[llvm-dev] Implement VLIW Backend on LLVM (Assembler Related Questions)

Krzysztof Parzyszek via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 10 11:19:46 PST 2018

In the intermediate language that assembler works on an instruction is 
represented by an MCInst. An MCInst can have other instructions as 
operands, and this is how the Hexagon backend implements bundles.

A top-level MCInst (i.e. the entire bundle) is encoded all at once from 
the point of view of the target-independent mechanisms. Those mechanisms 
use target-specific code that each implementation needs to provide, and 
in your code you can handle each bundle as you want.

Check MCCodeEmitter and how different targets implement it.

As for the syntax---the parser needs to be able to determine the bundle 
boundary. (For example Hexagon uses braces {} to enclose each bundle.) 
The way the assembler works is that it constructs an instruction and 
passes it to the associated streamer. The streamer is typically an 
assembly streamer (i.e. printing the instruction assembly), or an object 
file streamer (e.g. ELF, etc.)

The answers to all your questions are "yes", or "it's doable", but the 
degree of complexity may vary between different choices.

The major suggestion that I have is to make sure that the syntax is 
unambiguous, specifically when it comes to bundle boundaries. Another 
suggestion is to maintain the "mnemonic op, op, ..." syntax for 
individual instructions (i.e. mnemonic followed by a list of operands). 
Hexagon has its own assembly syntax that doesn't follow that, and it 
makes things a bit more complicated.


On 12/6/2018 7:46 PM, Cy Cheng via llvm-dev wrote:
> Hello,
> I want to implement LLVM backend for a specific VLIW hardware. I am 
> working on defining its instruction set, and assembly language.
> The hardware has two pipelines, int and float. Each pipeline can do 3 
> operations/cycle, 3 operations forms an instruction.
> One of the Integer Instruction looks like this:
>      add Ri, Rj, Rk; add Rl, Rm, Rn; add Ro, Rp, Rq
> An int instruction and a float instruction forms a VLIW instruction 
> (bundle), e.g.
> {
>      add Ri, Rj, Rk; add Rl, Rm, Rn; add Ro, Rp, Rq
>      fadd Fi, Fj, Fk; fadd Fl, Fm, Fn; fadd Fo, Fp, Fq
> }
> I want to express above concept in this way:
> // Assembly Language
> {
>      add Ri, Rj, Rk
>      add Rl, Rm, Rn
>      add Ro, Rp, Rq
>      fadd Fi, Fj, Fk
>      fadd Fl, Fm, Fn
>      fadd Fo, Fp, Fq
> }
> Q1:
> My first question is, the instruction encoding can only be determined 
> after parser has finished parsing the entire bundle.
> e.g. When parser see "add Ri, Rj, Rk", it generates one encoding, but 
> when parser see another "add Ri, Rj, Rk", it will modify previously 
> generated encoding.
> I would like to know can LLVM's assembler support this?
> Or I should define my instruction in this way:
>     add_type1 Ri, Rj, Rk
>     add_type2 Ri, Rj, Rk, Rl, Rm, Rn
>     add_type3 Ri, Rj, Rk, Rl, Rm, Rn, Ro, Rp, Rq
> Q2.
> Some of the instructions need to setup additional configuration, e.g.
> {
>      scache wa   ; Set cache mode: write allocate
>      ssize 64    ; Set write size = 64 bits
>      sendian big ; Set big endian writing
>      store R0, 0x1000000 ; Write "R0" to 0x1000000
> }
> So, again, parser has to parse the entire bundle to generate correct 
> encoding.
> Or I should define my instruction in this way:
> store R0, 0x1000000, wa, 64, big, .... (10 options can be set)
> Q3.
> The destination register can be omitted, e.g.
>      add , Rj, Rk
> So can I use this form to express omitting destination, or I should 
> define new instruction for it?
> e.g.
>      add_no_dest Rj, Rk
> Q4.
> Can I define the instruction which has the same name but with different 
> count of operands, e.g.
>      fadd Fi, Fj, Fk
>      fadd Fl, Fm, Fn, rounding_mode
> So fadd has two versions
> (a) normal rounding
> (b) special rounding mode
> Or I should define it in this way:
> fadd
> fadd_round_mode1
> fadd_round_mode2
> ..
> fadd_round_mode15
> (16 rounding mode)
> Thank You,
> CY
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