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[CodeGen] Fix lpad padding at section start after empty block #112595

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[CodeGen] Fix lpad padding at section start after empty block #112595

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10 changes: 9 additions & 1 deletion llvm/lib/CodeGen/BasicBlockSections.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -265,8 +265,16 @@ void llvm::sortBasicBlocksAndUpdateBranches(
// zero implies "no landing pad." This function inserts a NOP just before the EH
// pad label to ensure a nonzero offset.
void llvm::avoidZeroOffsetLandingPad(MachineFunction &MF) {
std::optional<MBBSectionID> CurrentSection;
auto IsFirstNonEmptyBBInSection = [&](const MachineBasicBlock &MBB) {
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@dhoekwater dhoekwater Oct 22, 2024

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This function may give false positives in the following situation:

bb0 (bbsections 0)
bb1 (bbsections cold)
bb2 (bbsections 0)

If false positives are fine (since we're just inserting a nop), then you can disregard this feedback. Otherwise, you may want to do something like

llvm::DenseSet<MBBSectionID> SectionsWithNonEmptyBlocks;
auto IsFirstNonEmptyBBInSection = [&](const MachineBasicBlock &MBB) {
  if (MBB.empty() || SectionsWithNonEmptyBlocks.contains(MBB.getSectionID()))
    return false;

  SectionsWithNonEmptyBlocks.insert(MBB.getSectionID());

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@pzfl pzfl Oct 31, 2024 •
edited
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Good catch! I was under the impression that basic blocks are assumed to be sorted. Both callsites avoidZeroOffsetLandingPad are preceded by a call to sortBasicBlocksAndUpdateBranches. However, I see now that finishAdjustingBasicBlocksAndLandingPads sorts only by section type, but not by section number. I am not sure how section numbers are used.

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@dhoekwater dhoekwater Nov 1, 2024

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Ahh, no you're right, sortBasicBlocksAndUpdateBranches guarantees the basic block sections will be contiguous. Looks good!

if (MBB.empty() || MBB.getSectionID() == CurrentSection)
return false;
CurrentSection = MBB.getSectionID();
return true;
};

for (auto &MBB : MF) {
if (MBB.isBeginSection() && MBB.isEHPad()) {
if (IsFirstNonEmptyBBInSection(MBB) && MBB.isEHPad()) {
MachineBasicBlock::iterator MI = MBB.begin();
while (!MI->isEHLabel())
++MI;
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30 changes: 30 additions & 0 deletions llvm/test/CodeGen/Generic/machine-function-splitter.ll
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Expand Up @@ -674,6 +674,36 @@ define void @foo22(i1 zeroext %0) nounwind !prof !14 !section_prefix !15 {
ret void
}

define i32 @foo23(i1 zeroext %0) personality ptr @__gxx_personality_v0 !prof !14 {
;; Check that nop is inserted just before the EH pad if it is the first
;; instruction in a section (but is preceeded by another empty block).
; MFS-DEFAULTS-LABEL: foo23
; MFS-DEFAULTS-X86-LABEL: callq baz
; MFS-DEFAULTS-X86: .section .text.split.foo23,"ax",@progbits
; MFS-DEFAULTS-X86-NEXT: foo23.cold:
; MFS-DEFAULTS-X86: nop
; MFS-DEFAULTS-X86: callq _Unwind_Resume@PLT
entry:
br i1 %0, label %try, label %unreachable, !prof !17

try:
invoke void @_Z1fv()
to label %try.cont unwind label %lpad, !prof !17

try.cont:
%1 = call i32 @baz()
ret i32 %1

unreachable:
unreachable

lpad:
%2 = landingpad { ptr, i32 }
cleanup
catch ptr @_ZTIi
resume { ptr, i32 } %2
}

declare i32 @bar()
declare i32 @baz()
declare i32 @bam()
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