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

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2023-01-26 at 01:42 +0200, Eduard Zingerman wrote:
> On Wed, 2023-01-25 at 22:52 +0000, Alan Maguire wrote:
> [...]
> > 
> > Thanks for this - I tried it, and we spot the optimization once we update
> > die__create_new_parameter() as follows:
> > 
> > diff --git a/dwarf_loader.c b/dwarf_loader.c
> > index f96b6ff..605ad45 100644
> > --- a/dwarf_loader.c
> > +++ b/dwarf_loader.c
> > @@ -1529,6 +1530,8 @@ static struct tag *die__create_new_parameter(Dwarf_Die *di
> >  
> >         if (ftype != NULL) {
> >                 ftype__add_parameter(ftype, parm);
> > +               if (parm->optimized)
> > +                       ftype->optimized_parms = 1;
> >                 if (param_idx >= 0) {
> >                         if (add_child_llvm_annotations(die, param_idx, conf, &(t
> >                                 return NULL;
> > 
> 
> Great, looks good.
> 
> > With that change, I see:
> > 
> > $ 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
> > added local function 'f', optimized-out params
> > skipping addition of 'f' due to optimized-out parameters
> 
> Sorry, I have one more silly program.
> 
> I talked to Yonghong today and we discussed if compiler can change a
> type of a function parameter as a result of some optimization.
> Consider the following example:
> 
>     $ cat test.c
>     struct st {
>       int a;
>       int b;
>     };
>     
>     __attribute__((noinline))
>     static int f(struct st *s) {
>       return s->a + s->b;
>     }
>     
>     int main(int argc, char *argv[]) {
>       struct st s = {
>         .a = (long)argv[0],
>         .b = (long)argv[1]
>       };
>       return f(&s);
>     }
> 
> When compiled by `clang` with -O3 the prototype of `f` is changed from
> `int f(struct *st)` to `int f(int, int)`:
> 
>     $ clang -O3 -g -c test.c -o test.o && llvm-objdump -d test.o
>     ...
>     0000000000000000 <main>:
>            0: 8b 3e                        	movl	(%rsi), %edi
>            2: 8b 76 08                     	movl	0x8(%rsi), %esi
>            5: eb 09                        	jmp	0x10 <f>
>            7: 66 0f 1f 84 00 00 00 00 00   	nopw	(%rax,%rax)
>     
>     0000000000000010 <f>:
>           10: 8d 04 37                     	leal	(%rdi,%rsi), %eax
>           13: c3                           	retq
>     
> But generated DWARF hides this information:

Actually, I'm not correct. The information is present because
`DW_AT_location` attribute is not present (just as 4.1.4 says).
So I think that the condition for optimized parameters detection has
to be adjusted one more time:

			has_location = attr_location(die, &loc.expr, &loc.exprlen) == 0;
			has_const_value = dwarf_attr(die, DW_AT_const_value, &attr) != NULL;

			if (has_location && 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 (!has_location || has_const_value) {
				parm->optimized = true;
			}

(But again, the parameter is marked as optimized but the function is
 not skipped in the final BTF, so either I applied our last change
 incorrectly or something additional should be done).
 
wdyt?

>     $ llvm-dwarfdump test.o
>     ...
>     0x0000005c:   DW_TAG_subprogram
>                     DW_AT_low_pc	(0x0000000000000010)
>                     DW_AT_high_pc	(0x0000000000000014)
>                     DW_AT_frame_base	(DW_OP_reg7 RSP)
>                     DW_AT_call_all_calls	(true)
>                     DW_AT_name	("f")
>                     DW_AT_decl_file	("/home/eddy/work/tmp/test.c")
>                     DW_AT_decl_line	(7)
>                     DW_AT_prototyped	(true)
>                     DW_AT_type	(0x00000074 "int")
>     
>     0x0000006b:     DW_TAG_formal_parameter
>                       DW_AT_name	("s")
>                       DW_AT_decl_file	("/home/eddy/work/tmp/test.c")
>                       DW_AT_decl_line	(7)
>                       DW_AT_type	(0x0000009e "st *")
>     
>     0x00000073:     NULL
>     ...
> 
> Is this important?
> (gcc does not do this for the particular example, but I don't know if
>  it could be tricked to under some conditions).
> 
> Thanks,
> Eduard
> 
> [...]