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[BranchFolding] Avoid moving blocks to fall through to an indirect target #152916

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[BranchFolding] Avoid moving blocks to fall through to an indirect target #152916

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[BranchFolding] Avoid moving blocks to fall through to an indirect ta…
XChy Aug 7, 2025
818ab09
fix comment
XChy Aug 13, 2025
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12 changes: 10 additions & 2 deletions llvm/lib/CodeGen/BranchFolding.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1787,10 +1787,18 @@ bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
// below were performed for EH "FallThrough" blocks. Therefore, even if
// that appears not to be happening anymore, we should assume that it is
// possible and not remove the "!FallThrough()->isEHPad" condition below.
//
// Similarly, the analyzeBranch call does not consider callbr, which also
// introduces the possibility of infinite rotation, as there may be
// multiple successors of PrevBB. Thus we check such case by
// FallThrough->isInlineAsmBrIndirectTarget().
// NOTE: Checking if PrevBB contains callbr is more precise, but much
// more expensive.
MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
SmallVector<MachineOperand, 4> PrevCond;
if (FallThrough != MF.end() &&
!FallThrough->isEHPad() &&

if (FallThrough != MF.end() && !FallThrough->isEHPad() &&
!FallThrough->isInlineAsmBrIndirectTarget() &&
!TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
PrevBB.isSuccessor(&*FallThrough)) {
MBB->moveAfter(&MF.back());
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17 changes: 5 additions & 12 deletions llvm/test/CodeGen/X86/callbr-asm-loop.ll
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@@ -1,35 +1,28 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5

; RUN: llc -O0 -mtriple=i686-- < %s | FileCheck %s
; RUN: llc -O1 -mtriple=i686-- < %s | FileCheck %s

; Test that causes multiple defs of %eax.
; FIXME: The testcase hangs with -O1/2/3 enabled.
define i32 @loop1() nounwind {
; CHECK-LABEL: loop1:
; CHECK: # %bb.0: # %entry
; CHECK-NEXT: pushl %esi
; CHECK-NEXT: jmp .LBB0_1
; CHECK-NEXT: .p2align 4
; CHECK-NEXT: .LBB0_1: # %tailrecurse
; CHECK-NEXT: # =>This Inner Loop Header: Depth=1
; CHECK-NEXT: xorl %eax, %eax
; CHECK-NEXT: movl $1, %edx
; CHECK-NEXT: #APP
; CHECK-NEXT: #NO_APP
; CHECK-NEXT: movl %eax, %ecx
; CHECK-NEXT: movl %edx, %esi
; CHECK-NEXT: jmp .LBB0_3
; CHECK-NEXT: jmp .LBB0_1
; CHECK-NEXT: .LBB0_2: # Inline asm indirect target
; CHECK-NEXT: # %tailrecurse.tailrecurse.backedge_crit_edge
; CHECK-NEXT: # %tailrecurse.tailrecurse_crit_edge
; CHECK-NEXT: # in Loop: Header=BB0_1 Depth=1
; CHECK-NEXT: # Label of block must be emitted
; CHECK-NEXT: .LBB0_3: # %tailrecurse.backedge
; CHECK-NEXT: # in Loop: Header=BB0_1 Depth=1
; CHECK-NEXT: jmp .LBB0_1
; CHECK-NEXT: .LBB0_4: # Inline asm indirect target
; CHECK-NEXT: .LBB0_3: # Inline asm indirect target
; CHECK-NEXT: # %lab2.split
; CHECK-NEXT: # Label of block must be emitted
; CHECK-NEXT: movl %edx, %eax
; CHECK-NEXT: popl %esi
; CHECK-NEXT: retl
entry:
br label %tailrecurse
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