Re: [PATCH dwarves 1/2] dwarf_loader: Trust the entry value register over the first location entry

["Alexei Starovoitov" <alexei.starovoitov@gmail.com>]

On Fri, Sep 25, 2026 at 02:36 PM Yonghong Song <yonghong.song@linux.dev> 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.