bpf, riscv: Optimize BPF_JSET BPF_K using andi on RV64

This patch optimizes BPF_JSET BPF_K by using a RISC-V andi instruction
when the BPF immediate fits in 12 bits, instead of first loading the
immediate to a temporary register.

Examples of generated code with and without this optimization:

BPF_JMP_IMM(BPF_JSET, R1, 2, 1) without optimization:

  20: li    t1,2
  24: and   t1,a0,t1
  28: bnez  t1,0x30

BPF_JMP_IMM(BPF_JSET, R1, 2, 1) with optimization:

  20: andi  t1,a0,2
  24: bnez  t1,0x2c

BPF_JMP32_IMM(BPF_JSET, R1, 2, 1) without optimization:

  20: li    t1,2
  24: mv    t2,a0
  28: slli  t2,t2,0x20
  2c: srli  t2,t2,0x20
  30: slli  t1,t1,0x20
  34: srli  t1,t1,0x20
  38: and   t1,t2,t1
  3c: bnez  t1,0x44

BPF_JMP32_IMM(BPF_JSET, R1, 2, 1) with optimization:

  20: andi  t1,a0,2
  24: bnez  t1,0x2c

In these examples, because the upper 32 bits of the sign-extended
immediate are 0, BPF_JMP BPF_JSET and BPF_JMP32 BPF_JSET are equivalent
and therefore the JIT produces identical code for them.

Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Björn Töpel <bjorn.topel@gmail.com>
Acked-by: Björn Töpel <bjorn.topel@gmail.com>
Link: https://lore.kernel.org/bpf/20200506000320.28965-5-luke.r.nels@gmail.com

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index b07cef9..6cfd164 100644 (file)
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -792,8 +792,6 @@ out_be:
        case BPF_JMP32 | BPF_JSGE | BPF_K:
        case BPF_JMP | BPF_JSLE | BPF_K:
        case BPF_JMP32 | BPF_JSLE | BPF_K:
-       case BPF_JMP | BPF_JSET | BPF_K:
-       case BPF_JMP32 | BPF_JSET | BPF_K:
                rvoff = rv_offset(i, off, ctx);
                s = ctx->ninsns;
                if (imm) {
@@ -813,15 +811,28 @@ out_be:
 
                /* Adjust for extra insns */
                rvoff -= (e - s) << 2;
+               emit_branch(BPF_OP(code), rd, rs, rvoff, ctx);
+               break;
 
-               if (BPF_OP(code) == BPF_JSET) {
-                       /* Adjust for and */
-                       rvoff -= 4;
-                       emit(rv_and(rs, rd, rs), ctx);
-                       emit_branch(BPF_JNE, rs, RV_REG_ZERO, rvoff, ctx);
+       case BPF_JMP | BPF_JSET | BPF_K:
+       case BPF_JMP32 | BPF_JSET | BPF_K:
+               rvoff = rv_offset(i, off, ctx);
+               s = ctx->ninsns;
+               if (is_12b_int(imm)) {
+                       emit(rv_andi(RV_REG_T1, rd, imm), ctx);
                } else {
-                       emit_branch(BPF_OP(code), rd, rs, rvoff, ctx);
+                       emit_imm(RV_REG_T1, imm, ctx);
+                       emit(rv_and(RV_REG_T1, rd, RV_REG_T1), ctx);
                }
+               /* For jset32, we should clear the upper 32 bits of t1, but
+                * sign-extension is sufficient here and saves one instruction,
+                * as t1 is used only in comparison against zero.
+                */
+               if (!is64 && imm < 0)
+                       emit(rv_addiw(RV_REG_T1, RV_REG_T1, 0), ctx);
+               e = ctx->ninsns;
+               rvoff -= (e - s) << 2;
+               emit_branch(BPF_JNE, RV_REG_T1, RV_REG_ZERO, rvoff, ctx);
                break;
 
        /* function call */