https://gcc.gnu.org/g:a1a0026c659196928113bad1c7889f5ca0999d06
commit r15-9242-ga1a0026c659196928113bad1c7889f5ca0999d06
Author: Richard Sandiford <richard.sandif...@arm.com>
Date: Mon Apr 7 08:03:49 2025 +0100
combine: Limit insn searchs for 2->2 combinations [PR116398]
As noted in the previous patch, combine still takes >30% of
compile time in the original testcase for PR101523. The problem
is that try_combine uses linear insn searches for some dataflow
queries, so in the worst case, an unlimited number of 2->2
combinations for the same i2 can lead to quadratic behaviour.
This patch limits distribute_links to a certain number
of instructions when i2 is unchanged. As Segher said in the PR trail,
it would make more conceptual sense to apply the limit unconditionally,
but I thought it would be better to change as little as possible at
this development stage. Logically, in stage 1, the --param should
be applied directly by distribute_links with no input from callers.
As I mentioned in:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116398#c28
I think it's safe to drop log links even if a use exists. All
processing of log links seems to handle the absence of a link
for a particular register in a conservative way.
The initial set-up errs on the side of dropping links, since for example
create_log_links has:
/* flow.c claimed:
We don't build a LOG_LINK for hard registers contained
in ASM_OPERANDs. If these registers get replaced,
we might wind up changing the semantics of the insn,
even if reload can make what appear to be valid
assignments later. */
if (regno < FIRST_PSEUDO_REGISTER
&& asm_noperands (PATTERN (use_insn)) >= 0)
continue;
which excludes combinations by dropping log links, rather than during
try_combine. And:
/* If this register is being initialized using itself, and the
register is uninitialized in this basic block, and there are
no LOG_LINKS which set the register, then part of the
register is uninitialized. In that case we can't assume
anything about the number of nonzero bits.
??? We could do better if we checked this in
reg_{nonzero_bits,num_sign_bit_copies}_for_combine. Then we
could avoid making assumptions about the insn which initially
sets the register, while still using the information in other
insns. We would have to be careful to check every insn
involved in the combination. */
if (insn
&& reg_referenced_p (x, PATTERN (insn))
&& !REGNO_REG_SET_P (DF_LR_IN (BLOCK_FOR_INSN (insn)),
REGNO (x)))
{
struct insn_link *link;
FOR_EACH_LOG_LINK (link, insn)
if (dead_or_set_p (link->insn, x))
break;
if (!link)
{
rsp->nonzero_bits = GET_MODE_MASK (mode);
rsp->sign_bit_copies = 1;
return;
}
}
treats the lack of a log link as a possible sign of uninitialised data,
but that would be a missed optimisation rather than a correctness issue.
One question is what the default --param value should be. I went with
Jakub's suggestion of 3000 from:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116398#c25
Also, to answer Jakub's question in that comment, I tried bisecting:
int limit = atoi (getenv ("BISECT"));
(so applying the limit for all calls from try_combine) with an
abort in distribute_links if the limit caused a link to be skipped.
The minimum BISECT value that allowed an aarch64-linux-gnu bootstrap
to succeed with --enable-languages=all --enable-checking=yes,rtl,extra
was 142, so much lower than the parameter value. I realised too late
that --enable-checking=release would probably have been a more
interesting test.
The previous patch meant that distribute_links itself is now linear
for a given i2 definition, since each search starts at the previous
last use, rather than at i2 itself. This means that the limit has
to be applied cumulatively across all searches for the same link.
The patch does that by storing a counter in the insn_link structure.
There was a 32-bit hole there on LP64 hosts.
gcc/
PR testsuite/116398
* params.opt (-param=max-combine-search-insns=): New param.
* doc/invoke.texi: Document it.
* combine.cc (insn_link::insn_count): New field.
(alloc_insn_link): Initialize it.
(distribute_links): Add a limit parameter.
(try_combine): Use the new param to limit distribute_links
when only i3 has changed.
Diff:
---
gcc/combine.cc | 21 +++++++++++++++++----
gcc/doc/invoke.texi | 9 +++++++++
gcc/params.opt | 4 ++++
3 files changed, 30 insertions(+), 4 deletions(-)
diff --git a/gcc/combine.cc b/gcc/combine.cc
index e99b064c98d4..5f085187cfef 100644
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -309,6 +309,7 @@ static int *uid_insn_cost;
struct insn_link {
rtx_insn *insn;
unsigned int regno;
+ int insn_count;
struct insn_link *next;
};
@@ -342,6 +343,7 @@ alloc_insn_link (rtx_insn *insn, unsigned int regno, struct
insn_link *next)
sizeof (struct insn_link));
l->insn = insn;
l->regno = regno;
+ l->insn_count = 0;
l->next = next;
return l;
}
@@ -472,7 +474,8 @@ static void move_deaths (rtx, rtx, int, rtx_insn *, rtx *);
static bool reg_bitfield_target_p (rtx, rtx);
static void distribute_notes (rtx, rtx_insn *, rtx_insn *, rtx_insn *,
rtx, rtx, rtx);
-static void distribute_links (struct insn_link *, rtx_insn * = nullptr);
+static void distribute_links (struct insn_link *, rtx_insn * = nullptr,
+ int limit = INT_MAX);
static void mark_used_regs_combine (rtx);
static void record_promoted_value (rtx_insn *, rtx);
static bool unmentioned_reg_p (rtx, rtx);
@@ -4593,7 +4596,7 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1,
rtx_insn *i0,
}
if (only_i3_changed)
- distribute_links (i3links, i3);
+ distribute_links (i3links, i3, param_max_combine_search_insns);
else
{
distribute_links (i3links);
@@ -14994,10 +14997,12 @@ distribute_notes (rtx notes, rtx_insn *from_insn,
rtx_insn *i3, rtx_insn *i2,
pointing at I3 when I3's destination is changed.
If START is nonnull and an insn, we know that the next location for each
- link is no earlier than START. */
+ link is no earlier than START. LIMIT is the maximum number of nondebug
+ instructions that can be scanned when looking for the next use of a
+ definition. */
static void
-distribute_links (struct insn_link *links, rtx_insn *start)
+distribute_links (struct insn_link *links, rtx_insn *start, int limit)
{
struct insn_link *link, *next_link;
@@ -15063,9 +15068,12 @@ distribute_links (struct insn_link *links, rtx_insn
*start)
I3 to I2. Also note that not much searching is typically done here
since most links don't point very far away. */
+ int count = 0;
insn = start;
if (!insn || NOTE_P (insn))
insn = NEXT_INSN (link->insn);
+ else
+ count = link->insn_count;
for (;
(insn && (this_basic_block->next_bb == EXIT_BLOCK_PTR_FOR_FN (cfun)
|| BB_HEAD (this_basic_block->next_bb) != insn));
@@ -15086,6 +15094,11 @@ distribute_links (struct insn_link *links, rtx_insn
*start)
}
else if (INSN_P (insn) && reg_set_p (reg, insn))
break;
+ else if (count >= limit)
+ break;
+ else
+ count += 1;
+ link->insn_count = count;
/* If we found a place to put the link, place it there unless there
is already a link to the same insn as LINK at that point. */
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index d5103f461dfd..4f0d3a72a8f8 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -16516,6 +16516,15 @@ in combiner for a pseudo register as last known value
of that register.
@item max-combine-insns
The maximum number of instructions the RTL combiner tries to combine.
+@item max-combine-search-insns
+The maximum number of instructions that the RTL combiner searches in order
+to find the next use of a given register definition. If this limit is reached
+without finding such a use, the combiner will stop trying to optimize the
+definition.
+
+Currently this limit only applies after certain successful combination
+attempts, but it could be extended to other cases in future.
+
@item integer-share-limit
Small integer constants can use a shared data structure, reducing the
compiler's memory usage and increasing its speed. This sets the maximum
diff --git a/gcc/params.opt b/gcc/params.opt
index 4f4eb4d7a2a5..422d082b3557 100644
--- a/gcc/params.opt
+++ b/gcc/params.opt
@@ -477,6 +477,10 @@ The maximum number of instructions to consider to unroll
in a loop on average.
Common Joined UInteger Var(param_max_combine_insns) Init(4) IntegerRange(2, 4)
Param Optimization
The maximum number of insns combine tries to combine.
+-param=max-combine-search-insns=
+Common Joined UInteger Var(param_max_combine_search_insns) Init(3000) Param
Optimization
+The maximum number of instructions that combine searches in order to find the
next use of a particular register.
+
-param=max-completely-peel-loop-nest-depth=
Common Joined UInteger Var(param_max_unroll_iterations) Init(8) Param
Optimization
The maximum depth of a loop nest we completely peel.
