llvm dev - Aug 2019 - Manually insert an instruction in SelectionDAG

Hello,
I am trying to insert a .byte/.word in the beginning of a specific LLVM IR
instruction when it prints out in assembly (the inserted ‘instruction' only
appears in assembly, not in LLVM IR), and I am guessing the best way to do that
is to insert it in SelectionDAG as it strips down some LLVM IR instructions when
it’s lowered. Can I get some guidance on what function I should use to insert a
.byte/.word in SelectionDAG so it’s printed out in assembly?

Thank you,
Kaarthik A. Alagappan
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Hi Kaarthik,

On Sat, 3 Aug 2019 at 22:57, Kaarthik Alagapan via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Can I get some guidance on what function I should use to insert a
.byte/.word in SelectionDAG so it’s printed out in assembly?
This kind of problem is often solved by selecting the original IR
instruction to a target-specific pseudo-instruction (one with
"isCodeGenOnly = 1" in TableGen) and then expanding it to the real
instruction sequence later.

In most cases that would be a special XYZExpandPseudoInsts.cpp pass
that runs very late and looks through a function replacing any
MachineInstrs with the correct sequence. But in your case .byte and
.word aren't even instructions so you probably want to do it during
conversion to MCInsts in XYZAsmPrinter.cpp (or possibly
XYZMCInstLower.cpp, different targets handle that step slightly
differently). There you can detect your special pseudo and call
EmitIntValue or something to get your directives.

Cheers.

Tim.

I’ll look into that file, thank you for the clarification!

Kaarthik A.
On Aug 4, 2019, 2:19 AM -0400, Tim Northover <t.p.northover at gmail.com>,
wrote:
Hi Kaarthik,

On Sat, 3 Aug 2019 at 22:57, Kaarthik Alagapan via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
Can I get some guidance on what function I should use to insert a .byte/.word in
SelectionDAG so it’s printed out in assembly?

This kind of problem is often solved by selecting the original IR
instruction to a target-specific pseudo-instruction (one with
"isCodeGenOnly = 1" in TableGen) and then expanding it to the real
instruction sequence later.

In most cases that would be a special XYZExpandPseudoInsts.cpp pass
that runs very late and looks through a function replacing any
MachineInstrs with the correct sequence. But in your case .byte and
.word aren't even instructions so you probably want to do it during
conversion to MCInsts in XYZAsmPrinter.cpp (or possibly
XYZMCInstLower.cpp, different targets handle that step slightly
differently). There you can detect your special pseudo and call
EmitIntValue or something to get your directives.

Cheers.

Tim.
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Hi Tim,
I was looking into what you said but then realized that I am working with jump
and branch instructions (not pseudo instructions), which I believe are all
translated to jumps by the AsmPrinter and InstLower phase, so  I tried to see
where I can detect any type of jumps so I call EmitIntValue but wasn’t able to
find any other than TAILJMP*. Any tips on where I can find jumps being detected
for their code emission?

Kaarthik A.
On Aug 4, 2019, 2:19 AM -0400, Tim Northover <t.p.northover at gmail.com>,
wrote:
Hi Kaarthik,

On Sat, 3 Aug 2019 at 22:57, Kaarthik Alagapan via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
Can I get some guidance on what function I should use to insert a .byte/.word in
SelectionDAG so it’s printed out in assembly?

This kind of problem is often solved by selecting the original IR
instruction to a target-specific pseudo-instruction (one with
"isCodeGenOnly = 1" in TableGen) and then expanding it to the real
instruction sequence later.

In most cases that would be a special XYZExpandPseudoInsts.cpp pass
that runs very late and looks through a function replacing any
MachineInstrs with the correct sequence. But in your case .byte and
.word aren't even instructions so you probably want to do it during
conversion to MCInsts in XYZAsmPrinter.cpp (or possibly
XYZMCInstLower.cpp, different targets handle that step slightly
differently). There you can detect your special pseudo and call
EmitIntValue or something to get your directives.

Cheers.

Tim.
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