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Sat, 26 Sep 2026 00:45:04 -0700 (PDT) Content-Type: text/plain; charset=UTF-8 Date: Sat, 26 Sep 2026 07:45:02 +0000 Message-Id: From: "Alexei Starovoitov" To: "Yonghong Song" , "Alan Maguire" , "Arnaldo Carvalho de Melo" , Cc: "Andrii Nakryiko" , , , "Tejun Heo" Subject: Re: [PATCH dwarves 1/2] dwarf_loader: Trust the entry value register over the first location entry In-Reply-To: <20260925213619.2187751-1-yonghong.song@linux.dev> References: <20260925213619.2187751-1-yonghong.song@linux.dev> X-Mailer: mkdraft (claude review draft; edit before sending) Content-Transfer-Encoding: 8bit Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, Sep 25, 2026 at 02:36 PM Yonghong Song wrote: > DW_OP_entry_value(DW_OP_regN) says the parameter still holds the value regN > had on entry to the function, so regN is by definition the register the > parameter was passed in. That is better evidence than the first location > list entry, so prefer it. my bot is saying that this is not quite the case. gcc 13 -O2 for long f6(long a, long b) { b = a; ext(); ext(); return 0; } describes 'b' as: [0x40, 0x44): DW_OP_reg4 RSI [0x44, 0x4c): DW_OP_reg5 RDI [0x4c, 0x59): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value pahole encodes f6() today. With this patch loc_reg of 'b' becomes RDI, RSI is expected, and f6() is dropped. > + if (entry_value_reg != PARAMETER_UNKNOWN_REG && !parameter__has_piece_info(parm)) > + parm->loc_reg = entry_value_reg; This fixes only the functions where clang happened to emit an entry value. clang -O2 for bool g_lazy(struct task *p, bool lazy) { rcu_lock(); __atomic_store_n(&p->lazy, lazy, __ATOMIC_RELAXED); rcu_unlock(); return lazy; } describes 'lazy' as: [0x09, 0x1e): DW_OP_reg3 RBX [0x1e, 0x21): DW_OP_reg0 RAX low_pc is 0. There is no entry value, so g_lazy() is still dropped. With 'int lazy' the list starts at low_pc with RSI. Looks like clang loses the entry range for every bool arg.