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[BOLT] Don't terminate on trap instruction for Linux kernel #87021

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Merged
merged 1 commit into from
Mar 29, 2024
Merged

[BOLT] Don't terminate on trap instruction for Linux kernel #87021

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4 changes: 1 addition & 3 deletions bolt/include/bolt/Core/MCPlusBuilder.h
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Original file line number Diff line number Diff line change
Expand Up @@ -533,9 +533,7 @@ class MCPlusBuilder {
return Analysis->isReturn(Inst);
}

virtual bool isTerminator(const MCInst &Inst) const {
return Analysis->isTerminator(Inst);
}
virtual bool isTerminator(const MCInst &Inst) const;

virtual bool isNoop(const MCInst &Inst) const {
llvm_unreachable("not implemented");
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14 changes: 14 additions & 0 deletions bolt/lib/Core/MCPlusBuilder.cpp
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Expand Up @@ -12,12 +12,14 @@

#include "bolt/Core/MCPlusBuilder.h"
#include "bolt/Core/MCPlus.h"
#include "bolt/Utils/CommandLineOpts.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCInstrAnalysis.h"
#include "llvm/MC/MCInstrDesc.h"
#include "llvm/MC/MCInstrInfo.h"
#include "llvm/MC/MCRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include <cstdint>
#include <queue>
Expand All @@ -28,6 +30,13 @@ using namespace llvm;
using namespace bolt;
using namespace MCPlus;

namespace opts {
cl::opt<bool>
TerminalTrap("terminal-trap",
cl::desc("Assume that execution stops at trap instruction"),
cl::init(true), cl::Hidden, cl::cat(BoltCategory));
}

bool MCPlusBuilder::equals(const MCInst &A, const MCInst &B,
CompFuncTy Comp) const {
if (A.getOpcode() != B.getOpcode())
Expand Down Expand Up @@ -121,6 +130,11 @@ bool MCPlusBuilder::equals(const MCTargetExpr &A, const MCTargetExpr &B,
llvm_unreachable("target-specific expressions are unsupported");
}

bool MCPlusBuilder::isTerminator(const MCInst &Inst) const {
return Analysis->isTerminator(Inst) ||
(opts::TerminalTrap && Info->get(Inst.getOpcode()).isTrap());
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@yota9 The new code makes brk 0x3e8 Arm instruction a basic block terminator. If it's not the desired behavior, we can turn off --terminal-trap on Arm.

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@maksfb Hi! Currently I don't think it is a huge problem

}

void MCPlusBuilder::setTailCall(MCInst &Inst) const {
assert(!hasAnnotation(Inst, MCAnnotation::kTailCall));
setAnnotationOpValue(Inst, MCAnnotation::kTailCall, true);
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11 changes: 9 additions & 2 deletions bolt/lib/Rewrite/RewriteInstance.cpp
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Expand Up @@ -84,6 +84,7 @@ extern cl::opt<JumpTableSupportLevel> JumpTables;
extern cl::opt<bool> KeepNops;
extern cl::list<std::string> ReorderData;
extern cl::opt<bolt::ReorderFunctions::ReorderType> ReorderFunctions;
extern cl::opt<bool> TerminalTrap;
extern cl::opt<bool> TimeBuild;

cl::opt<bool> AllowStripped("allow-stripped",
Expand Down Expand Up @@ -2033,8 +2034,14 @@ void RewriteInstance::adjustCommandLineOptions() {
if (opts::Lite)
BC->outs() << "BOLT-INFO: enabling lite mode\n";

if (BC->IsLinuxKernel && !opts::KeepNops.getNumOccurrences())
opts::KeepNops = true;
if (BC->IsLinuxKernel) {
if (!opts::KeepNops.getNumOccurrences())
opts::KeepNops = true;

// Linux kernel may resume execution after a trap instruction in some cases.
if (!opts::TerminalTrap.getNumOccurrences())
opts::TerminalTrap = false;
}
}

namespace {
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7 changes: 0 additions & 7 deletions bolt/lib/Target/X86/X86MCPlusBuilder.cpp
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Expand Up @@ -211,13 +211,6 @@ class X86MCPlusBuilder : public MCPlusBuilder {
return false;
}

// FIXME: For compatibility with old LLVM only!
bool isTerminator(const MCInst &Inst) const override {
unsigned Opcode = Inst.getOpcode();
return Info->get(Opcode).isTerminator() || X86::isUD1(Opcode) ||
X86::isUD2(Opcode);
Comment on lines -217 to -218
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Assuming you tested that it's safe to drop ud1/ud2 from terminators.

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I'm dropping them only for the Linux kernel. For the rest, by default they will still terminate, but the condition check is modified with isTrap() check instead of the opcode lookup.

}

bool isIndirectCall(const MCInst &Inst) const override {
return isCall(Inst) &&
((getMemoryOperandNo(Inst) != -1) || Inst.getOperand(0).isReg());
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4 changes: 4 additions & 0 deletions bolt/test/X86/linux-bug-table.s
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Expand Up @@ -40,6 +40,10 @@ _start:
# CHECK-REOPT-SAME: BugEntry: 2

ret
## The return instruction is reachable only via preceding ud2. Test that it is
## treated as a reachable instruction in the Linux kernel mode.

# CHECK-REOPT-NEXT: ret
.size _start, .-_start


Expand Down