On Thu, 9 Sept 2021 at 15:38, Roger Sayle <ro...@nextmovesoftware.com> wrote: > > > As observed by Jakub in comment #2 of PR 98865, the expression -(a>>63) > is optimized in GENERIC but not in GIMPLE. Investigating further it > turns out that this is one of a few transformations performed by > fold_negate_expr in fold-const.c that aren't yet performed by match.pd. > This patch moves/duplicates them there, and should be relatively safe > as these transformations are already performed by the compiler, but > just in different passes. IIRC, we should probably remove the code from fold-const.c while porting the pattern to match.pd, which I suppose is case RSHIFT_EXPR block in fold_negate_expr_1 near the end of the function ? > > Alas the one minor complication is that some of these transformations > are only wins, if the intermediate result (of the multiplication or > division) is only used once, to avoid duplication/performing them again. > See gcc.dg/tree-ssa/ssa-free-88.c. Normally, this is the perfect usage > of match's single_use (aka SSA's has_single_use). Alas, single_use is > not always accurate in match.pd, as some passes will construct and > simplify an expression/stmt before inserting it into GIMPLE, and folding > during this process sees the temporary undercount from the data-flow. > To solve this, this patch introduces a new single_use_is_op_p that > double checks that the single_use has the expected tree_code/operation > and skips the transformation if we can tell single_use might be invalid. > > A follow-up patch might be to investigate whether genmatch.c can be > tweaked to use this new helper function to implement the :s qualifier > when the enclosing context is/should be known, but that's overkill > to just unblock Jakub and Andrew on 98865. > > This patch has been tested on x86_64-pc-linux-gnu with a "make bootstrap" > and "make -k check" with no new failures. Ok for mainline? > > > 2021-09-09 Roger Sayle <ro...@nextmovesoftware.com> > > gcc/ChangeLog > * generic-match-head.c (single_use_is_op_p): New helper function. > * gimple-match-head.c (single_use_is_op_p): New helper function. > * match.pd (negation simplifications): Implement some negation > folding transformations from fold-const.c's fold_negate_expr. > > gcc/testsuite/ChangeLog > * gcc.dg/fold-negate-1.c: New test case. Just a small nit -- please include the test-case as part of the patch (instead of separate attachment), thanks!
Thanks, Prathamesh > > Roger > -- >
