llvm dev - Sep 2012 - [LLVMdev] Fwd: Need Help Understanding Operands in X86 MachineFunctionPass

Dear All,

I'm working on an X86 MachineFunctionPass that adds prefetch 
instructions to a function.  I have code that adds a "prefetchnta 
<constant address>" instruction to x86 32-bit code.  What I want to
do
is to add a "prefetchnta <constant address>" instruction to
x86_64 code.

The code for adding the 32-bit instruction is:

BuildMI(MBB,MI,dl,TII->get(X86::PREFETCHNTA)).addReg(0).addImm(0).addReg(0).addImm(<constant>).addReg(0);

The code above doesn't work for x86_64, so I think I need to change the 
operands to the instruction.  However, I have no idea what the different 
register and immediate arguments to this instruction represent.  Do they 
somehow encode the Mod/RM bits used in x86 instruction encoding?  Are 
they offset and register operands?  I've checked the TableGen files, but 
their meaning appears cryptic to me.

Can someone explain to me what these operands are meant to represent?

Thanks in advance,

-- John T.

On Sep 11, 2012, at 10:57 AM, John Criswell wrote:
> I'm working on an X86 MachineFunctionPass that adds prefetch
instructions to a function.  I have code that adds a "prefetchnta
<constant address>" instruction to x86 32-bit code.  What I want to
do is to add a "prefetchnta <constant address>" instruction to
x86_64 code.
Given that you don't actually want to execute this instruction ever, is
there a reason to even emit the instruction? Why not just stick your ID directly
into the code and then jump beyond it. Executing the instruction causes a slow
down and it requires a bunch of extra bytes. If you can live with a 32-bit ID,
it'll take 4 bytes. If you go the prefetchnta (or any of the other prefetch
instructions) route, then on the x86_64, it looks like you're going to have
2 bytes of opcode, a mod r/m byte, and 8 bytes of address and look something
like 0f 18 00 dd cc bb aa 00 00 00 00 (I probably got the mod r/m byte wrong,
it's a /0 for prefetchnta at any rate). That's just going to add extra
pressure to the icache.
> The code for adding the 32-bit instruction is:
> 
>
BuildMI(MBB,MI,dl,TII->get(X86::PREFETCHNTA)).addReg(0).addImm(0).addReg(0).addImm(<constant>).addReg(0);
That's surprising to me. What are all of those registers and immediates for?


-- 
Stephen Checkoway

Trying again from the correct email address, sorry if you get this twice.

On Sep 11, 2012, at 10:57 AM, John Criswell wrote:
> I'm working on an X86 MachineFunctionPass that adds prefetch
instructions to a function.  I have code that adds a "prefetchnta
<constant address>" instruction to x86 32-bit code.  What I want to
do is to add a "prefetchnta <constant address>" instruction to
x86_64 code.
Given that you don't actually want to execute this instruction ever, is
there a reason to even emit the instruction? Why not just stick your ID directly
into the code and then jump beyond it. Executing the instruction causes a slow
down and it requires a bunch of extra bytes. If you can live with a 32-bit ID,
it'll take 4 bytes. If you go the prefetchnta (or any of the other prefetch
instructions) route, then on the x86_64, it looks like you're going to have
2 bytes of opcode, a mod r/m byte, and 8 bytes of address and look something
like 0f 18 00 dd cc bb aa 00 00 00 00 (I probably got the mod r/m byte wrong,
it's a /0 for prefetchnta at any rate). That's just going to add extra
pressure to the icache.
> The code for adding the 32-bit instruction is:
> 
>
BuildMI(MBB,MI,dl,TII->get(X86::PREFETCHNTA)).addReg(0).addImm(0).addReg(0).addImm(<constant>).addReg(0);
That's surprising to me. What are all of those registers and immediates for?


-- 
Stephen Checkoway

On 9/11/12 12:10 PM, Stephen Checkoway wrote:
> On Sep 11, 2012, at 10:57 AM, John Criswell wrote:
>
>> I'm working on an X86 MachineFunctionPass that adds prefetch
instructions to a function.  I have code that adds a "prefetchnta
<constant address>" instruction to x86 32-bit code.  What I want to
do is to add a "prefetchnta <constant address>" instruction to
x86_64 code.
> Given that you don't actually want to execute this instruction ever, is
there a reason to even emit the instruction?
It's true that I don't want to emit the instruction, but it may be 
easier to emit an instruction than to emit a constant into the 
instruction stream.

Also, I imagine that if I don't solve this problem with prefetchnta that 
I might run into it again when I write some other machine-level 
instrumentation.  The prefetchnta is just a concrete example; more 
generally, I just need to understand how the operands to the x86 
instructions work in the LLVM code generator.  I figure that if I 
understand prefetchnta I'll have an easier time figuring out other 
instructions.

> Why not just stick your ID directly into the code and then jump beyond it.
Executing the instruction causes a slow down and it requires a bunch of extra
bytes. If you can live with a 32-bit ID, it'll take 4 bytes. If you go the
prefetchnta (or any of the other prefetch instructions) route, then on the
x86_64, it looks like you're going to have 2 bytes of opcode, a mod r/m
byte, and 8 bytes of address and look something like 0f 18 00 dd cc bb aa 00 00
00 00 (I probably got the mod r/m byte wrong, it's a /0 for prefetchnta at
any rate). That's just going to add extra pressure to the icache.
I think the instrumentation already adds a jump around the prefetchnta.
>
>> The code for adding the 32-bit instruction is:
>>
>>
BuildMI(MBB,MI,dl,TII->get(X86::PREFETCHNTA)).addReg(0).addImm(0).addReg(0).addImm(<constant>).addReg(0);
> That's surprising to me. What are all of those registers and immediates
for?
That is precisely my question.  What do all of those register and 
immediate arguments do?

-- John T.
>
>