[LLVMdev] Pseudo instructions expansion

[Medic, Vladimir]

Hi all,
I'm trying to solve a problem that we have in implementation of the assembler for Mips platform in llvm. Mips has some pseudo instructions that, depending on the arguments can be emitted as one or more real instructions by the assembler.

For example load immediate instruction can have multiple expansions depending on a size of immediate operand:
This expansion is for 0 ≤ j ≤ 65535.
li d,j =>
ori d,$zero,j
This one is for −32768 ≤ j < 0.
li d,j =>
addiu d,$zero,j
This one is for any other value of j that is representable as a 32-bit integer.
li d,j =>
lui d,hi16(j)
ori d,d,lo16(j)

I have found that class PseudoLoweringEmitter emits code which deals with PseudoInstExpansion. This sounds like exactly what we need , but, as stated in comment in PseudoLoweringEmitter.cpp:
// FIXME: This pass currently can only expand a pseudo to a single instruction.
// The pseudo expansion really should take a list of dags, not just
// a single dag, so we can do fancier things.

Are the 'fancier things' mentioned in the comment things that we need, expansion to a multiple instructions and could they use operand values as conditions for different expansions?
Could it work for inline asm in C code, with/without direct-object emitter?

Kind Regards

Vladimir

[Jim Grosbach]

Hi Vladimir,

The pass you refer to isn't used by the assembler at all. That's strictly a compiler codegen thing. The assembler equivalents are expressed via InstAlias constructions. Again, though, those are for a single output instruction, so you need something more. Sprecifically, you can handle assembly pseudo-instructions in C++ code. Something like the ARM assembler's processInstruction() hook would be an appropriate place.

-Jim

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[Medic, Vladimir]

Hi Jim,
thank you for the quick response. I have used InstAlias in some cases, but these are really simple pseudo instructions where the  pseudo instruction is more like a special case of existing one, like using fixed operand or simply a more human understandable way of presenting an operation. I know that there are predicates available to improve matching, but can InstAlias use conditions to choose between different replacements, like the ones I stated for load immediate? I have thought of expanding instructions after successful match but I was hoping that there is some kind of mechanism from tableGen that can be used for the purpose as the former way seems to me more like a hack.

Kind regards

Vladimir

---

From: Jim Grosbach [gros...@apple.com]
Sent: Thursday, August 09, 2012 6:18 PM
To: Medic, Vladimir
Cc: llv...@cs.uiuc.edu
Subject: Re: [LLVMdev] Pseudo instructions expansion

[Peter Cooper]

Hi Vladimir

I thought you were expanding this very late (after regalloc), but if you want to custom match your pseudo in tablegen based on the value of the immediate then one solution is to mimic x86's fpimm0.

This is from X86InstrFragmentsSIMD.td

def fp32imm0 : PatLeaf<(f32 fpimm), [{

  return N->isExactlyValue(+0.0);

}]>;

You can write a similar one to check the range of your immediate (an i32 i imagine, not f32) and match patterns against it.

Thanks,

Pete