of the control flow graph for the sake of efficiency, it does not handle or value number the conditional expression of GIMPLE_COND statement. As a result, FRE/PRE does not simplify conditional expression, as reported in bug 30997.
Since it would be complicate and difficult to process conditional expression in currently SCCVN algorithm, how about following method? STEP1 Before starting FRE/PRE, we can factor out the conditional expression, like change following codes: -------------------------------- if (cond_expr) goto lable_a else goto label_b into codes: -------------------------------- tmp = cond_expr if (tmp == 1) goto label_a else goto label_b STEP2 Let SCCVN/FRE/PRE do its job on value numbering cond_expr and redundancy elimination; STEP3 After FRE/PRE, for those "tmp=cond_expr" not used in any redundancy elimination, we can forward it to the corresponding GIMPLE_COND statement, just like tree-ssa-forwprop.c. In this way, the conditional expression will be handled as other expressions and no redundant assignment generated. Most important,this does not affect the current implementation of SCCVN/FRE/PRE. The only problem is the method cannot work on reversion of conditional expression. For example: -------------------------------- x = a > 2; if (a<=2) goto label_a else goto lable_b could be optimized as: -------------------------------- x = a > 2 if (x == 0) goto label_a else goto label_b I have worked a draft patch to do the work and would like to hear your comments on this. Thanks very much. -- Best Regards.
