[llvm-dev] Quick question: How to BuildMI mov64mi32 arbitrary MMB address to memory (original) (raw)

K Jelesnianski via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 24 22:44:18 PDT 2018

In the mean time I thought I could do the inverse and "sum" the size of each instruction between the entry MBB and the target .LBB0_0 by doing a double for loop for each MBB for each MInstr currInstrBytes = getInstSizeInBytes(MInstr); totalAsmbytes += currInstrBytes;

Unfortunately X86-64 is missing a very convenient function called "getInstSizeInBytes( MachineInstr* ) that I found in AAarch64InstrInfo. ;/ So I'm still unsure how to proceed.

On Mon, Sep 24, 2018 at 7:28 PM K Jelesnianski <kjski at vt.edu> wrote:

Dear Dr. Northover,

You've still got to access that symbol, and it's not obvious how a block that's moving around in memory could do that. The same arguments that it can't use %rip relative addressing for a local BB would seem to apply to any other entity.

Agreed, for now I will probably perform need to perform load-time analysis, grab some info, and patch the binary to get around this. I have gone ahead and begun implementing your proposed work around. My question now is how would you create the BuildMI for the second assembly instr (addq (LBB00 - func), %rax) you proposed? I know I can get the global address of the MF we are currently in with .addGlobalAddress(M->getNamedValue(MF.getName())) but how do we take that and make an expression out of it. I am not sure the MBB object gives us any way to get its address and perform the given subtraction expression of LBB00 - func. So far I have the first instruction working: movq func at GOTPCREL(%rip), %rax const Module *M = MF.getMMI().getModule(); /* movq func at GOTPCREL(%rip), %rax */ BuildMI(MBB, MBIt, DL, TII->get(X86::MOV64rm)) .addReg(X86::RAX) //dest .addReg(X86::RIP) //base .addImm(0x1) //scale .addReg(0x0) //index .addGlobalAddress(M->getNamedValue(MF.getName())) //Disp .addReg(0x0); //seg addq (.LBB00-func), %rax /* addq (.LBB00-func), %rax ???? */ BuildMI(MBB, MBIt, DL, TII->get(X86::ADD64ri32)) .addReg(X86:RAX) //destination .addReg(X86::RAX) //base .addImm(0x1) //scale .addReg(0x0) //index .addImm(<<<< I assume expression is related to_ _displacement and goes here >>>>>>>) .addReg(0x0); //segment If I try to put a simple asm.s into llvm-mc -show-inst, it tells me to use MCExpr, but I am not sure that is correct (shown below). There does exist .addExpr but it only valid for MCInstBuilder, not MachineInstrBuilder::BuildMI. $ llvm-mc -show-inst asm.s foo: .LBB00: movq 2099957(%rip), %rax # <MCInst #1810 MOV64rm_ _# # # # # # > addq (.LBB00 - foo) , %rax # <MCInst #202 ADD64rm_ _# # # # # # <MCOperand Expr:(.LBB00-foo)> # > retq # <MCInst #2601 RETQ> > Do I need to make these symbols for the trampoline BBs as an IR opt pass, can I get away with it using a MachineModule Pass to add the trampolines per module (file) (so far I have only created BasicBlock, MachineBasicBlock, and MachineFunction passes)?? If you go that route you can probably add entries to the MachineConstantPool with a MachineModule pass. The same addressing concerns seem to apply though. I actually forgot about this constraint, we already have experienced some unintended side-effects when attempting to reference .rodata information (e.g. printf printing garbage). So that is something to look fix in the near future once this part is done. :) Thanks again for your reply! Sincerely, K Jelesnianski -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180925/ffffa3c0/attachment.html>

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