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. --- 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 9eae1a0111e..197c19f6bdf 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); @@ -4591,7 +4594,7 @@ 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); @@ -14992,10 +14995,12 @@ 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; @@ -15061,9 +15066,12 @@ 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)); @@ -15084,6 +15092,11 @@ 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 0da3f296beb..4c1f903d370 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -16467,6 +16467,15 @@ 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 4f4eb4d7a2a..422d082b355 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. -- 2.25.1
