https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121439
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2025-08-07
Status|UNCONFIRMED |NEW
--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
For machine generated code we suggest to use -O1
For me GCC 15 at -O0 takes ~3GB memory and 35s, the interesing part is -O1,
there I also see combine taking ages.
Note combine is inherently quadratic in particular with large BBs and it
does not have any limiting implemented. In particular LOG_LINK distance
isn't limited (and we do xyz-inbetween walks) and the retry extra walks
of insns makes it even cubic.
Something like the following tackles the latter, but I suspect for this
testcase it's also the former. See 2nd hunk below.
diff --git a/gcc/combine.cc b/gcc/combine.cc
index 4dbc1f6a4a4..40bb93755fb 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -1239,6 +1239,18 @@ combine_instructions (rtx_insn *f, unsigned int nregs)
label_tick_ebb_start = label_tick;
last_bb = this_basic_block;
+ rtx_insn *tem = BB_HEAD (this_basic_block);
+ if (!NONDEBUG_INSN_P (tem))
+ tem = next_nondebug_insn (tem);
+ int bb_first_luid = DF_INSN_LUID (tem);
+ tem = BB_END (this_basic_block);
+ if (!NONDEBUG_INSN_P (tem))
+ tem = prev_nondebug_insn (tem);
+ int bb_last_luid = DF_INSN_LUID (tem);
+ int extra_walk_budget
+ = bb_first_luid < bb_last_luid ? bb_last_luid - bb_first_luid : 0;
+ extra_walk_budget += 256;
+
rtl_profile_for_bb (this_basic_block);
for (insn = BB_HEAD (this_basic_block);
insn != NEXT_INSN (BB_END (this_basic_block));
@@ -1432,6 +1444,17 @@ combine_instructions (rtx_insn *f, unsigned int nregs)
record_dead_and_set_regs (insn);
retry:
+ if (next)
+ {
+ if (DF_INSN_LUID (next) < DF_INSN_LUID (insn))
+ {
+ if (DF_INSN_LUID (insn) - DF_INSN_LUID (next)
+ < extra_walk_budget)
+ next = NULL;
+ else
+ extra_walk_budget -= DF_INSN_LUID (insn) - DF_INSN_LUID
(next);
+ }
+ }
;
}
}
diff --git a/gcc/combine.cc b/gcc/combine.cc
index 4dbc1f6a4a4..c9e5c91c973 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -1079,6 +1079,10 @@ create_log_links (void)
&& asm_noperands (PATTERN (use_insn)) >= 0)
continue;
+ /* Don't add far away links. */
+ if (DF_INSN_LUID (use_insn) - DF_INSN_LUID (insn) > 256)
+ continue;
+
/* Don't add duplicate links between instructions. */
struct insn_link *links;
FOR_EACH_LOG_LINK (links, use_insn)
that get's combine in check but then afer a few minutes LRA blows up
with >25GB memory use so I had to kill it.
