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btf: change encoding of forward-declared enums [PR111735]

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The BTF specification does not formally define a representation for
forward-declared enum types such as:

  enum Foo;

Forward-declarations for struct and union types are represented by
BTF_KIND_FWD, which has a 1-bit flag distinguishing the two.

The de-facto standard format used by other tools like clang and pahole
is to represent forward-declared enums as BTF_KIND_ENUM with vlen=0,
i.e. as a regular enum type with no enumerators.  This patch changes
GCC to adopt that format, and makes a couple of minor cleanups in
btf_asm_type ().

gcc/

	PR debug/111735
	* btfout.cc (btf_fwd_to_enum_p): New.
	(btf_asm_type_ref): Special case references to enum forwards.
	(btf_asm_type): Special case enum forwards. Rename btf_size_type to
	btf_size, and change chained ifs switching on btf_kind into else ifs.

gcc/testsuite/

	PR debug/111735
	* gcc.dg/debug/btf/btf-forward-2.c: New test.
This commit is contained in:
David Faust 2023-12-12 13:55:59 -08:00
parent 0a37463758
commit 1502d724df
2 changed files with 53 additions and 11 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

46
gcc/btfout.cc
View file

@ -268,6 +268,17 @@ btf_emit_id_p (ctf_id_t id)
&& (btf_id_map[id] <= BTF_MAX_TYPE));
}
/* Return true if DTD is a forward-declared enum. The BTF representation
of forward declared enums is not formally defined. */
static bool
btf_fwd_to_enum_p (ctf_dtdef_ref dtd)
{
uint32_t btf_kind = get_btf_kind (CTF_V2_INFO_KIND (dtd->dtd_data.ctti_info));
return (btf_kind == BTF_KIND_FWD && dtd->dtd_data.ctti_type == CTF_K_ENUM);
}
/* Each BTF type can be followed additional, variable-length information
completing the description of the type. Calculate the number of bytes
of variable information required to encode a given type. */
@ -753,8 +764,12 @@ btf_asm_type_ref (const char *prefix, ctf_container_ref ctfc, ctf_id_t ref_id)
uint32_t ref_kind
= get_btf_kind (CTF_V2_INFO_KIND (ref_type->dtd_data.ctti_info));
const char *kind_name = btf_fwd_to_enum_p (ref_type)
? btf_kind_name (BTF_KIND_ENUM)
: btf_kind_name (ref_kind);
dw2_asm_output_data (4, ref_id, "%s: (BTF_KIND_%s '%s')",
prefix, btf_kind_name (ref_kind),
prefix, kind_name,
get_btf_type_name (ref_type));
}
}
@ -765,11 +780,11 @@ btf_asm_type_ref (const char *prefix, ctf_container_ref ctfc, ctf_id_t ref_id)
static void
btf_asm_type (ctf_container_ref ctfc, ctf_dtdef_ref dtd)
{
uint32_t btf_kind, btf_kflag, btf_vlen, btf_size_type;
uint32_t btf_kind, btf_kflag, btf_vlen, btf_size;
uint32_t ctf_info = dtd->dtd_data.ctti_info;
btf_kind = get_btf_kind (CTF_V2_INFO_KIND (ctf_info));
btf_size_type = dtd->dtd_data.ctti_type;
btf_size = dtd->dtd_data.ctti_size;
btf_vlen = CTF_V2_INFO_VLEN (ctf_info);
/* By now any unrepresentable types have been removed. */
@ -777,7 +792,7 @@ btf_asm_type (ctf_container_ref ctfc, ctf_dtdef_ref dtd)
/* Size 0 integers are redundant definitions of void. None should remain
in the types list by this point. */
gcc_assert (btf_kind != BTF_KIND_INT || btf_size_type >= 1);
gcc_assert (btf_kind != BTF_KIND_INT || btf_size >= 1);
/* Re-encode the ctti_info to BTF. */
/* kflag is 1 for structs/unions with a bitfield member.
@ -810,16 +825,26 @@ btf_asm_type (ctf_container_ref ctfc, ctf_dtdef_ref dtd)
structs and forwards to unions. The dwarf2ctf conversion process stores
the kind of the forward in ctti_type, but for BTF this must be 0 for
forwards, with only the KIND_FLAG to distinguish.
At time of writing, BTF forwards to enums are unspecified. */
if (btf_kind == BTF_KIND_FWD)
Forwards to enum types are special-cased below. */
else if (btf_kind == BTF_KIND_FWD)
{
if (dtd->dtd_data.ctti_type == CTF_K_UNION)
btf_kflag = 1;
btf_size_type = 0;
/* PR debug/111735. Encode foward-declared enums as BTF_KIND_ENUM
with vlen=0. A representation for these is not formally defined;
this is the de-facto standard used by other tools like clang
and pahole. */
else if (dtd->dtd_data.ctti_type == CTF_K_ENUM)
{
btf_kind = BTF_KIND_ENUM;
btf_vlen = 0;
}
btf_size = 0;
}
if (btf_kind == BTF_KIND_ENUM)
else if (btf_kind == BTF_KIND_ENUM)
{
btf_kflag = dtd->dtd_enum_unsigned
? BTF_KF_ENUM_UNSIGNED
@ -829,7 +854,7 @@ btf_asm_type (ctf_container_ref ctfc, ctf_dtdef_ref dtd)
}
/* PR debug/112656. BTF_KIND_FUNC_PROTO is always anonymous. */
if (btf_kind == BTF_KIND_FUNC_PROTO)
else if (btf_kind == BTF_KIND_FUNC_PROTO)
dtd->dtd_data.ctti_name = 0;
dw2_asm_output_data (4, dtd->dtd_data.ctti_name,
@ -848,8 +873,7 @@ btf_asm_type (ctf_container_ref ctfc, ctf_dtdef_ref dtd)
case BTF_KIND_ENUM:
case BTF_KIND_DATASEC:
case BTF_KIND_ENUM64:
dw2_asm_output_data (4, dtd->dtd_data.ctti_size, "btt_size: %uB",
dtd->dtd_data.ctti_size);
dw2_asm_output_data (4, btf_size, "btt_size: %uB", btf_size);
return;
case BTF_KIND_ARRAY:
case BTF_KIND_FWD:

18
gcc/testsuite/gcc.dg/debug/btf/btf-forward-2.c Normal file
View file

@ -0,0 +1,18 @@
/* PR111735. Test BTF generation for forward-declared enum.
The BTF representation for forward-declared enums is not formally
defined, but the de-facto representation used by various tools is
to encode them as a BTF_KIND_ENUM type with vlen=0 and size=0. */
/* { dg-do compile } */
/* { dg-options "-O0 -gbtf -dA" } */
enum Foo;
enum Foo *pfoo;
/* Check that there is one BTF_KIND_ENUM with vlen=0, and no BTF_KIND_FWD. */
/* { dg-final { scan-assembler-times "\[\t \]0x6000000\[\t \]+\[^\n\]*btt_info" 1 } } */
/* { dg-final { scan-assembler-times "\[\t \]0x7000000\[\t \]+\[^\n\]*btt_info" 0 } } */
/* Verify the reference to the enum-forward. */
/* { dg-final { scan-assembler-times "btt_type: \\(BTF_KIND_ENUM 'Foo'\\)" 1 } } */