release/20.x: [TailDuplicator] Do not restrict the computed gotos (#114990) #130585

llvmbot · 2025-03-10T11:40:42Z

Backport dd21aac

Requested by: @dianqk

llvmbot · 2025-03-10T11:40:48Z

@nikic What do you think about merging this PR to the release branch?

llvmbot · 2025-03-10T11:41:16Z

@llvm/pr-subscribers-backend-x86

Author: None (llvmbot)

Changes

Backport dd21aac

Requested by: @DianQK

Full diff: https://github.com/llvm/llvm-project/pull/130585.diff

3 Files Affected:

(modified) llvm/include/llvm/CodeGen/MachineInstr.h (+11-2)
(modified) llvm/lib/CodeGen/TailDuplicator.cpp (+10-2)
(added) llvm/test/CodeGen/X86/tail-dup-computed-goto.mir (+255)

diff --git a/llvm/include/llvm/CodeGen/MachineInstr.h b/llvm/include/llvm/CodeGen/MachineInstr.h
index 102b1eb07358e..b26cabe801ee8 100644
--- a/llvm/include/llvm/CodeGen/MachineInstr.h
+++ b/llvm/include/llvm/CodeGen/MachineInstr.h
@@ -994,8 +994,17 @@ class MachineInstr
 
   /// Return true if this is an indirect branch, such as a
   /// branch through a register.
-  bool isIndirectBranch(QueryType Type = AnyInBundle) const {
-    return hasProperty(MCID::IndirectBranch, Type);
+  bool isIndirectBranch(QueryType Type = AnyInBundle,
+                        bool IncludeJumpTable = true) const {
+    return hasProperty(MCID::IndirectBranch, Type) &&
+           (IncludeJumpTable || !llvm::any_of(operands(), [](const auto &Op) {
+              return Op.isJTI();
+            }));
+  }
+
+  bool isComputedGoto(QueryType Type = AnyInBundle) const {
+    // Jump tables are not considered computed gotos.
+    return isIndirectBranch(Type, /*IncludeJumpTable=*/false);
   }
 
   /// Return true if this is a branch which may fall
diff --git a/llvm/lib/CodeGen/TailDuplicator.cpp b/llvm/lib/CodeGen/TailDuplicator.cpp
index 6c6d38462484a..21f75458c90f3 100644
--- a/llvm/lib/CodeGen/TailDuplicator.cpp
+++ b/llvm/lib/CodeGen/TailDuplicator.cpp
@@ -601,8 +601,11 @@ bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
   // that rearrange the predecessors of the indirect branch.
 
   bool HasIndirectbr = false;
-  if (!TailBB.empty())
+  bool HasComputedGoto = false;
+  if (!TailBB.empty()) {
     HasIndirectbr = TailBB.back().isIndirectBranch();
+    HasComputedGoto = TailBB.back().isComputedGoto();
+  }
 
   if (HasIndirectbr && PreRegAlloc)
     MaxDuplicateCount = TailDupIndirectBranchSize;
@@ -660,7 +663,12 @@ bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
   // Duplicating a BB which has both multiple predecessors and successors will
   // may cause huge amount of PHI nodes. If we want to remove this limitation,
   // we have to address https://github.com/llvm/llvm-project/issues/78578.
-  if (TailBB.pred_size() > TailDupPredSize &&
+  // NB. This basically unfactors computed gotos that were factored early on in
+  // the compilation process to speed up edge based data flow. If we do not
+  // unfactor them again, it can seriously pessimize code with many computed
+  // jumps in the source code, such as interpreters. Therefore we do not
+  // restrict the computed gotos.
+  if (!HasComputedGoto && TailBB.pred_size() > TailDupPredSize &&
       TailBB.succ_size() > TailDupSuccSize) {
     // If TailBB or any of its successors contains a phi, we may have to add a
     // large number of additional phis with additional incoming values.
diff --git a/llvm/test/CodeGen/X86/tail-dup-computed-goto.mir b/llvm/test/CodeGen/X86/tail-dup-computed-goto.mir
new file mode 100644
index 0000000000000..a472dc67d8d51
--- /dev/null
+++ b/llvm/test/CodeGen/X86/tail-dup-computed-goto.mir
@@ -0,0 +1,255 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# 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
+# Check that only the computed goto is not be restrict by tail-dup-pred-size and tail-dup-succ-size.
+--- |
+  declare i64 @f0()
+  declare i64 @f1()
+  declare i64 @f2()
+  declare i64 @f3()
+  declare i64 @f4()
+  declare i64 @f5()
+  @computed_goto.dispatch = external global [5 x ptr]
+  define void @computed_goto() { ret void }
+  define void @jump_table() { ret void }
+...
+---
+name:            computed_goto
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: computed_goto
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f0, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:gr64_nosp = COPY [[COPY]]
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:gr64_nosp = COPY [[COPY1]]
+  ; CHECK-NEXT:   JMP64m $noreg, 8, [[COPY1]], @computed_goto.dispatch, $noreg
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f1, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY4:%[0-9]+]]:gr64_nosp = COPY [[COPY3]]
+  ; CHECK-NEXT:   [[COPY5:%[0-9]+]]:gr64_nosp = COPY [[COPY4]]
+  ; CHECK-NEXT:   JMP64m $noreg, 8, [[COPY4]], @computed_goto.dispatch, $noreg
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f2, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY6:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY7:%[0-9]+]]:gr64_nosp = COPY [[COPY6]]
+  ; CHECK-NEXT:   [[COPY8:%[0-9]+]]:gr64_nosp = COPY [[COPY7]]
+  ; CHECK-NEXT:   JMP64m $noreg, 8, [[COPY7]], @computed_goto.dispatch, $noreg
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f3, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY9:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY10:%[0-9]+]]:gr64_nosp = COPY [[COPY9]]
+  ; CHECK-NEXT:   [[COPY11:%[0-9]+]]:gr64_nosp = COPY [[COPY10]]
+  ; CHECK-NEXT:   JMP64m $noreg, 8, [[COPY10]], @computed_goto.dispatch, $noreg
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4:
+  ; CHECK-NEXT:   successors: %bb.1(0x20000000), %bb.2(0x20000000), %bb.3(0x20000000), %bb.4(0x20000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f4, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY12:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY13:%[0-9]+]]:gr64_nosp = COPY [[COPY12]]
+  ; CHECK-NEXT:   [[COPY14:%[0-9]+]]:gr64_nosp = COPY [[COPY13]]
+  ; CHECK-NEXT:   JMP64m $noreg, 8, [[COPY13]], @computed_goto.dispatch, $noreg
+  bb.0:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f0, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %6:gr64 = COPY $rax
+    %0:gr64 = COPY %6
+    JMP_1 %bb.5
+
+  bb.1:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f1, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %10:gr64 = COPY $rax
+    %1:gr64 = COPY %10
+    JMP_1 %bb.5
+
+  bb.2:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f2, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %9:gr64 = COPY $rax
+    %2:gr64 = COPY %9
+    JMP_1 %bb.5
+
+  bb.3:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f3, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %8:gr64 = COPY $rax
+    %3:gr64 = COPY %8
+    JMP_1 %bb.5
+
+  bb.4:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f4, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %7:gr64 = COPY $rax
+    %4:gr64 = COPY %7
+
+  bb.5:
+    successors: %bb.1, %bb.2, %bb.3, %bb.4
+
+    %5:gr64_nosp = PHI %0, %bb.0, %4, %bb.4, %3, %bb.3, %2, %bb.2, %1, %bb.1
+    JMP64m $noreg, 8, %5, @computed_goto.dispatch, $noreg
+
+...
+---
+name:            jump_table
+tracksRegLiveness: true
+jumpTable:
+  kind:            block-address
+  entries:
+    - id:              0
+      blocks:          [ '%bb.2', '%bb.3', '%bb.4', '%bb.5', '%bb.6' ]
+body:             |
+  ; CHECK-LABEL: name: jump_table
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f0, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:gr64 = COPY [[COPY]]
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.3(0x1999999a), %bb.4(0x1999999a), %bb.5(0x1999999a), %bb.6(0x1999999a), %bb.7(0x1999999a)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[PHI:%[0-9]+]]:gr64 = PHI [[COPY1]], %bb.0, %3, %bb.7, %4, %bb.6, %5, %bb.5, %6, %bb.4, %7, %bb.3
+  ; CHECK-NEXT:   [[DEC64r:%[0-9]+]]:gr64_nosp = DEC64r [[PHI]], implicit-def dead $eflags
+  ; CHECK-NEXT:   JMP64m $noreg, 8, [[DEC64r]], %jump-table.0, $noreg :: (load (s64) from jump-table)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f1, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:gr64 = COPY [[COPY2]]
+  ; CHECK-NEXT:   JMP_1 %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f2, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY4:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY5:%[0-9]+]]:gr64 = COPY [[COPY4]]
+  ; CHECK-NEXT:   JMP_1 %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.5:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f3, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY6:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY7:%[0-9]+]]:gr64 = COPY [[COPY6]]
+  ; CHECK-NEXT:   JMP_1 %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.6:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f4, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY8:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY9:%[0-9]+]]:gr64 = COPY [[COPY8]]
+  ; CHECK-NEXT:   JMP_1 %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.7:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   CALL64pcrel32 target-flags(x86-plt) @f5, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+  ; CHECK-NEXT:   ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+  ; CHECK-NEXT:   [[COPY10:%[0-9]+]]:gr64 = COPY $rax
+  ; CHECK-NEXT:   [[COPY11:%[0-9]+]]:gr64 = COPY [[COPY10]]
+  ; CHECK-NEXT:   JMP_1 %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.8:
+  bb.0:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f0, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %7:gr64 = COPY $rax
+    %0:gr64 = COPY %7
+
+  bb.1:
+    %1:gr64 = PHI %0, %bb.0, %6, %bb.6, %5, %bb.5, %4, %bb.4, %3, %bb.3, %2, %bb.2
+    %8:gr64_nosp = DEC64r %1, implicit-def dead $eflags
+
+  bb.8:
+    successors: %bb.2(0x1999999a), %bb.3(0x1999999a), %bb.4(0x1999999a), %bb.5(0x1999999a), %bb.6(0x1999999a)
+
+    JMP64m $noreg, 8, %8, %jump-table.0, $noreg :: (load (s64) from jump-table)
+
+  bb.2:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f1, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %13:gr64 = COPY $rax
+    %2:gr64 = COPY %13
+    JMP_1 %bb.1
+
+  bb.3:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f2, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %12:gr64 = COPY $rax
+    %3:gr64 = COPY %12
+    JMP_1 %bb.1
+
+  bb.4:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f3, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %11:gr64 = COPY $rax
+    %4:gr64 = COPY %11
+    JMP_1 %bb.1
+
+  bb.5:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f4, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %10:gr64 = COPY $rax
+    %5:gr64 = COPY %10
+    JMP_1 %bb.1
+
+  bb.6:
+    ADJCALLSTACKDOWN64 0, 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    CALL64pcrel32 target-flags(x86-plt) @f5, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
+    ADJCALLSTACKUP64 0, 0, implicit-def dead $rsp, implicit-def dead $eflags, implicit-def dead $ssp, implicit $rsp, implicit $ssp
+    %9:gr64 = COPY $rax
+    %6:gr64 = COPY %9
+    JMP_1 %bb.1
+
+  bb.7:
+
+...

Fixes llvm#106846. This is what I learned from GCC. I found that GCC does not duplicate the BB that has indirect jumps with the jump table. I believe GCC has provided a clear explanation here: > Duplicate the blocks containing computed gotos. This basically unfactors computed gotos that were factored early on in the compilation process to speed up edge based data flow. We used to not unfactor them again, which can seriously pessimize code with many computed jumps in the source code, such as interpreters. (cherry picked from commit dd21aac)

github-actions · 2025-03-11T21:04:10Z

@dianqk (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.

dianqk · 2025-03-12T01:39:52Z

@dianqk (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.

Fix a performance regression related to computed gotos, which usually affects interpreter projects. If you are using Clang 19, adding -mllvm -tail-dup-pred-size=100000 should restore the original performance.

chris-eibl · 2025-03-12T07:02:18Z

Does this mean this won't be backported to 19?

dianqk · 2025-03-12T07:35:09Z

@tstellar
Since this affects many interpreter projects, could we release an additional LLVM 19 version?

llvmbot added this to the LLVM 20.X Release milestone Mar 10, 2025

github-project-automation bot added this to LLVM Release Status Mar 10, 2025

github-project-automation bot moved this to Needs Triage in LLVM Release Status Mar 10, 2025

llvmbot requested a review from nikic March 10, 2025 11:40

llvmbot mentioned this pull request Mar 10, 2025

performance regression in clang-19 when using computed goto #106846

Closed

llvmbot added the backend:X86 label Mar 10, 2025

nikic approved these changes Mar 10, 2025

View reviewed changes

github-project-automation bot moved this from Needs Triage to Needs Merge in LLVM Release Status Mar 10, 2025

llvmbot force-pushed the issue106846 branch from 5449b17 to 0412f70 Compare March 11, 2025 21:03

tstellar merged commit 0412f70 into llvm:release/20.x Mar 11, 2025
7 of 9 checks passed

github-project-automation bot moved this from Needs Merge to Done in LLVM Release Status Mar 11, 2025

dianqk added release:note and removed backend:X86 labels Mar 12, 2025

tstellar added the release:note:published label Mar 19, 2025

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

release/20.x: [TailDuplicator] Do not restrict the computed gotos (#114990) #130585

release/20.x: [TailDuplicator] Do not restrict the computed gotos (#114990) #130585

llvmbot commented Mar 10, 2025

llvmbot commented Mar 10, 2025

llvmbot commented Mar 10, 2025

github-actions bot commented Mar 11, 2025

dianqk commented Mar 12, 2025

chris-eibl commented Mar 12, 2025

dianqk commented Mar 12, 2025

release/20.x: [TailDuplicator] Do not restrict the computed gotos (#114990) #130585

release/20.x: [TailDuplicator] Do not restrict the computed gotos (#114990) #130585

Conversation

llvmbot commented Mar 10, 2025

llvmbot commented Mar 10, 2025

llvmbot commented Mar 10, 2025

github-actions bot commented Mar 11, 2025

dianqk commented Mar 12, 2025

chris-eibl commented Mar 12, 2025

dianqk commented Mar 12, 2025