On 12/8/22 22:55, Indu Bhagat wrote:
> Hi David,
>
> On 12/7/22 12:57, David Faust wrote:
>> Add support for the 'extern' linkage value for BTF_KIND_VAR records,
>> which is used for variables declared as extern in the source file.
>>
>> PR target/106773
>>
>> gcc/
>>
>> * btfout.cc (BTF_LINKAGE_STATIC): New define.
>> (BTF_LINKAGE_GLOBAL): Likewise.
>> (BTF_LINKAGE_EXTERN): Likewise.
>> (btf_collect_datasec): Mark extern variables as such.
>> (btf_asm_varent): Accomodate 'extern' linkage.
>>
>> gcc/testsuite/
>>
>> * gcc.dg/debug/btf/btf-variables-4.c: New test.
>>
>> include/
>>
>> * btf.h (struct btf_var): Update comment to note 'extern' linkage.
>> ---
>> gcc/btfout.cc | 9 ++++++-
>> .../gcc.dg/debug/btf/btf-variables-4.c | 24 +++++++++++++++++++
>> include/btf.h | 2 +-
>> 3 files changed, 33 insertions(+), 2 deletions(-)
>> create mode 100644 gcc/testsuite/gcc.dg/debug/btf/btf-variables-4.c
>>
>> diff --git a/gcc/btfout.cc b/gcc/btfout.cc
>> index aef9fd70a28..a1c6266a7db 100644
>> --- a/gcc/btfout.cc
>> +++ b/gcc/btfout.cc
>> @@ -66,6 +66,10 @@ static char btf_info_section_label[MAX_BTF_LABEL_BYTES];
>>
>> #define BTF_INVALID_TYPEID 0xFFFFFFFF
>>
>> +#define BTF_LINKAGE_STATIC 0
>> +#define BTF_LINKAGE_GLOBAL 1
>> +#define BTF_LINKAGE_EXTERN 2
>> +
>
> I was about to suggest to rename these to use the same name as used in
> the kernel btf.h. What is used there is:
> BTF_VAR_STATIC = 0,
> BTF_VAR_GLOBAL_ALLOCATED = 1,
> BTF_VAR_GLOBAL_EXTERN = 2,
>
> But after looking at the Patch 3/3, I see you reuse these definitions
> for functions as well. I just find the names confusing on the first look
> - "BTF_LINKAGE_STATIC".
>
> Naming aside, what do you think about adding the defines to
> include/btf.h instead ?
Actually, I forgot these are defined (separately for both VARs and FUNCs)
in the kernel uapi/linux/btf.h. It would probably be best to mirror that
approach and use a separate enum for each, in include/btf.h. WDYT?
>
>> /* Mapping of CTF variables to the IDs they will be assigned when they are
>> converted to BTF_KIND_VAR type records. Strictly accounts for the index
>> from the start of the variable type entries, does not include the number
>> @@ -314,6 +318,9 @@ btf_collect_datasec (ctf_container_ref ctfc)
>> continue;
>>
>> const char *section_name = node->get_section ();
>> + /* Mark extern variables. */
>> + if (DECL_EXTERNAL (node->decl))
>> + dvd->dvd_visibility = BTF_LINKAGE_EXTERN;
>>
>
> This made me think about the following case.
>
> extern const char a[];
> const char a[] = "foo";
>
> What is the expected BTF for this? Since BTF can differentiate between
> the non-defining extern variable declaration, I expected to see two
> variables with different "linkage". At this time I see, two variables
> with global linkage but different types:
>
> .long 0xe000000 # btv_info
> .long 0x4 # btv_type
> .long 0x1 # btv_linkage
> .long 0x1f # btv_name
> .long 0xe000000 # btv_info
> .long 0x7 # btv_type
> .long 0x1 # btv_linkage
> .long 0x60 # btt_name
>
The BTF documentation in the kernel does not clarify this case.
Going off the implementation in clang as a reference, it looks like
only one VAR record is expected, with 'global' linkage:
$ cat extdef.c
extern const char a[];
const char a[] = "foo";
$ clang -target bpf -c -g extdef.c -o extdef.o
$ /usr/sbin/bpftool btf dump file extdef.o
[1] CONST '(anon)' type_id=2
[2] INT 'char' size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
[3] ARRAY '(anon)' type_id=1 index_type_id=4 nr_elems=4
[4] INT '__ARRAY_SIZE_TYPE__' size=4 bits_offset=0 nr_bits=32 encoding=(none)
[5] VAR 'a' type_id=3, linkage=global
[6] DATASEC '.rodata' size=0 vlen=1
type_id=5 offset=0 size=4 (VAR 'a')
In GCC we have two records since we have two DIEs for "a" in the
DWARF. One has type "const char[4]" and the other has type
"const char[]", so the BTF records point to two different types
as well.
I guess we should find a way in BTF to identify this and
emit only the defining definition as clang does.
>> if (section_name == NULL)
>> {
>> @@ -676,7 +683,7 @@ btf_asm_varent (ctf_dvdef_ref var)
>> dw2_asm_output_data (4, var->dvd_name_offset, "btv_name");
>> dw2_asm_output_data (4, BTF_TYPE_INFO (BTF_KIND_VAR, 0, 0), "btv_info");
>> dw2_asm_output_data (4, get_btf_id (var->dvd_type), "btv_type");
>> - dw2_asm_output_data (4, (var->dvd_visibility ? 1 : 0), "btv_linkage");
>> + dw2_asm_output_data (4, var->dvd_visibility, "btv_linkage");
>> }
>>
>> /* Asm'out a member description following a BTF_KIND_STRUCT or
>> diff --git a/gcc/testsuite/gcc.dg/debug/btf/btf-variables-4.c
>> b/gcc/testsuite/gcc.dg/debug/btf/btf-variables-4.c
>> new file mode 100644
>> index 00000000000..d77600bae1c
>> --- /dev/null
>> +++ b/gcc/testsuite/gcc.dg/debug/btf/btf-variables-4.c
>> @@ -0,0 +1,24 @@
>> +/* Test BTF generation for extern variables. */
>> +
>> +/* { dg-do compile } */
>> +/* { dg-options "-O0 -gbtf -dA" } */
>> +
>> +/* Expect 4 variables. */
>> +/* { dg-final { scan-assembler-times "\[\t \]0xe000000\[\t
>> \]+\[^\n\]*btv_info" 4 } } */
>> +
>> +/* 2 extern, 1 global, 1 static. */
>> +/* { dg-final { scan-assembler-times "\[\t \]0\[\t \]+\[^\n\]*btv_linkage"
>> 1 } } */
>> +/* { dg-final { scan-assembler-times "\[\t \]0x1\[\t
>> \]+\[^\n\]*btv_linkage" 1 } } */
>> +/* { dg-final { scan-assembler-times "\[\t \]0x2\[\t
>> \]+\[^\n\]*btv_linkage" 2 } } */
>> +
>> +extern int a;
>> +extern const int b;
>> +int c;
>> +static const int d = 5;
>> +
>> +int foo (int x)
>> +{
>> + c = a + b + x;
>> +
>> + return c + d;
>> +}
>> diff --git a/include/btf.h b/include/btf.h
>> index eba67f9d599..9a757ce5bc9 100644
>> --- a/include/btf.h
>> +++ b/include/btf.h
>> @@ -182,7 +182,7 @@ struct btf_param
>> information about the variable. */
>> struct btf_var
>> {
>> - uint32_t linkage; /* Currently only 0=static or 1=global. */
>> + uint32_t linkage; /* 0=static, 1=global, 2=extern. */
>> };
>>
>> /* BTF_KIND_DATASEC is followed by VLEN struct btf_var_secinfo entries,
>
