[PATCH 1/3] btf: add 'extern' linkage for variables [PR106773]
Indu Bhagat
indu.bhagat@oracle.com
Fri Dec 9 06:55:14 GMT 2022
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 ?
> /* 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
> 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,
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