Re: [PATCH dwarves 1/5] dwarves: help dwarf loader spot functions with optimized-out parameters

[Alan Maguire <alan.maguire@oracle.com>]

On 25/01/2023 17:47, Eduard Zingerman wrote:
> On Tue, 2023-01-24 at 13:45 +0000, Alan Maguire wrote:
>> Compilation generates DWARF at several stages, and often the
>> later DWARF representations more accurately represent optimizations
>> that have occurred during compilation.
>>
>> In particular, parameter representations can be spotted by their
>> abstract origin references to the original parameter, but they
>> often have more accurate location information.  In most cases,
>> the parameter locations will match calling conventions, and be
>> registers for the first 6 parameters on x86_64, first 8 on ARM64
>> etc.  If the parameter is not a register when it should be however,
>> it is likely passed via the stack or the compiler has used a
>> constant representation instead.
>>
>> This change adds a field to parameters and their associated
>> ftype to note if a parameter has been optimized out.  Having
>> this information allows us to skip such functions, as their
>> presence in CUs makes BTF encoding impossible.
>>
>> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
>> ---
>>  dwarf_loader.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>>  dwarves.h      |  4 +++-
>>  2 files changed, 77 insertions(+), 3 deletions(-)
>>
>> diff --git a/dwarf_loader.c b/dwarf_loader.c
>> index 5a74035..0220f1d 100644
>> --- a/dwarf_loader.c
>> +++ b/dwarf_loader.c
>> @@ -992,13 +992,67 @@ static struct class_member *class_member__new(Dwarf_Die *die, struct cu *cu,
>>  	return member;
>>  }
>>  
>> -static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu, struct conf_load *conf)
>> +/* How many function parameters are passed via registers?  Used below in
>> + * determining if an argument has been optimized out or if it is simply
>> + * an argument > NR_REGISTER_PARAMS.  Setting NR_REGISTER_PARAMS to 0
>> + * allows unsupported architectures to skip tagging optimized-out
>> + * values.
>> + */
>> +#if defined(__x86_64__)
>> +#define NR_REGISTER_PARAMS      6
>> +#elif defined(__s390__)
>> +#define NR_REGISTER_PARAMS	5
>> +#elif defined(__aarch64__)
>> +#define NR_REGISTER_PARAMS      8
>> +#elif defined(__mips__)
>> +#define NR_REGISTER_PARAMS	8
>> +#elif defined(__powerpc__)
>> +#define NR_REGISTER_PARAMS	8
>> +#elif defined(__sparc__)
>> +#define NR_REGISTER_PARAMS	6
>> +#elif defined(__riscv) && __riscv_xlen == 64
>> +#define NR_REGISTER_PARAMS	8
>> +#elif defined(__arc__)
>> +#define NR_REGISTER_PARAMS	8
>> +#else
>> +#define NR_REGISTER_PARAMS      0
>> +#endif
>> +
>> +static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
>> +					struct conf_load *conf, int param_idx)
>>  {
>>  	struct parameter *parm = tag__alloc(cu, sizeof(*parm));
>>  
>>  	if (parm != NULL) {
>> +		struct location loc;
>> +
>>  		tag__init(&parm->tag, cu, die);
>>  		parm->name = attr_string(die, DW_AT_name, conf);
>> +
>> +		/* Parameters which use DW_AT_abstract_origin to point at
>> +		 * the original parameter definition (with no name in the DIE)
>> +		 * are the result of later DWARF generation during compilation
>> +		 * so often better take into account if arguments were
>> +		 * optimized out.
>> +		 *
>> +		 * By checking that locations for parameters that are expected
>> +		 * to be passed as registers are actually passed as registers,
>> +		 * we can spot optimized-out parameters.
>> +		 */
>> +		if (param_idx < NR_REGISTER_PARAMS && !parm->name &&
>> +		    attr_location(die, &loc.expr, &loc.exprlen) == 0 &&
>> +		    loc.exprlen != 0) {
>> +			Dwarf_Op *expr = loc.expr;
>> +
>> +			switch (expr->atom) {
>> +			case DW_OP_reg1 ... DW_OP_reg31:
>> +			case DW_OP_breg0 ... DW_OP_breg31:
>> +				break;
>> +			default:
>> +				parm->optimized = true;
>> +				break;
>> +			}
>> +		}
> 
> Hi Alan,
> 
> I looked through the DWARF standard and found two relevant entries:
> 
>> 4.1.4
>>
>> If no location attribute is present in a variable entry representing
>> the definition of a variable (...), or if the location attribute is
>> present but has an empty location description (...), the variable is
>> assumed to exist in the source code but not in the executable program
>> (but see number 10, below).
> 
> This paragraph implies that parameter name presence or absence is
> irrelevant, but I don't have any examples when parameter name is
> present for a removed parameter.
> 
>> 4.1.10
>>
>> A DW_AT_const_value attribute for an entry describing a variable or formal
>> parameter whose value is constant and not represented by an object in the
>> address space of the program, or an entry describing a named constant. (Note
>> that such an entry does not have a location attribute.)
> 
> For this paragraph I have an example:
> 
>     $ cat test.c
>     __attribute__((noinline))
>     static int f(int x, int y) {
>         return x + y;
>     }
>     
>     int main(int argc, char *argv[]) {
>         return f(1, 2) + f(1, 3);
>     }
>     
>     $ gcc --version | head -n1
>     gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
>     $ gcc -O2 -g -c test.c -o test.o
>     
> The objdump shows that constant propagation removed the first
> parameter of the function `f`:
> 
>     $ llvm-objdump -d test.o 
>     
>     test.o:	file format elf64-x86-64
>     
>     Disassembly of section .text:
>     
>     0000000000000000 <f.constprop.0>:
>            0: 8d 47 01                     	leal	0x1(%rdi), %eax
>            3: c3                           	retq
>     
>     Disassembly of section .text.startup:
>     
>     0000000000000000 <main>:
>            0: f3 0f 1e fa                  	endbr64
>            4: bf 02 00 00 00               	movl	$0x2, %edi
>            9: e8 00 00 00 00               	callq	0xe <main+0xe>
>            e: bf 03 00 00 00               	movl	$0x3, %edi
>           13: 89 c2                        	movl	%eax, %edx
>           15: e8 00 00 00 00               	callq	0x1a <main+0x1a>
>           1a: 01 d0                        	addl	%edx, %eax
>           1c: c3                           	retq
>     
> However, the information about this parameter is still present in the DWARF:
> 
>     $ llvm-dwarfdump test.o
>     ...
>     0x000000c1:   DW_TAG_subprogram
>                     DW_AT_name	("f")
>                     DW_AT_decl_file	("/home/eddy/work/tmp/test.c")
>                     DW_AT_decl_line	(2)
>                     DW_AT_decl_column	(0x0c)
>                     DW_AT_prototyped	(true)
>                     DW_AT_type	(0x000000a9 "int")
>                     DW_AT_inline	(DW_INL_inlined)
>                     DW_AT_sibling	(0x000000e1)
>     
>     0x000000d0:     DW_TAG_formal_parameter
>                       DW_AT_name	("x")
>                       DW_AT_decl_file	("/home/eddy/work/tmp/test.c")
>                       DW_AT_decl_line	(2)
>                       DW_AT_decl_column	(0x12)
>                       DW_AT_type	(0x000000a9 "int")
>     
>     0x000000d8:     DW_TAG_formal_parameter
>                       DW_AT_name	("y")
>                       DW_AT_decl_file	("/home/eddy/work/tmp/test.c")
>                       DW_AT_decl_line	(2)
>                       DW_AT_decl_column	(0x19)
>                       DW_AT_type	(0x000000a9 "int")
>     
>     0x000000e0:     NULL
>     
>     0x000000e1:   DW_TAG_subprogram
>                     DW_AT_abstract_origin	(0x000000c1 "f")
>                     DW_AT_low_pc	(0x0000000000000000)
>                     DW_AT_high_pc	(0x0000000000000004)
>                     DW_AT_frame_base	(DW_OP_call_frame_cfa)
>                     DW_AT_call_all_calls	(true)
>     
>     0x000000f8:     DW_TAG_formal_parameter
>                       DW_AT_abstract_origin	(0x000000d8 "y")
>                       DW_AT_location	(DW_OP_reg5 RDI)
>     
>     0x000000ff:     DW_TAG_formal_parameter
>                       DW_AT_abstract_origin	(0x000000d0 "x")
>                       DW_AT_const_value	(0x01)
>     
>     0x00000105:     NULL
>     
> When I ask pahole with this patch-set applied to generate BTF I see
> the following output:
> 
>     $ pahole --verbose --btf_encode_detached=test.btf test.o
>     btf_encoder__new: 'test.o' doesn't have '.data..percpu' section
>     Found 0 per-CPU variables!
>     Found 2 functions!
>     File test.o:
>     [1] INT int size=4 nr_bits=32 encoding=SIGNED
>     [2] PTR (anon) type_id=3
>     [3] PTR (anon) type_id=4
>     [4] INT char size=1 nr_bits=8 encoding=SIGNED
>     [5] FUNC_PROTO (anon) return=1 args=(1 argc, 2 argv)
>     [6] FUNC main type_id=5
>     matched function 'f' with 'f.constprop.0'
>     added local function 'f'
>     matched function 'f' with 'f.constprop.0'
>     [7] FUNC_PROTO (anon) return=1 args=(1 x, 1 y)
>     [8] FUNC f type_id=7
>     
> Meaning that function `f` had not been skipped.
> A trivial modification overcomes this:
> 
> 		if (param_idx < NR_REGISTER_PARAMS && !parm->name) {
> 			if (attr_location(die, &loc.expr, &loc.exprlen) == 0 &&
> 			    loc.exprlen != 0) {
> 				Dwarf_Op *expr = loc.expr;
> 
> 				switch (expr->atom) {
> 				case DW_OP_reg1 ... DW_OP_reg31:
> 				case DW_OP_breg0 ... DW_OP_breg31:
> 					break;
> 				default:
> 					parm->optimized = true;
> 					break;
> 				}
> 			} else if (dwarf_attr(die, DW_AT_const_value, &attr) != NULL) {
> 					parm->optimized = true;
> 			}
> 
> With it pahole seem to work as intended (if I understand the intention correctly):
> 
>     $ pahole --verbose --btf_encode_detached=test.btf test.o
>     btf_encoder__new: 'test.o' doesn't have '.data..percpu' section
>     Found 0 per-CPU variables!
>     Found 2 functions!
>     File test.o:
>     [1] INT int size=4 nr_bits=32 encoding=SIGNED
>     [2] PTR (anon) type_id=3
>     [3] PTR (anon) type_id=4
>     [4] INT char size=1 nr_bits=8 encoding=SIGNED
>     [5] FUNC_PROTO (anon) return=1 args=(1 argc, 2 argv)
>     [6] FUNC main type_id=5
>     matched function 'f' with 'f.constprop.0', has optimized-out parameters
>     added local function 'f', optimized-out params
>     matched function 'f' with 'f.constprop.0', has optimized-out parameters
>     skipping addition of 'f' due to optimized-out parameters
> 
> wdyt?
> 

This is great, thanks Eduard! I can add an additional patch
for the else clause code above, attributing that to you in v2 if
you like?

Alan

> Thanks,
> Eduard
> 
>>  
>>  	return parm;
>> @@ -1450,7 +1504,7 @@ static struct tag *die__create_new_parameter(Dwarf_Die *die,
>>  					     struct cu *cu, struct conf_load *conf,
>>  					     int param_idx)
>>  {
>> -	struct parameter *parm = parameter__new(die, cu, conf);
>> +	struct parameter *parm = parameter__new(die, cu, conf, param_idx);
>>  
>>  	if (parm == NULL)
>>  		return NULL;
>> @@ -2209,6 +2263,10 @@ static void ftype__recode_dwarf_types(struct tag *tag, struct cu *cu)
>>  			}
>>  			pos->name = tag__parameter(dtype->tag)->name;
>>  			pos->tag.type = dtype->tag->type;
>> +			if (pos->optimized) {
>> +				tag__parameter(dtype->tag)->optimized = pos->optimized;
>> +				type->optimized_parms = 1;
>> +			}
>>  			continue;
>>  		}
>>  
>> @@ -2219,6 +2277,20 @@ static void ftype__recode_dwarf_types(struct tag *tag, struct cu *cu)
>>  		}
>>  		pos->tag.type = dtype->small_id;
>>  	}
>> +	/* if parameters were optimized out, set flag for the ftype this
>> +	 * function tag referred to via abstract origin.
>> +	 */
>> +	if (type->optimized_parms) {
>> +		struct dwarf_tag *dtype = type->tag.priv;
>> +		struct dwarf_tag *dftype;
>> +
>> +		dftype = dwarf_cu__find_tag_by_ref(dcu, &dtype->abstract_origin);
>> +		if (dftype && dftype->tag) {
>> +			struct ftype *ftype = tag__ftype(dftype->tag);
>> +
>> +			ftype->optimized_parms = 1;
>> +		}
>> +	}
>>  }
>>  
>>  static void lexblock__recode_dwarf_types(struct lexblock *tag, struct cu *cu)
>> diff --git a/dwarves.h b/dwarves.h
>> index 589588e..1ad1b3b 100644
>> --- a/dwarves.h
>> +++ b/dwarves.h
>> @@ -808,6 +808,7 @@ size_t lexblock__fprintf(const struct lexblock *lexblock, const struct cu *cu,
>>  struct parameter {
>>  	struct tag tag;
>>  	const char *name;
>> +	bool optimized;
>>  };
>>  
>>  static inline struct parameter *tag__parameter(const struct tag *tag)
>> @@ -827,7 +828,8 @@ struct ftype {
>>  	struct tag	 tag;
>>  	struct list_head parms;
>>  	uint16_t	 nr_parms;
>> -	uint8_t		 unspec_parms; /* just one bit is needed */
>> +	uint8_t		 unspec_parms:1; /* just one bit is needed */
>> +	uint8_t		 optimized_parms:1;
>>  };
>>  
>>  static inline struct ftype *tag__ftype(const struct tag *tag)
>