[BOLT] Fix NOP instruction emission on x86 #72186
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Use MCAsmBackend::writeNopData() interface to emit NOP instructions on x86. There are multiple forms of NOP instruction on x86 with different sizes. Currently, LLVM's assembly/disassembly does not support all forms correctly which can lead to a breakage of input code semantics, e.g. if the program relies on NOP instructions for reserving a patch space. Add "--keep-nops" option to preserve NOP instructions.
aaupov
approved these changes
Nov 14, 2023
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Yes, it's needed for the Linux kernel support, but should be applicable to other use cases as well. |
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Note that with this change, NOP instruction encoding could be different. E.g., 4-byte NOP is now encoded as: compared to: previously. This is also required to match Linux kernel expectations for NOP encoding. |
yota9
reviewed
Nov 14, 2023
| @@ -4366,6 +4367,10 @@ MCInst *BinaryFunction::getInstructionAtOffset(uint64_t Offset) { | |||
| } | |||
| } | |||
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| bool BinaryFunction::shouldPreserveNops() const { | |||
| return PreserveNops || opts::KeepNops; | |||
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Small nit: Maybe add option check to RemoveNops::runOnFunctions too/instead?
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What's the rationale behind that extra check?
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Not to loop though the functions in pass. It is minor thing
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Ah - we can skip the pass all over: #72228.
zahiraam
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Nov 20, 2023
Use MCAsmBackend::writeNopData() interface to emit NOP instructions on x86. There are multiple forms of NOP instruction on x86 with different sizes. Currently, LLVM's assembly/disassembly does not support all forms correctly which can lead to a breakage of input code semantics, e.g. if the program relies on NOP instructions for reserving a patch space. Add "--keep-nops" option to preserve NOP instructions.
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Use MCAsmBackend::writeNopData() interface to emit NOP instructions on x86. There are multiple forms of NOP instruction on x86 with different sizes. Currently, LLVM's assembly/disassembly does not support all forms correctly which can lead to a breakage of input code semantics, e.g. if the program relies on NOP instructions for reserving a patch space.
Add "--keep-nops" option to preserve NOP instructions.