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LoongArch: BPF: Optimize redundant TCC loads in epilogue

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The legacy epilogue implementation pops the tail call counter (TCC)
context via a redundant double-load pattern. It first decrements the
load_offset by 2 slots to fetch 'tcc_ptr', and then immediately bumps
it back up by 1 slot to load the original 'tcc' value into REG_TCC,
unnecessarily overwriting the register.

Optimize this sequence by adjusting the load_offset by only 1 slot.
This aligns the offset directly with the higher stack slot containing
the entry TCC counter (or caller state), allowing us to restore the
REG_TCC register safely with a single load.

This removes one redundant instruction from the epilogue hot path,
improves code readability, and ensures the correct TCC register context
is handed back cleanly upon normal return.

Cc: stable@vger.kernel.org
Fixes: c0fcc955ff ("LoongArch: BPF: Fix the tailcall hierarchy")
Fixes: ef54c517a9 ("LoongArch: BPF: Implement PROBE_MEM32 pseudo instructions")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
This commit is contained in:
Tiezhu Yang 2026-08-17 22:07:23 +08:00 committed by Huacai Chen
parent 29479b1431
commit fd3cb1bfeb
1 changed files with 2 additions and 8 deletions
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10
arch/loongarch/net/bpf_jit.c
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@ -246,14 +246,8 @@ static void __build_epilogue(struct jit_ctx *ctx, bool is_tail_call)
emit_insn(ctx, ldd, REG_ARENA, LOONGARCH_GPR_SP, load_offset);
}
/*
* When push into the stack, follow the order of tcc then tcc_ptr.
* When pop from the stack, first pop tcc_ptr then followed by tcc.
*/
load_offset -= 2 * sizeof(long);
emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_SP, load_offset);
load_offset += sizeof(long);
/* Only restore the TCC state into REG_TCC from the higher slot */
load_offset -= sizeof(long);
emit_insn(ctx, ldd, REG_TCC, LOONGARCH_GPR_SP, load_offset);
emit_insn(ctx, addid, LOONGARCH_GPR_SP, LOONGARCH_GPR_SP, stack_adjust);