llvm dev - Dec 2017 - Convert MachineInstr to MCInst in AsmPrinter.cpp

Hello everyone,

In the file *lib/CodeGen/AsmPrinter/AsmPrinter.cpp*, I would like to obtain
an MCInst corresponding to its MachineInstr. Can anyone tell me a way to do
that?

If that is not possible, then, I would like to know if a given MachineInstr
is an *lea *instruction and I would like to know if the symbol involved
with this lea instruction is a jump-table.
For instance, given a MachineInstr, I would like to know if it is of the
following form.

*leaq LJTI0_0(%rip), %rax*

Also, say, I want to add custom labels (some string) while generating
assembly right before emitting such *lea *instructions, how do I do that
given that OutStreamer->EmitLabel() takes MCSymbol as an argument and not a
string/StringRef?

*My goal: *To find lea instructions corresponding to jump-tables and emit a
custom label before such instructions and also maintain a count of how many
such lea instructions are there for each jump-table (using a DenseMap).

I have a workaround for all the problems stated above but I want to write a
cleaner solution.

   - For finding an *lea *instruction and emitting a label before actually
   emitting it, I have modified the EmitInstruction() function in
   MCAsmStreamer.cpp wherein I keep track of how many lea instructions are
   there for each jump-table using a DenseMap.
   - As I require the number of lea instructions per jump-table in
   AsmPrinter.cpp, I have created a function in MCAsmStreamer.cpp which
   returns the DenseMap (added a declaration of this function in MCStreamer.h).


Is there a way to do both of the aforementioned bullet points inside
AsmPrinter.cpp without having to call a function in MCAsmStreamer?

Thank you.

Regards,
Malhar

ᐧ
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Hi Malhar,
> On 27 Dec 2017, at 00:56, Malhar Thakkar via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hello everyone,
> 
> In the file lib/CodeGen/AsmPrinter/AsmPrinter.cpp, I would like to obtain
an MCInst corresponding to its MachineInstr. Can anyone tell me a way to do
that?
Usually, it happens in lib/Target/X86/X86MCInstLower.cpp, the X86MCInstLower
class is used by the X86AsmPrinter (subclass of the AsmPrinter).
> 
> If that is not possible, then, I would like to know if a given MachineInstr
is an lea instruction and I would like to know if the symbol involved with this
lea instruction is a jump-table.
You can identify if a MachineInstr is a lea instruction by using something
similar to the function isLEA from lib/Target/X86/X86OptimizeLEAs.cpp.
You can also identify if any of the operands of a MachineInstruction is a
jump-table reference by iterating over all the MachineOperands and checking if
it’s a jump-table index using the function isJTI.
> For instance, given a MachineInstr, I would like to know if it is of the
following form.
> 
> leaq	LJTI0_0(%rip), %rax
> 
> Also, say, I want to add custom labels (some string) while generating
assembly right before emitting such lea instructions, how do I do that given
that OutStreamer->EmitLabel() takes MCSymbol as an argument and not a
string/StringRef?
You will need to create an MCSymbol from a string. The class MCContext is able
to create symbols and manage them, so you should be able to create a symbol and
pass it to EmitLabel using getOrCreateSymbol or any of the functions in
include/llvm/MC/MCContext.h.
> 
> My goal: To find lea instructions corresponding to jump-tables and emit a
custom label before such instructions and also maintain a count of how many such
lea instructions are there for each jump-table (using a DenseMap).
> 
> I have a workaround for all the problems stated above but I want to write a
cleaner solution.
> For finding an lea instruction and emitting a label before actually
emitting it, I have modified the EmitInstruction() function in MCAsmStreamer.cpp
wherein I keep track of how many lea instructions are there for each jump-table
using a DenseMap.
How are you planning to consume that DenseMap? If you only want to dump it you
can probably keep track of everything in X86AsmPrinter::EmitInstruction
(lib/Target/X86/X86MCInstLower.cpp).

I recommend doing everything in the target-specific code as you are only
interested in x86 specifics.

Cheers,

— 
Francis Visoiu Mistrih
> As I require the number of lea instructions per jump-table in
AsmPrinter.cpp, I have created a function in MCAsmStreamer.cpp which returns the
DenseMap (added a declaration of this function in MCStreamer.h).
> 
> Is there a way to do both of the aforementioned bullet points inside
AsmPrinter.cpp without having to call a function in MCAsmStreamer?
> 
> Thank you.
> 
> Regards,
> Malhar
> 
> ᐧ
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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