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[BOLT] Keep X86 HLT instruction as a terminator in user mode #154402

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Merged
merged 1 commit into from
Aug 19, 2025
Merged

[BOLT] Keep X86 HLT instruction as a terminator in user mode #154402

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16 changes: 12 additions & 4 deletions bolt/lib/Core/MCPlusBuilder.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,11 @@ using namespace bolt;
using namespace MCPlus;

namespace opts {
cl::opt<bool>
TerminalHLT("terminal-x86-hlt",
cl::desc("Assume that execution stops at x86 HLT instruction"),
cl::init(true), cl::Hidden, cl::cat(BoltCategory));

cl::opt<bool>
TerminalTrap("terminal-trap",
cl::desc("Assume that execution stops at trap instruction"),
Expand Down Expand Up @@ -132,10 +137,13 @@ bool MCPlusBuilder::equals(const MCSpecifierExpr &A, const MCSpecifierExpr &B,
}

bool MCPlusBuilder::isTerminator(const MCInst &Inst) const {
return (opts::TerminalTrap && Info->get(Inst.getOpcode()).isTrap()) ||
Analysis->isTerminator(Inst)
? !isX86HLT(Inst)
: false;
if (isX86HLT(Inst))
return opts::TerminalHLT;

if (Info->get(Inst.getOpcode()).isTrap())
return opts::TerminalTrap;

return Analysis->isTerminator(Inst);
}

void MCPlusBuilder::setTailCall(MCInst &Inst) const {
Expand Down
5 changes: 4 additions & 1 deletion bolt/lib/Rewrite/RewriteInstance.cpp
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Expand Up @@ -84,6 +84,7 @@ extern cl::opt<bool> KeepNops;
extern cl::opt<bool> Lite;
extern cl::list<std::string> ReorderData;
extern cl::opt<bolt::ReorderFunctions::ReorderType> ReorderFunctions;
extern cl::opt<bool> TerminalHLT;
extern cl::opt<bool> TerminalTrap;
extern cl::opt<bool> TimeBuild;
extern cl::opt<bool> TimeRewrite;
Expand Down Expand Up @@ -2177,7 +2178,9 @@ void RewriteInstance::adjustCommandLineOptions() {
if (!opts::KeepNops.getNumOccurrences())
opts::KeepNops = true;

// Linux kernel may resume execution after a trap instruction in some cases.
// Linux kernel may resume execution after a trap or x86 HLT instruction.
if (!opts::TerminalHLT.getNumOccurrences())
opts::TerminalHLT = false;
if (!opts::TerminalTrap.getNumOccurrences())
opts::TerminalTrap = false;
}
Expand Down
17 changes: 0 additions & 17 deletions bolt/test/X86/cfg_build_hlt.s
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24 changes: 24 additions & 0 deletions bolt/test/X86/hlt-terminator.s
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Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
## Check that HLT instruction is handled differently depending on the flags.
## It's a terminator in the user-level code, but the execution can resume in
## ring 0.

# RUN: %clang %cflags %s -static -o %t.exe -nostdlib
# RUN: llvm-bolt %t.exe --print-cfg --print-only=main --terminal-x86-hlt=0 \
# RUN: -o %t.ring0 2>&1 | FileCheck %s --check-prefix=CHECK-RING0
# RUN: llvm-bolt %t.exe --print-cfg --print-only=main \
# RUN: -o %t.ring3 2>&1 | FileCheck %s --check-prefix=CHECK-RING3
# RUN: llvm-objdump -d %t.ring0 --print-imm-hex | FileCheck %s --check-prefix=CHECK-BIN

# CHECK-RING0: BB Count : 1
# CHECK-RING3: BB Count : 2

# CHECK-BIN: <main>:
# CHECK-BIN-NEXT: f4 hlt
# CHECK-BIN-NEXT: c3 retq

.global main
.type main, %function
main:
hlt
retq
.size main, .-main
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