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@xxxxxxxxxx>
> ---
> 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?
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)
