[LLVMdev] RFC building a target MCAsmParser

[Colin LeMahieu]

An assembler frontend for converting to MCInsts is a possibility.  The parser for that would probably be generated using a third party parser generator.  At that point we could just substitute use of the TableGen match tables for the generated library.  Do you know if anyone’s considered including a parser lib like this before?

 

The TableGen changes I alluded to would get us close to parsing without hacks though it would still be deficient compared to a full parser.  These changes would probably be the least amount of deviation from other targets depending on what everyone wants long-term.

 

From: Reid Kleckner [mailto:rnk at google.com] 
Sent: Tuesday, April 14, 2015 1:17 PM
To: Colin LeMahieu
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] RFC building a target MCAsmParser

 

Frankly, the MC assembler infrastructure is pretty weak. I've spoken personally to some of the developers who wrote it, and they said they know it's bad and suggested that it be rewritten. My experience from attempting to make MC parse Intel x86 asm confirms this.

 

Here's some questions for you:

Do you need a high-level macro assembler designed for human consumption?
Do you need Clang to understand inline assembly that uses these high-level assembly constructs?

Do you need the assembler to parse the output of `clang -S`?

 

It might be best to have your own small frontend for the macro assembler language that produces MCInsts which can be printed using a more traditional gas-like mneumonic. Running llvm-mc on a hexagon assembly file and re-emitting assembly would parse the high-level assembly and produce the low-level assembly.

 

Alternatively, you can do as you say and try to make MC into more of a real language frontend. The major problem is that lexing rules are not the same across all targets, even when they all use a gas-like syntax.

 

On Tue, Apr 14, 2015 at 10:58 AM, Colin LeMahieu <colinl at codeaurora.org <mailto:colinl at codeaurora.org> > wrote:

Hi everyone.  We’re interested in contributing a Hexagon assembler to MC and we’re looking for comments on a good way to integrate the grammar in to the infrastructure.

 

We rely on having a robust assembler because we have a large base of developers that write in assembly due to low power requirements for mobile devices.  We put in some C-like concepts to make the syntax easier and this design is fairly well received by users.

 

The following is a list of grammar snippets we’ve had trouble integrating in to the asm parser framework.

 

Instruction packets are optionally enclosed in braces.

    { r0 = add(r1, r2) r1 = add(r2, r0) }

 

Register can be the beginning of a statement.  Register transfers have no mnemonic.

    r0 = r1

 

Double registers have a colon in the middle which can look like a label

    r1:0 = add(r3:2, r5:4)

 

Predicated variants for many instructions

    if(p1) r0 = add(r1, r2)

 

Dense semantics for DSP applications.  Complex multiply optionally shifting result left by 1 with optional rounding and optional saturation

    r0 = cmpy(r1, r2):<<1:rnd:sat

 

Hardware loops ended by optional packet suffix

    { r0 = r1 }:endloop0:endloop1

 

We found the Hexagon grammar to be straight forward to implement using plain lex / parse but harder within the MCTargetAsmParser. 

 

We were thinking a way to get the grammar to work would involve modifying tablegen and the main asm parser loop.  We’d have to make tablegen break down each instructions in to a sequence of tokens and build a sorted matching table based on the set of these sequences.  The matching loop would bisect this sorted list looking for a match.  We think existing grammars would be unaffected; all existing instructions start with a mnemonic so their first token would be an identifier followed by the same sequence of tokens they currently have.

 

Let us know if we’re likely to run in to any issues making these changes or if there are other recommendations on what we could do.  Thanks!

 

Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, 
a Linux Foundation Collaborative Project

 


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