[PATCH] Improve debug info in ivopts optimized loops (PR debug/90231)
Jakub Jelinek
jakub@redhat.com
Sat Oct 19 07:06:00 GMT 2019
Hi!
As mentioned in the PR, the following patch attempts to address two issues.
In remove_unused_ivs we first find the best iv_cand (we prefer primarily the
same step, next same mode and lastly constant base) and only then call
get_computation_at to determine the replacement expression. Unfortunately,
in various cases get_computation_at can return NULL_TREE and in that case we
don't try any other candidate and just leave the vars for debug as is, which
results in => NULL and the IVs <optimized away>.
The following patch will only consider candidates for which
get_computation_at succeeds, while it can be slower, it can handle more
cases. Perhaps alternative would be to have two passes, pick up the best
candidate without calling get_computation_at, calling it on the best
candidate and if that fails, retry the best candidate search with calling
get_computation_at.
Another thing is that get_computation_at can only handle cases where
use can be expressed as ubase + (var - cbase) * ratio for integral ratio.
In the PR testcase we don't have any, one (the one we pick as best)
has a ratio 1/15 and the other 1/4. Using a division in ivopts at runtime
is hardly ever desirable, but for debug info we don't mind that, all we need
to ensure is that we don't result in wrong-debug.
The patch implements expressing use as ubase + (var - cbase) / ratio for
these debug info only uses, but so far requires that if the use IV couldn't
wrap around, that the candidate (var - cbase) can't wrap around either,
which should be true whenever the candidate type is IMHO at least
ceil_log2 (ratio) bits larger than the use type. Do I need to punt if
!use->iv->no_overflow, or is
ubase + (utype) ((var - cbase) / ratio) computation safe even if it wraps?
And as questioned in the PR, are there other cases where we can safely
assume no wrap (e.g. use it if use->iv->no_overflow && cand->iv->no_overflow
without those extra precision checks)?
Anyway, bootstrapped/regtested successfully on x86_64-linux and i686-linux.
Comparing bootstrapped cc1plus without and with this patch (the former with
the patch applied after bootstrap and stage3 rebuilt, so that .text etc. is
identical) shows:
- [33] .debug_info PROGBITS 0000000000000000 22a4788 749275e 00 0 0 1
- [34] .debug_abbrev PROGBITS 0000000000000000 9736ee6 204aad 00 0 0 1
- [35] .debug_line PROGBITS 0000000000000000 993b993 1688464 00 0 0 1
- [36] .debug_str PROGBITS 0000000000000000 afc3df7 6f65aa 01 MS 0 0 1
- [37] .debug_loc PROGBITS 0000000000000000 b6ba3a1 71a2dde 00 0 0 1
- [38] .debug_ranges PROGBITS 0000000000000000 1285d17f 16414d0 00 0 0 1
- [39] .symtab SYMTAB 0000000000000000 13e9e650 166ff8 18 40 38193 8
- [40] .strtab STRTAB 0000000000000000 14005648 2ad809 00 0 0 1
- [41] .shstrtab STRTAB 0000000000000000 142b2e51 0001a0 00 0 0 1
+ [33] .debug_info PROGBITS 0000000000000000 22a4788 749365e 00 0 0 1
+ [34] .debug_abbrev PROGBITS 0000000000000000 9737de6 204a9f 00 0 0 1
+ [35] .debug_line PROGBITS 0000000000000000 993c885 1688f0c 00 0 0 1
+ [36] .debug_str PROGBITS 0000000000000000 afc5791 6f65aa 01 MS 0 0 1
+ [37] .debug_loc PROGBITS 0000000000000000 b6bbd3b 71cd404 00 0 0 1
+ [38] .debug_ranges PROGBITS 0000000000000000 1288913f 16414b0 00 0 0 1
+ [39] .symtab SYMTAB 0000000000000000 13eca5f0 166ff8 18 40 38193 8
+ [40] .strtab STRTAB 0000000000000000 140315e8 2ad809 00 0 0 1
+ [41] .shstrtab STRTAB 0000000000000000 142dedf1 0001a0 00 0 0 1
so .debug_info is 3840 bytes larger and .debug_loc is 173606 bytes larger
(0.15%), so there are some changes with this, but not huge amount of them,
though .debug_loc size changed in 217 gcc/*.o files out of 474.
2019-10-18 Jakub Jelinek <jakub@redhat.com>
PR debug/90231
* tree-ssa-loop-ivopts.c (get_debug_computation_at): New function.
(remove_unused_ivs): Use it instead of get_computation_at. When
choosing best candidate, only consider candidates where
get_debug_computation_at actually returns non-NULL.
--- gcc/tree-ssa-loop-ivopts.c.jj 2019-09-20 12:25:26.810718338 +0200
+++ gcc/tree-ssa-loop-ivopts.c 2019-10-18 23:55:10.054026219 +0200
@@ -4089,6 +4089,83 @@ get_computation_at (class loop *loop, gi
return fold_convert (type, aff_combination_to_tree (&aff));
}
+/* Like get_computation_at, but try harder, even if the computation
+ is more expensive. Intended for debug stmts. */
+
+static tree
+get_debug_computation_at (class loop *loop, gimple *at,
+ struct iv_use *use, struct iv_cand *cand)
+{
+ if (tree ret = get_computation_at (loop, at, use, cand))
+ return ret;
+
+ tree ubase = use->iv->base, ustep = use->iv->step;
+ tree cbase = cand->iv->base, cstep = cand->iv->step;
+ tree var;
+ tree utype = TREE_TYPE (ubase), ctype = TREE_TYPE (cbase);
+ widest_int rat;
+
+ /* We must have a precision to express the values of use. */
+ if (TYPE_PRECISION (utype) >= TYPE_PRECISION (ctype))
+ return NULL_TREE;
+
+ /* Try to handle the case that get_computation_at doesn't,
+ try to express
+ use = ubase + (var - cbase) / ratio. */
+ if (!constant_multiple_of (cstep, fold_convert (TREE_TYPE (cstep), ustep),
+ &rat))
+ return NULL_TREE;
+
+ bool neg_p = false;
+ if (wi::neg_p (rat))
+ {
+ if (TYPE_UNSIGNED (ctype))
+ return NULL_TREE;
+ neg_p = true;
+ rat = wi::neg (rat);
+ }
+
+ int bits = wi::exact_log2 (rat);
+ if (bits == -1)
+ bits = wi::floor_log2 (rat) + 1;
+ if (TYPE_PRECISION (utype) + bits > TYPE_PRECISION (ctype))
+ return NULL_TREE;
+
+ var = var_at_stmt (loop, cand, at);
+
+ if (POINTER_TYPE_P (ctype))
+ {
+ ctype = unsigned_type_for (ctype);
+ cbase = fold_convert (ctype, cbase);
+ cstep = fold_convert (ctype, cstep);
+ var = fold_convert (ctype, var);
+ }
+
+ ubase = unshare_expr (ubase);
+ cbase = unshare_expr (cbase);
+ if (stmt_after_increment (loop, cand, at))
+ var = fold_build2 (MINUS_EXPR, TREE_TYPE (var), var,
+ unshare_expr (cstep));
+
+ var = fold_build2 (MINUS_EXPR, TREE_TYPE (var), var, cbase);
+ var = fold_build2 (EXACT_DIV_EXPR, TREE_TYPE (var), var,
+ wide_int_to_tree (TREE_TYPE (var), rat));
+ if (POINTER_TYPE_P (utype))
+ {
+ var = fold_convert (sizetype, var);
+ if (neg_p)
+ var = fold_build1 (NEGATE_EXPR, sizetype, var);
+ var = fold_build2 (POINTER_PLUS_EXPR, utype, ubase, var);
+ }
+ else
+ {
+ var = fold_convert (utype, var);
+ var = fold_build2 (neg_p ? MINUS_EXPR : PLUS_EXPR, utype,
+ ubase, var);
+ }
+ return var;
+}
+
/* Adjust the cost COST for being in loop setup rather than loop body.
If we're optimizing for space, the loop setup overhead is constant;
if we're optimizing for speed, amortize it over the per-iteration cost.
@@ -7523,6 +7600,7 @@ remove_unused_ivs (struct ivopts_data *d
struct iv_use dummy_use;
struct iv_cand *best_cand = NULL, *cand;
unsigned i, best_pref = 0, cand_pref;
+ tree comp = NULL_TREE;
memset (&dummy_use, 0, sizeof (dummy_use));
dummy_use.iv = info->iv;
@@ -7543,20 +7621,22 @@ remove_unused_ivs (struct ivopts_data *d
? 1 : 0;
if (best_cand == NULL || best_pref < cand_pref)
{
- best_cand = cand;
- best_pref = cand_pref;
+ tree this_comp
+ = get_debug_computation_at (data->current_loop,
+ SSA_NAME_DEF_STMT (def),
+ &dummy_use, cand);
+ if (this_comp)
+ {
+ best_cand = cand;
+ best_pref = cand_pref;
+ comp = this_comp;
+ }
}
}
if (!best_cand)
continue;
- tree comp = get_computation_at (data->current_loop,
- SSA_NAME_DEF_STMT (def),
- &dummy_use, best_cand);
- if (!comp)
- continue;
-
if (count > 1)
{
tree vexpr = make_node (DEBUG_EXPR_DECL);
Jakub
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