Re: [PATCH bpf-next] bpf: Fold LSH and ARSH pair to a single MOVSX for sign-extension

[Yonghong Song <yonghong.song@linux.dev>]

On 4/29/24 8:20 AM, Xu Kuohai wrote:
> From: Xu Kuohai <xukuohai@huawei.com>
>
> As shown in the ExpandSEXTINREG function in [1], LLVM generates SRL and
> SRA instruction pair to implement sign-extension. For x86 and arm64,
> this instruction pair will be folded to a single instruction, but the
> LLVM BPF backend does not do such folding.

With -mcpu=v4, sign-extention will be generated and in selftest
test_progs-cpuv4 will test with -mcpu=v4. The cpu v4 support
is added in llvm18, and I hope once llvm18 is widely available, we
might be able to make test_progs-cpuv4 as the default test_progs.

So I think this optimization is not needed.

>
> For example, the following C code:
>
> long f(int x)
> {
> 	return x;
> }
>
> will be compiled to:
>
> r0 = r1
> r0 <<= 0x20
> r0 s>>= 0x20
> exit
>
> Since 32-bit to 64-bit sign-extension is a common case and we already
> have MOVSX instruction for sign-extension, this patch tries to fold
> the 32-bit to 64-bit LSH and ARSH pair to a single MOVSX instruction.
>
> [1] https://github.com/llvm/llvm-project/blob/4523a267829c807f3fc8fab8e5e9613985a51565/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp#L1228
>
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
> ---
>   include/linux/filter.h |  8 ++++++++
>   kernel/bpf/verifier.c  | 46 ++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 54 insertions(+)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 7a27f19bf44d..7cc90a32ed9a 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -173,6 +173,14 @@ struct ctl_table_header;
>   		.off   = 0,					\
>   		.imm   = 0 })
>   
> +#define BPF_MOV64_SEXT_REG(DST, SRC, OFF)			\
> +	((struct bpf_insn) {					\
> +		.code  = BPF_ALU64 | BPF_MOV | BPF_X,		\
> +		.dst_reg = DST,					\
> +		.src_reg = SRC,					\
> +		.off   = OFF,					\
> +		.imm   = 0 })
> +
>   #define BPF_MOV32_REG(DST, SRC)					\
>   	((struct bpf_insn) {					\
>   		.code  = BPF_ALU | BPF_MOV | BPF_X,		\
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 4e474ef44e9c..6bcee052d90d 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -20659,6 +20659,49 @@ static int optimize_bpf_loop(struct bpf_verifier_env *env)
>   	return 0;
>   }
>   
> +static bool is_sext32(struct bpf_insn *insn1, struct bpf_insn *insn2)
> +{
> +	if (insn1->code != (BPF_ALU64 | BPF_K | BPF_LSH) || insn1->imm != 32)
> +		return false;
> +
> +	if (insn2->code != (BPF_ALU64 | BPF_K | BPF_ARSH) || insn2->imm != 32)
> +		return false;
> +
> +	if (insn1->dst_reg != insn2->dst_reg)
> +		return false;
> +
> +	return true;
> +}
> +
> +/* LLVM generates sign-extension with LSH and ARSH pair, replace it with MOVSX.
> + *
> + * Before:
> + * DST <<= 32
> + * DST s>>= 32
> + *
> + * After:
> + * DST = (s32)DST
> + */
> +static int optimize_sext32_insns(struct bpf_verifier_env *env)
> +{
> +	int i, err;
> +	int insn_cnt = env->prog->len;
> +	struct bpf_insn *insn = env->prog->insnsi;
> +
> +	for (i = 0; i < insn_cnt; i++, insn++) {
> +		if (i + 1 >= insn_cnt || !is_sext32(insn, insn + 1))
> +			continue;
> +		/* patch current insn to MOVSX */
> +		*insn = BPF_MOV64_SEXT_REG(insn->dst_reg, insn->dst_reg, 32);
> +		/* remove next insn */
> +		err = verifier_remove_insns(env, i + 1, 1);
> +		if (err)
> +			return err;
> +		insn_cnt--;
> +	}
> +	return 0;
> +}
> +
>   static void free_states(struct bpf_verifier_env *env)
>   {
>   	struct bpf_verifier_state_list *sl, *sln;
> @@ -21577,6 +21620,9 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr, __u3
>   	if (ret == 0)
>   		ret = optimize_bpf_loop(env);
>   
> +	if (ret == 0)
> +		ret = optimize_sext32_insns(env);
> +
>   	if (is_priv) {
>   		if (ret == 0)
>   			opt_hard_wire_dead_code_branches(env);