Re: Re: Re: Re: [PATCH V2 1/3] riscv: Add Zicbop instruction definitions & cpufeature

[Andrew Jones]

On Thu, Jan 04, 2024 at 12:03:57PM -0300, Leonardo Bras wrote:
...
> > > > > What we need here is something like:
> > > > > + enum {
> > > > > + 	PREFETCH_I,
> > > > > + 	PREFETCH_R,
> > > > > + 	PREFETCH_W,
> > > > > + }	 
> > > > 
> > > > Can't use enum. This header may be included in assembly.
> > > 
> > > Oh, I suggest defines then, since it's better to make it clear instead of 
> > > using 0, 1, 3.
> > 
> > I don't think we gain anything by adding another define in order to create
> > the instruction define. We have to review the number sooner or later. I'd
> > prefer we use the number inside the instruction define so we only need
> > to look one place, which is also consistent with how we use FUNC fields.
> > 
> 
> Sorry, I was unable to understand the reasoning.
> 
> If we are going to review the numbers sooner or later, would not it be 
> better to have the instruction define to have "PREFETCH_W" instead of a 
> number, and a unified list of defines for instructions.
> 
> This way we don't need to look into the code for 0's 1's and 3's, but 
> instead just replace the number in the define list.
> 
> What am I missing?  

PREFETCH_W isn't defined as just 3, it's defined as
   INSN_S(OPCODE_OP_IMM, FUNC3(6), __RS2(3), ...)

Adding a define (PREFETCH_W_RS2?) for the 3 just bloats the code and
requires reviewers of PREFETCH_W to go look up another define.
OPCODE_OP_IMM gets a define because it's used in multiple instructions,
but everything else in an instruction definition should be a number
exactly matching the spec, making it easy to review, or be an argument
passed into the instruction macro.

> 
> > > 
> > > > 
> > > > > +
> > > > > + #define CBO_PREFETCH(type, base, offset)                      \
> > > > > +     INSN_S(OPCODE_OP_IMM, FUNC3(6), __RS2(type),              \
> > > > > +            SIMM12(offset & ~0x1f), RS1(base))
> > > > 
> > > > Yes. I suggested we mask offset as well, but ideally we'd detect a caller
> > > > using an offset with nonzero lower 5 bits at compile time.
> > > 
> > > I would suggest the compiler would take care of this, but I am not sure 
> > > about the assembly, since I am not sure if it gets any optimization.
> > > 
> > > I don't think we can detect a caller with non-zero offset at compile time, 
> > > since it will be used in locks which can be at (potentially) any place in 
> > > the block size. (if you have any idea though, please let me know :) )

I forgot to reply to this before. The reason I think it may be possible to
validate offset at compile time is because it must be a constant, i.e.
__builtin_constant_p(offset) must return true. So maybe something like

 static_assert(__builtin_constant_p(offset) && !(offset & 0x1f))

I'll try to find time to play with it.

> > > 
> > > On the other hand, we could create a S-Type macro which deliberately 
> > > ignores imm[4:0], like  
> > > 
> > > + INSN_S_TRUNCATE(OPCODE_OP_IMM, FUNC3(6), __RS2(3),               \
> > > +                 SIMM12(offset), RS1(base))
> > > 
> > > Which saves the bits 11:5 of offset  into imm[11:5], and zero-fill 
> > > imm[4:0], like
> > > 
> > > +#define DEFINE_INSN_S                                                    \
> > > + __DEFINE_ASM_GPR_NUMS                                           \
> > > +"        .macro insn_s, opcode, func3, rs2, simm12, rs1\n"               \
> > > +"        .4byte  ((\\opcode << " __stringify(INSN_S_OPCODE_SHIFT) ") |"  \
> > > +"                 (\\func3 << " __stringify(INSN_S_FUNC3_SHIFT) ") |"    \
> > > +"                 (.L__gpr_num_\\rs2 << " __stringify(INSN_S_RS2_SHIFT) ") |" \
> > > +"                 (.L__gpr_num_\\rs1 << " __stringify(INSN_S_RS1_SHIFT) ") |" \
> > > +"                 (((\\simm12 >> 5) & 0x7f) << " __stringify(INSN_S_SIMM7_SHIFT) "))\n" \
> > > +"        .endm\n"
> > > +
> > > 
> > > Does this make sense?
> > 
> > If we create a special version of INSN_S, then I suggest we create one
> > where its two SIMM fields are independent and then define prefetch
> > instructions like this
> > 
> >  #define PREFETCH_W(base, offset) \
> >      INSN_S_SPLIT_IMM(OPCODE_OP_IMM, FUNC3(6), __RS2(3), \
> >          SIMM_11_5(offset >> 5), SIMM_4_0(0), RS1(base))
> > 
> > which would allow simple review against the spec and potentially
> > support other instructions which use hard coded values in the
> > immediate fields.
> > 
> 
> I agree, it looks better this way.
> 
> We could have:
> INSN_S_SPLIT_IMM(OPCODE, FUNC3, RS1, RS2, SIMM_11_5, SIMM_4_0)
> 
> and implement INSN_S like:
> #define INSN_S(OPCODE, FUNC3, RS1, RS2, SIMM_11_0) \
> 	INSN_S_SPLIT_IMM(OPCODE, FUNC3, RS1, RS2,  \
> 		SIMM_11_0 >> 5, SIMM_11_0 & 0x1f)

That won't work since SIMM_11_0 will be a string. Actually, with
stringification in mind, I don't think defining INSN_S_SPLIT_IMM()
is a good idea.

Thanks,
drew