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</o:shapelayout></xml><![endif]--></head><body lang=EN-GB link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Hi,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>As discussed on llvm-dev recently, this patch adds support for “soft-failing” on disassembly of certain ARM instructions.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Certain instructions are marked as “UNPREDICTABLE” in the ARMARM – the disassembly of these instructions may still have value however, especially to a debugger or debugging code generated by another toolchain.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This patch promotes the return value of all Decode* operations in the ARM disassembly to a ternary value (Success, Unpredictable, Fail) and touches a lot of the error handling code to perform status demotion from Success to Unpredictable, and returning early on Fail. It also touches the FixedLenDecoderEmitter to facilitate the same changes in autogenerated code.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The major changelist is:<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Add a new type “DecodeStatus”, local to ARMDisassembler.cpp, and add a macro CHECK(S, X) that will possibly demote a DecodeStatus S based on a function X’s return value, and exit early if the value becomes “Fail”.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Modify the return value of Decode* from bool to “DecodeStatus”, modify “if(!X()) return false;” to “CHECK(S, X());”, assuming a local accumulator “DecodeStatus S = Success;”.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Parameterise FixedLenDecoderEmitter to take as extra (optional) arguments:<o:p></o:p></p><p class=MsoListParagraph style='margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo1'><![if !supportLists]><span style='font-family:"Courier New"'><span style='mso-list:Ignore'>o<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>The return value of the functions to emit (default “bool”)<o:p></o:p></p><p class=MsoListParagraph style='margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo1'><![if !supportLists]><span style='font-family:"Courier New"'><span style='mso-list:Ignore'>o<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]> “Guard” prefix and postfix to wrap any function call that may fail (default “if (!” and “) return false;” respectively)<o:p></o:p></p><p class=MsoListParagraph style='margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo1'><![if !supportLists]><span style='font-family:"Courier New"'><span style='mso-list:Ignore'>o<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Value to return if everything went OK (default “true”)<o:p></o:p></p><p class=MsoListParagraph style='margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo1'><![if !supportLists]><span style='font-family:"Courier New"'><span style='mso-list:Ignore'>o<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Value to return on failure (default “false”)<o:p></o:p></p><p class=MsoListParagraph style='margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo1'><![if !supportLists]><span style='font-family:"Courier New"'><span style='mso-list:Ignore'>o<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Any extra local variables to add (default “”)<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>These defaults maintain the current behaviour of FixedLenDecoderEmitter. ARM/Thumb has been special cased in DisassemblerEmitter.cpp.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The patch as is does not change the user-seen behaviour of the disassembler at all. I’m still not sure the best way to expose this information to the user.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>AFAIK, Intel or most other architectures don't have the same concept as ARM of a valid but unpredictable instruction. I'm therefore fearful of changing MCDisassembler.h to cope with it. At the moment I have an alternate getInstruction() function that also takes a bool& to which it writes if the insn was predictable or not.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>The problem with this obviously is that it requires the user to #include <ARMDisassembler.h>, which is impossible in the general case as it is a hidden header.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I can see three options, none of which being particularly elegant:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Make ARMDisassembler.h a public header so the user can cast<ARMDisassembler> and access an alternate getInstruction() function.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Add a bool* parameter to getInstruction() for UNPREDICTABLE; default is NULL and the other backends ignore it.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>Add an "wasInsnUnpredictable()" function to MCDisassembler, which returns false for all other backends and only the ARM backend deals with. Nastily non-reentrant.<o:p></o:p></p><p class=MsoListParagraph style='text-indent:-18.0pt;mso-list:l0 level1 lfo1'><![if !supportLists]><span style='font-family:Symbol'><span style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'> </span></span></span><![endif]>??? An alternative.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>What would you suggest?<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Cheers,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>James<o:p></o:p></p></div></body></html>