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[TailDup] Delay aggressive computed-goto taildup to after RegAlloc.
#114990 allowed more aggressive tail duplication for computed-gotos in both pre- and post-regalloc tail duplication. In some cases, performing tail-duplication too early can lead to worse results, especially if we duplicate blocks with a number of phi nodes. This is causing a ~3% performance regression in some workloads using Python 3.12. This patch updates TailDup to delay aggressive tail-duplication for computed gotos to after register allocation. This means we can keep the non-duplicated version for a bit longer throughout the backend, which should reduce compile-time as well as allowing a number of optimizations and simplifications to trigger before drastically expanding the CFG. For the case in #106846, I get the same performance with and without this patch on Skylake.
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‎llvm/lib/CodeGen/TailDuplicator.cpp

Lines changed: 10 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -610,6 +610,15 @@ bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
610610
if (HasIndirectbr && PreRegAlloc)
611611
MaxDuplicateCount = TailDupIndirectBranchSize;
612612

613+
// Allow higher limits when the block has computed-gotos and running after
614+
// register allocation. NB. This basically unfactors computed gotos that were
615+
// factored early on in the compilation process to speed up edge based data
616+
// flow. If we do not unfactor them again, it can seriously pessimize code
617+
// with many computed jumps in the source code, such as interpreters.
618+
// Therefore we do not restrict the computed gotos.
619+
if (HasComputedGoto && !PreRegAlloc)
620+
MaxDuplicateCount = std::max(MaxDuplicateCount, 10u);
621+
613622
// Check the instructions in the block to determine whether tail-duplication
614623
// is invalid or unlikely to be profitable.
615624
unsigned InstrCount = 0;
@@ -663,12 +672,7 @@ bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
663672
// Duplicating a BB which has both multiple predecessors and successors will
664673
// may cause huge amount of PHI nodes. If we want to remove this limitation,
665674
// we have to address https://github.com/llvm/llvm-project/issues/78578.
666-
// NB. This basically unfactors computed gotos that were factored early on in
667-
// the compilation process to speed up edge based data flow. If we do not
668-
// unfactor them again, it can seriously pessimize code with many computed
669-
// jumps in the source code, such as interpreters. Therefore we do not
670-
// restrict the computed gotos.
671-
if (!HasComputedGoto && TailBB.pred_size() > TailDupPredSize &&
675+
if (PreRegAlloc && TailBB.pred_size() > TailDupPredSize &&
672676
TailBB.succ_size() > TailDupSuccSize) {
673677
// If TailBB or any of its successors contains a phi, we may have to add a
674678
// large number of additional phis with additional incoming values.

‎llvm/test/CodeGen/X86/tail-dup-computed-goto.mir renamed to ‎llvm/test/CodeGen/X86/early-tail-dup-computed-goto.mir

Lines changed: 23 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -1,6 +1,8 @@
11
# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
22
# RUN: llc -mtriple=x86_64-unknown-linux-gnu -run-pass=early-tailduplication -tail-dup-pred-size=1 -tail-dup-succ-size=1 %s -o - | FileCheck %s
3-
# Check that only the computed goto is not be restrict by tail-dup-pred-size and tail-dup-succ-size.
3+
#
4+
# Check that only the computed goto and others are restricted by tail-dup-pred-size and tail-dup-succ-size.
5+
#
46
--- |
57
@computed_goto.dispatch = constant [5 x ptr] [ptr null, ptr blockaddress(@computed_goto, %bb1), ptr blockaddress(@computed_goto, %bb2), ptr blockaddress(@computed_goto, %bb3), ptr blockaddress(@computed_goto, %bb4)]
68
declare i64 @f0()
@@ -30,54 +32,54 @@ tracksRegLiveness: true
3032
body: |
3133
; CHECK-LABEL: name: computed_goto
3234
; CHECK: bb.0:
33-
; CHECK-NEXT: successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
35+
; CHECK-NEXT: successors: %bb.5(0x80000000)
3436
; CHECK-NEXT: {{ $}}
3537
; CHECK-NEXT: ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
3638
; CHECK-NEXT: CALL64pcrel32 target-flags(x86-plt) @f0, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
3739
; CHECK-NEXT: ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
38-
; CHECK-NEXT: [[COPY:%[0-9]+]]:gr64_nosp = COPY $rax
39-
; CHECK-NEXT: [[COPY1:%[0-9]+]]:gr64_nosp = COPY [[COPY]]
40-
; CHECK-NEXT: JMP64m $noreg, 8, [[COPY]], @computed_goto.dispatch, $noreg
40+
; CHECK-NEXT: [[COPY:%[0-9]+]]:gr64 = COPY $rax
41+
; CHECK-NEXT: JMP_1 %bb.5
4142
; CHECK-NEXT: {{ $}}
4243
; CHECK-NEXT: bb.1.bb1 (ir-block-address-taken %ir-block.bb1):
43-
; CHECK-NEXT: successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
44+
; CHECK-NEXT: successors: %bb.5(0x80000000)
4445
; CHECK-NEXT: {{ $}}
4546
; CHECK-NEXT: ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
4647
; CHECK-NEXT: CALL64pcrel32 target-flags(x86-plt) @f1, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
4748
; CHECK-NEXT: ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
48-
; CHECK-NEXT: [[COPY2:%[0-9]+]]:gr64_nosp = COPY $rax
49-
; CHECK-NEXT: [[COPY3:%[0-9]+]]:gr64_nosp = COPY [[COPY2]]
50-
; CHECK-NEXT: JMP64m $noreg, 8, [[COPY2]], @computed_goto.dispatch, $noreg
49+
; CHECK-NEXT: [[COPY1:%[0-9]+]]:gr64 = COPY $rax
50+
; CHECK-NEXT: JMP_1 %bb.5
5151
; CHECK-NEXT: {{ $}}
5252
; CHECK-NEXT: bb.2.bb2 (ir-block-address-taken %ir-block.bb2):
53-
; CHECK-NEXT: successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
53+
; CHECK-NEXT: successors: %bb.5(0x80000000)
5454
; CHECK-NEXT: {{ $}}
5555
; CHECK-NEXT: ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
5656
; CHECK-NEXT: CALL64pcrel32 target-flags(x86-plt) @f2, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
5757
; CHECK-NEXT: ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
58-
; CHECK-NEXT: [[COPY4:%[0-9]+]]:gr64_nosp = COPY $rax
59-
; CHECK-NEXT: [[COPY5:%[0-9]+]]:gr64_nosp = COPY [[COPY4]]
60-
; CHECK-NEXT: JMP64m $noreg, 8, [[COPY4]], @computed_goto.dispatch, $noreg
58+
; CHECK-NEXT: [[COPY2:%[0-9]+]]:gr64 = COPY $rax
59+
; CHECK-NEXT: JMP_1 %bb.5
6160
; CHECK-NEXT: {{ $}}
6261
; CHECK-NEXT: bb.3.bb3 (ir-block-address-taken %ir-block.bb3):
63-
; CHECK-NEXT: successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
62+
; CHECK-NEXT: successors: %bb.5(0x80000000)
6463
; CHECK-NEXT: {{ $}}
6564
; CHECK-NEXT: ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
6665
; CHECK-NEXT: CALL64pcrel32 target-flags(x86-plt) @f3, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
6766
; CHECK-NEXT: ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
68-
; CHECK-NEXT: [[COPY6:%[0-9]+]]:gr64_nosp = COPY $rax
69-
; CHECK-NEXT: [[COPY7:%[0-9]+]]:gr64_nosp = COPY [[COPY6]]
70-
; CHECK-NEXT: JMP64m $noreg, 8, [[COPY6]], @computed_goto.dispatch, $noreg
67+
; CHECK-NEXT: [[COPY3:%[0-9]+]]:gr64 = COPY $rax
68+
; CHECK-NEXT: JMP_1 %bb.5
7169
; CHECK-NEXT: {{ $}}
7270
; CHECK-NEXT: bb.4.bb4 (ir-block-address-taken %ir-block.bb4):
73-
; CHECK-NEXT: successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
71+
; CHECK-NEXT: successors: %bb.5(0x80000000)
7472
; CHECK-NEXT: {{ $}}
7573
; CHECK-NEXT: ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
7674
; CHECK-NEXT: CALL64pcrel32 target-flags(x86-plt) @f4, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
7775
; CHECK-NEXT: ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
78-
; CHECK-NEXT: [[COPY8:%[0-9]+]]:gr64_nosp = COPY $rax
79-
; CHECK-NEXT: [[COPY9:%[0-9]+]]:gr64_nosp = COPY [[COPY8]]
80-
; CHECK-NEXT: JMP64m $noreg, 8, [[COPY8]], @computed_goto.dispatch, $noreg
76+
; CHECK-NEXT: [[COPY4:%[0-9]+]]:gr64 = COPY $rax
77+
; CHECK-NEXT: {{ $}}
78+
; CHECK-NEXT: bb.5:
79+
; CHECK-NEXT: successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
80+
; CHECK-NEXT: {{ $}}
81+
; CHECK-NEXT: [[PHI:%[0-9]+]]:gr64_nosp = PHI [[COPY]], %bb.0, [[COPY4]], %bb.4, [[COPY3]], %bb.3, [[COPY2]], %bb.2, [[COPY1]], %bb.1
82+
; CHECK-NEXT: JMP64m $noreg, 8, [[PHI]], @computed_goto.dispatch, $noreg
8183
bb.0:
8284
ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
8385
CALL64pcrel32 target-flags(x86-plt) @f0, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax

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