On Fri, May 05, 2023 at 11:04:09AM +0200, Jakub Jelinek via Gcc-patches wrote: > As mentioned in the PR, save_expr seems to be very optimistic when > some expression is invariant, which can result in various wrong-code > issues. > The problem is with the TREE_READONLY (t) && !TREE_SIDE_EFFECTS (t) > case in tree_invariant_p_1. TREE_READONLY (t) in that case says > that the object shouldn't be modified during its lifetime and > !TREE_SIDE_EFFECTS (t) that it can be evaluated safely multiple times, > but that doesn't mean we can avoid wrapping the expression into SAVE_EXPR > say for a TREE_READONLY COMPONENT_REF with INDIRECT_REF as first operand > - either the lifetime of the TREE_READONLY object could end earlier than > when we need to reevaluate the object (that happens in the > pr52339-1.c case where save_expr is called on p->a and then free (p) is > done or pr52339.C where delete a->b when calling ~B () dtor deallocates a), > or e.g. the pointer could change as in pr52339-2.c (so evaluating p->a again > after ++p yields a possibly different value than originally and again we need > a SAVE_EXPR). > > Attached are two patches which fix this, unfortunately both regress > FAIL: gnat.dg/loop_optimization21.adb scan-tree-dump-times optimized > "Index_Check" 1 > FAIL: gnat.dg/vect1.adb scan-tree-dump-times vect "vectorized 1 loops" 15 > FAIL: gnat.dg/vect2.adb scan-tree-dump-times vect "vectorized 1 loops" 15 > FAIL: gnat.dg/vect3.adb scan-tree-dump-times vect "vectorized 1 loops" 15 > FAIL: gnat.dg/vect4.adb scan-tree-dump-times vect "vectorized 1 loops" 15 > FAIL: gnat.dg/vect5.adb scan-tree-dump-times vect "vectorized 1 loops" 15 > FAIL: gnat.dg/vect6.adb scan-tree-dump-times vect "vectorized 1 loops" 15 > on x86_64-linux (the first scan triggers 2 times rather than once, > the next 3 13 times rather than 15 and the last 3 14 times rather than 15 > times). > The first patch has been otherwise successfully bootstrapped/regtested on > x86_64-linux and i686-linux (with that above regressions), the second one > is probably better but has been so far tested just on the new testcases and > verified to also cause the above Ada regressions.
Looking at the Ada cases (I admit I don't really understand why it isn't vectorized, the IL is so different from the start because of the extra SAVE_EXPRs that it is very hard to diff stuff), the case where save_expr used to return the argument and no longer does are those r.P_BOUNDS->LB0 etc. cases. Now, I wondered if (pre-gimplification) we couldn't make an exception and allow the base to be INDIRECT_REF or of a REFERENCE_TYPE with the idea that references are really imutable and can't be changed during its lifetime (after gimplification whether something is REFERENCE_TYPE or POINTER_TYPE is lost), but that isn't what Ada is using. So, another possibility would be to allow bases of TREE_READONLY (t) && !TREE_SIDE_EFFECTS (t) which are INDIRECT_REFs of tree_invariant_p_1 addresses. That doesn't work either, in the r.P_BOUNDS->LB0 case P_BOUNDS is a FIELD_DECL with POINTER_TYPE, LB0 is TREE_READONLY FIELD_DECL and that COMPONENT_REF is also TREE_READONLY, r is TREE_READONLY PARM_DECL, but unforuntately the r.P_BOUNDS COMPONENT_REF isn't marked TREE_READONLY. Thus, shall we treat as tree_invariant_p_1 also handled components which are !TREE_SIDE_EFFECTS (t), but not TREE_READONLY and only their base is TREE_READONLY? Or do that only during the recursion? Jakub