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Jessica Paquette
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[MachineOutliner][AArch64] Save + restore LR in noreturn functions
Conservatively always save + restore LR in noreturn functions. These functions do not end in a RET, and so they aren't guaranteed to have an instruction which uses LR in any way. So, as a result, you can end up in unfortunate situations where you can't backtrace out of these functions in a debugger. Remove the old noreturn test, and add a new one which is more descriptive. Remove the restriction that we can't outline from noreturn functions as well since we now do the right thing.
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4 files changed

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‎llvm/lib/CodeGen/MachineOutliner.cpp

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@@ -1306,12 +1306,6 @@ void MachineOutliner::populateMapper(InstructionMapper &Mapper, Module &M,
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if (F.empty())
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continue;
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// Disable outlining from noreturn functions right now. Noreturn requires
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// special handling for the case where what we are outlining could be a
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// tail call.
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if (F.hasFnAttribute(Attribute::NoReturn))
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continue;
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// There's something in F. Check if it has a MachineFunction associated with
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// it.
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MachineFunction *MF = MMI.getMachineFunction(F);

‎llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

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@@ -5868,11 +5868,21 @@ outliner::OutlinedFunction AArch64InstrInfo::getOutliningCandidateInfo(
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unsigned NumBytesNoStackCalls = 0;
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std::vector<outliner::Candidate> CandidatesWithoutStackFixups;
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// Check if we have to save LR.
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for (outliner::Candidate &C : RepeatedSequenceLocs) {
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C.initLRU(TRI);
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// If we have a noreturn caller, then we're going to be conservative and
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// say that we have to save LR. If we don't have a ret at the end of the
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// block, then we can't reason about liveness accurately.
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//
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// FIXME: We can probably do better than always disabling this in
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// noreturn functions by fixing up the liveness info.
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bool IsNoReturn =
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C.getMF()->getFunction().hasFnAttribute(Attribute::NoReturn);
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// Is LR available? If so, we don't need a save.
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if (C.LRU.available(AArch64::LR)) {
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if (C.LRU.available(AArch64::LR) && !IsNoReturn) {
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NumBytesNoStackCalls += 4;
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C.setCallInfo(MachineOutlinerNoLRSave, 4);
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CandidatesWithoutStackFixups.push_back(C);

‎llvm/test/CodeGen/AArch64/machine-outliner-noreturn-save-lr.mir

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@@ -0,0 +1,103 @@
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# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
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# RUN: llc -mtriple=aarch64-unknown-unknown -run-pass=machine-outliner -verify-machineinstrs %s -o - | FileCheck %s
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# Verify that we save + restore LR around calls to outlined functions from
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# noreturn parents. LR isn't actually used, but debuggers need LR in this case
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# to get a backtrace.
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--- |
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define void @save_lr_1() #0 { ret void }
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define void @save_lr_2() #0 { ret void }
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define void @save_lr_3() #0 { ret void }
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define void @save_lr_4() #0 { ret void }
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attributes #0 = { noredzone noreturn }
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...
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---
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name: save_lr_1
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alignment: 4
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tracksRegLiveness: true
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frameInfo:
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maxAlignment: 1
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maxCallFrameSize: 0
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machineFunctionInfo: {}
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body: |
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bb.0:
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liveins: $lr
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; CHECK-LABEL: name: save_lr_1
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; CHECK: liveins: $lr
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; CHECK: $x0 = ORRXrs $xzr, $lr, 0
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; CHECK: BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp, implicit-def $lr, implicit-def $lr, implicit-def $w3, implicit-def $w4, implicit-def $w5, implicit-def $w6
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; CHECK: $lr = ORRXrs $xzr, $x0, 0
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$w3 = ORRWri $wzr, 1
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$w4 = ORRWri $wzr, 1
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BRK 1
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$w5 = ORRWri $wzr, 1
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$w6 = ORRWri $wzr, 1
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...
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---
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name: save_lr_2
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alignment: 4
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tracksRegLiveness: true
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frameInfo:
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maxAlignment: 1
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maxCallFrameSize: 0
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machineFunctionInfo: {}
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body: |
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bb.0:
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liveins: $lr
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; CHECK-LABEL: name: save_lr_2
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; CHECK: liveins: $lr
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; CHECK: $x0 = ORRXrs $xzr, $lr, 0
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; CHECK: BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp, implicit-def $lr, implicit-def $lr, implicit-def $w3, implicit-def $w4, implicit-def $w5, implicit-def $w6
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; CHECK: $lr = ORRXrs $xzr, $x0, 0
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$w3 = ORRWri $wzr, 1
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$w4 = ORRWri $wzr, 1
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BRK 1
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$w5 = ORRWri $wzr, 1
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$w6 = ORRWri $wzr, 1
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...
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---
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name: save_lr_3
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alignment: 4
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tracksRegLiveness: true
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frameInfo:
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maxAlignment: 1
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maxCallFrameSize: 0
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machineFunctionInfo: {}
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body: |
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bb.0:
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liveins: $lr
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; CHECK-LABEL: name: save_lr_3
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; CHECK: liveins: $lr
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; CHECK: $x0 = ORRXrs $xzr, $lr, 0
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; CHECK: BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp, implicit-def $lr, implicit-def $lr, implicit-def $w3, implicit-def $w4, implicit-def $w5, implicit-def $w6
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; CHECK: $lr = ORRXrs $xzr, $x0, 0
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$w3 = ORRWri $wzr, 1
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$w4 = ORRWri $wzr, 1
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BRK 1
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$w5 = ORRWri $wzr, 1
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$w6 = ORRWri $wzr, 1
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...
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---
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name: save_lr_4
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alignment: 4
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tracksRegLiveness: true
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frameInfo:
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maxAlignment: 1
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maxCallFrameSize: 0
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machineFunctionInfo: {}
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body: |
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bb.0:
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liveins: $lr
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; CHECK-LABEL: name: save_lr_4
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; CHECK: liveins: $lr
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; CHECK: $x0 = ORRXrs $xzr, $lr, 0
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; CHECK: BL @OUTLINED_FUNCTION_0, implicit-def $lr, implicit $sp, implicit-def $lr, implicit-def $lr, implicit-def $w3, implicit-def $w4, implicit-def $w5, implicit-def $w6
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; CHECK: $lr = ORRXrs $xzr, $x0, 0
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$w3 = ORRWri $wzr, 1
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$w4 = ORRWri $wzr, 1
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BRK 1
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$w5 = ORRWri $wzr, 1
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$w6 = ORRWri $wzr, 1
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...

‎llvm/test/CodeGen/AArch64/machine-outliner-noreturn.mir

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