Re: [PATCH RFC v4 net-next 00/26] BPF syscall, maps, verifier, samples, llvm

On Wed, Aug 13, 2014 at 4:25 PM, David Miller [off-list ref] wrote:

From: David Laight <redacted>
Date: Wed, 13 Aug 2014 08:52:30 +0000

quoted

From: Of Alexei Starovoitov

quoted

one more RFC...

Major difference vs previous set is a new 'load 64-bit immediate' eBPF insn.
Which is first 16-byte instruction. It shows how eBPF ISA can be extended
while maintaining backward compatibility, but mainly it cleans up eBPF
program access to maps and improves run-time performance.

Wouldn't it be more sensible to follow the scheme used by a lot of cpus
and add a 'load high' instruction (follow with 'add' or 'or').
It still takes 16 bytes to load a 64bit immediate value, but the instruction
size remains constant.
There is nothing to stop any JIT software detecting the instruction pair.

The opposite argument is that JITs can expand the IMM64 load into whatever
sequence of instructions is most optimal.

My only real gripe with IMM64 loads is that it's not mainly for
loading an immediate, it's for loading a pointer.  And this
distinction is important for some JITs.

For example, on sparc64 all symbol based addresses are actually 32-bit
because of the code model we use to compile the kernel and all modules.
So if we knew this is a pointer load and it's to a symbol in a kernel
or module image, we could do a 32-bit load.

This is true for x86_64 as well, I think.

(Almost.  For x86_64 we have a choice between a sign-extended load of
a value in the top 2GB of the address space and lea reg,offset(%rip).)

--Andy