Re: [PATCH 07/48] perf dwarf-aux: Add die_find_variable_by_reg() helper

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 11 Oct 2023 20:50:30 -0700
Namhyung Kim <namhyung@kernel.org> wrote:

> The die_find_variable_by_reg() will search for a variable or a parameter
> sub-DIE in the given scope DIE where the location matches to the given
> register.
> 
> For the simpliest and most common case, memory access usually happens
> with a base register and an offset to the field so the register would
> hold a pointer in a variable or function parameter.  Then we can find
> one if it has a location expression at the (instruction) address.  So
> this function only handles such a simple case for now.
> 
> In this case, the expression would have a DW_OP_regN operation where
> N < 32.  If the register index (N) is greater than or equal to 32,
> DW_OP_regx operation with an operand which saves the value for the N
> would be used.  It would reject expressions with more operations.
> 

Nice! This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you,

> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  tools/perf/util/dwarf-aux.c | 67 +++++++++++++++++++++++++++++++++++++
>  tools/perf/util/dwarf-aux.h | 12 +++++++
>  2 files changed, 79 insertions(+)
> 
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index 10aa32334d6f..652e6e7368a2 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -1245,6 +1245,73 @@ int die_get_var_range(Dwarf_Die *sp_die, Dwarf_Die *vr_die, struct strbuf *buf)
>  out:
>  	return ret;
>  }
> +
> +/* Interval parameters for __die_find_var_reg_cb() */
> +struct find_var_data {
> +	/* Target instruction address */
> +	Dwarf_Addr pc;
> +	/* Target register */
> +	unsigned reg;
> +};
> +
> +/* Max number of registers DW_OP_regN supports */
> +#define DWARF_OP_DIRECT_REGS  32
> +
> +/* Only checks direct child DIEs in the given scope. */
> +static int __die_find_var_reg_cb(Dwarf_Die *die_mem, void *arg)
> +{
> +	struct find_var_data *data = arg;
> +	int tag = dwarf_tag(die_mem);
> +	ptrdiff_t off = 0;
> +	Dwarf_Attribute attr;
> +	Dwarf_Addr base, start, end;
> +	Dwarf_Op *ops;
> +	size_t nops;
> +
> +	if (tag != DW_TAG_variable && tag != DW_TAG_formal_parameter)
> +		return DIE_FIND_CB_SIBLING;
> +
> +	if (dwarf_attr(die_mem, DW_AT_location, &attr) == NULL)
> +		return DIE_FIND_CB_SIBLING;
> +
> +	while ((off = dwarf_getlocations(&attr, off, &base, &start, &end, &ops, &nops)) > 0) {
> +		/* Assuming the location list is sorted by address */
> +		if (end < data->pc)
> +			continue;
> +		if (start > data->pc)
> +			break;
> +
> +		/* Only match with a simple case */
> +		if (data->reg < DWARF_OP_DIRECT_REGS) {
> +			if (ops->atom == (DW_OP_reg0 + data->reg) && nops == 1)
> +				return DIE_FIND_CB_END;
> +		} else {
> +			if (ops->atom == DW_OP_regx && ops->number == data->reg &&
> +			    nops == 1)
> +				return DIE_FIND_CB_END;
> +		}
> +	}
> +	return DIE_FIND_CB_SIBLING;
> +}
> +
> +/**
> + * die_find_variable_by_reg - Find a variable saved in a register
> + * @sc_die: a scope DIE
> + * @pc: the program address to find
> + * @reg: the register number to find
> + * @die_mem: a buffer to save the resulting DIE
> + *
> + * Find the variable DIE accessed by the given register.
> + */
> +Dwarf_Die *die_find_variable_by_reg(Dwarf_Die *sc_die, Dwarf_Addr pc, int reg,
> +				    Dwarf_Die *die_mem)
> +{
> +	struct find_var_data data = {
> +		.pc = pc,
> +		.reg = reg,
> +	};
> +	return die_find_child(sc_die, __die_find_var_reg_cb, &data, die_mem);
> +}
>  #endif
>  
>  /*
> diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
> index f9d765f80fb0..b6f430730bd1 100644
> --- a/tools/perf/util/dwarf-aux.h
> +++ b/tools/perf/util/dwarf-aux.h
> @@ -137,6 +137,10 @@ int die_get_scopes(Dwarf_Die *cu_die, Dwarf_Addr pc, Dwarf_Die **scopes);
>  /* Get byte offset range of given variable DIE */
>  int die_get_var_range(Dwarf_Die *sp_die, Dwarf_Die *vr_die, struct strbuf *buf);
>  
> +/* Find a variable saved in the 'reg' at given address */
> +Dwarf_Die *die_find_variable_by_reg(Dwarf_Die *sc_die, Dwarf_Addr pc, int reg,
> +				    Dwarf_Die *die_mem);
> +
>  #else /*  HAVE_DWARF_GETLOCATIONS_SUPPORT */
>  
>  static inline int die_get_var_range(Dwarf_Die *sp_die __maybe_unused,
> @@ -146,6 +150,14 @@ static inline int die_get_var_range(Dwarf_Die *sp_die __maybe_unused,
>  	return -ENOTSUP;
>  }
>  
> +static inline Dwarf_Die *die_find_variable_by_reg(Dwarf_Die *sc_die __maybe_unused,
> +						  Dwarf_Addr pc __maybe_unused,
> +						  int reg __maybe_unused,
> +						  Dwarf_Die *die_mem __maybe_unused)
> +{
> +	return NULL;
> +}
> +
>  #endif /* HAVE_DWARF_GETLOCATIONS_SUPPORT */
>  
>  #endif /* _DWARF_AUX_H */
> -- 
> 2.42.0.655.g421f12c284-goog
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>