llvm dev - Jul 2021 - Address of instruction in codegen

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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On Wed, Jun 23, 2021 at 12:23 PM Krzysztof Parzyszek via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Check if MachineInstr::[sg]etPreInstrSymbol do what you need.
>
I have generated enough awareness of this API that other people mention it
now. Success. :)

A word of caution: depending on where you are in codegen, there are late
passes that can delete or duplicate MachineInstrs (tail duplication comes
to mind). You need to be reasonably confident that the annotated
instruction won't be affected by such passes, or you will get errors from
the assembler about an undefined label or duplicate label definitions.
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Reid Kleckner <rnk at google.com> writes:
> A word of caution: depending on where you are in codegen, there are late
> passes that can delete or duplicate MachineInstrs (tail duplication comes
> to mind). You need to be reasonably confident that the annotated
> instruction won't be affected by such passes, or you will get errors
from
> the assembler about an undefined label or duplicate label definitions.
I actually ran into that.  :)  Would a patch to fix the tail merging
issue be of interest?  As composed now I have it not allowing merging
where a pre- or post-symbol differs but I can also imagine enhancements
to update uses of the symbol.

                        -David