On Wed, May 08, 2019 at 03:45:12PM +0100, Jiong Wang wrote:
>
> I might be misunderstanding your points, please just shout if I am wrong.
>
> Suppose the following BPF code:
>
> unsigned helper(unsigned long long, unsigned long long);
> unsigned long long test(unsigned *a, unsigned int c)
> {
> unsigned int b = *a;
> c += 10;
> return helper(b, c);
> }
>
> We get the following instruction sequence by latest llvm
> (-O2 -mattr=+alu32 -mcpu=v3)
>
> test:
> 1: w1 = *(u32 *)(r1 + 0)
> 2: w2 += 10
> 3: call helper
> 4: exit
>
> Argument Types
> ===
> Now instruction 1 and 2 are sub-register defines, and instruction 3, the
> call, use them implicitly.
>
> Without the introduction of the new ARG_CONST_SIZE32 and
> ARG_CONST_SIZE32_OR_ZERO, we don't know what should be done with w1 and
> w2, zero-extend them should be fine for all cases, but could resulting in a
> few unnecessary zero-extension inserted.
I don't think we're on the same page.
The argument type is _const_.
In the example above they are not _const_.
>
> And that why I introduce these new argument types, without them, there
> could be more than 10% extra zext inserted on benchmarks like bpf_lxc.
10% extra ? so be it.
We're talking past each other here.
I agree with your optimization goal, but I think you're missing
the safety concerns I'm trying to explain.
>
> But for helper functions, they are done by native code which may not follow
> this convention. For example, on arm32, calling helper functions are just
> jump to and execute native code. And if the helper returns u32, it just set
> r0, no clearing of r1 which is the high 32-bit in the register pair
> modeling eBPF R0.
it's arm32 bug then. All helpers _must_ return 64-bit back to bpf prog
and _must_ accept 64-bit from bpf prog.
