llvm dev - Jun 2021 - Address of instruction in codegen

Hi everyone,

I have a need to generate the address of an instruction during codegen.
Is there any way to do this?  I was hoping that I could split the basic
block very late and then use an MO_BlockAddress MachineOperand, but
apparently generating such a thing requires a BlockAddress object, which
in turn would force splitting the basic block very early, at the IR
level.

I don't actually need the address until just before regalloc.  In fact,
generating a BlockAddress early would probably result in a dead
Instruction as I wouldn't have anything to actually connect it to as a
user.  I suppose I could use a dummy target intrinsic as a user but that
seems very hacky.

I also thought about doing this extremely late and encoding the address
generation during binary streaming in the MC layer but that also seems
very hacky and difficult to maintain, requiring various
pseudo-instructions to live until asm or object writing.

Any brilliant ideas out there?

                        -David

Check if MachineInstr::[sg]etPreInstrSymbol do what you need.

-- 
Krzysztof Parzyszek  kparzysz at quicinc.com   AI tools development

-----Original Message-----
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of David
Greene via llvm-dev
Sent: Wednesday, June 23, 2021 2:18 PM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] Address of instruction in codegen

Hi everyone,

I have a need to generate the address of an instruction during codegen.
Is there any way to do this?  I was hoping that I could split the basic block
very late and then use an MO_BlockAddress MachineOperand, but apparently
generating such a thing requires a BlockAddress object, which in turn would
force splitting the basic block very early, at the IR level.

I don't actually need the address until just before regalloc.  In fact,
generating a BlockAddress early would probably result in a dead Instruction as I
wouldn't have anything to actually connect it to as a user.  I suppose I
could use a dummy target intrinsic as a user but that seems very hacky.

I also thought about doing this extremely late and encoding the address
generation during binary streaming in the MC layer but that also seems very
hacky and difficult to maintain, requiring various pseudo-instructions to live
until asm or object writing.

Any brilliant ideas out there?

                        -David
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Krzysztof Parzyszek via llvm-dev <llvm-dev at lists.llvm.org> writes:
> Check if MachineInstr::[sg]etPreInstrSymbol do what you need.
It does!  As it happens I found the API just a little bit later.  :)

             -David