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. The latter can often be
spotted by checking for a DW_AT_const_value attribute,
as noted by Eduard.
In addition, absence of a location tag (either across
the abstract origin reference and the original parameter,
or in the standalone parameter description) is evidence of
an optimized-out parameter. Presence of a location tag
is stored in the parameter description and shared between
abstract tags and their original referents.
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 | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++++----
dwarves.h | 5 ++-
2 files changed, 122 insertions(+), 8 deletions(-)
diff --git a/dwarf_loader.c b/dwarf_loader.c
index 5a74035..93c2307 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -992,13 +992,98 @@ 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) {
+ bool has_const_value;
+ Dwarf_Attribute attr;
+ struct location loc;
+
tag__init(&parm->tag, cu, die);
parm->name = attr_string(die, DW_AT_name, conf);
+
+ if (param_idx >= NR_REGISTER_PARAMS)
+ return parm;
+ /* 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.
+ *
+ * It can also be the case that a parameter DIE has
+ * a constant value attribute reflecting optimization or
+ * has no location attribute.
+ *
+ * From the DWARF spec:
+ *
+ * "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.)"
+ *
+ * So we can also use the absence of a location for a parameter
+ * as evidence it has been optimized out. This info will
+ * need to be shared between a parameter and any abstract
+ * origin references however, since gcc can have location
+ * information in the parameter that refers back to the original
+ * via abstract origin, so we need to share location presence
+ * between these parameter representations. See
+ * ftype__recode_dwarf_types() below for how this is handled.
+ */
+ parm->has_loc = dwarf_attr(die, DW_AT_location, &attr) != NULL;
+ has_const_value = dwarf_attr(die, DW_AT_const_value, &attr) != NULL;
+ if (parm->has_loc &&
+ 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 = 1;
+ break;
+ }
+ } else if (has_const_value) {
+ parm->optimized = 1;
+ }
}
return parm;
@@ -1450,7 +1535,7 @@ 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;
@@ -2194,6 +2279,7 @@ static void ftype__recode_dwarf_types(struct tag *tag, struct cu *cu)
ftype__for_each_parameter(type, pos) {
struct dwarf_tag *dpos = pos->tag.priv;
+ struct parameter *opos;
struct dwarf_tag *dtype;
if (dpos->type.off == 0) {
@@ -2207,8 +2293,18 @@ static void ftype__recode_dwarf_types(struct tag *tag, struct cu *cu)
tag__print_abstract_origin_not_found(&pos->tag);
continue;
}
- pos->name = tag__parameter(dtype->tag)->name;
+ opos = tag__parameter(dtype->tag);
+ pos->name = opos->name;
pos->tag.type = dtype->tag->type;
+ /* share location information between parameter and
+ * abstract origin; if neither have location, we will
+ * mark the parameter as optimized out.
+ */
+ if (pos->has_loc)
+ opos->has_loc = pos->has_loc;
+
+ if (pos->optimized)
+ opos->optimized = pos->optimized;
continue;
}
@@ -2478,18 +2574,33 @@ out:
return 0;
}
-static int cu__resolve_func_ret_types(struct cu *cu)
+static int cu__resolve_func_ret_types_optimized(struct cu *cu)
{
struct ptr_table *pt = &cu->functions_table;
uint32_t i;
for (i = 0; i < pt->nr_entries; ++i) {
struct tag *tag = pt->entries[i];
+ struct parameter *pos;
+ struct function *fn = tag__function(tag);
+
+ /* mark function as optimized if parameter is, or
+ * if parameter does not have a location; at this
+ * point location presence has been marked in
+ * abstract origins for cases where a parameter
+ * location is not stored in the original function
+ * parameter tag.
+ */
+ ftype__for_each_parameter(&fn->proto, pos) {
+ if (pos->optimized || !pos->has_loc) {
+ fn->proto.optimized_parms = 1;
+ break;
+ }
+ }
if (tag == NULL || tag->type != 0)
continue;
- struct function *fn = tag__function(tag);
if (!fn->abstract_origin)
continue;
@@ -2612,7 +2723,7 @@ static int die__process_and_recode(Dwarf_Die *die, struct cu *cu, struct conf_lo
if (ret != 0)
return ret;
- return cu__resolve_func_ret_types(cu);
+ return cu__resolve_func_ret_types_optimized(cu);
}
static int class_member__cache_byte_size(struct tag *tag, struct cu *cu,
@@ -3132,7 +3243,7 @@ static int cus__merge_and_process_cu(struct cus *cus, struct conf_load *conf,
* encoded in another subprogram through abstract_origin
* tag. Let us visit all subprograms again to resolve this.
*/
- if (cu__resolve_func_ret_types(cu) != LSK__KEEPIT)
+ if (cu__resolve_func_ret_types_optimized(cu) != LSK__KEEPIT)
goto out_abort;
if (cus__finalize(cus, cu, conf, NULL) == LSK__STOP_LOADING)
diff --git a/dwarves.h b/dwarves.h
index 589588e..2723466 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -808,6 +808,8 @@ size_t lexblock__fprintf(const struct lexblock *lexblock, const struct cu *cu,
struct parameter {
struct tag tag;
const char *name;
+ uint8_t optimized:1;
+ uint8_t has_loc:1;
};
static inline struct parameter *tag__parameter(const struct tag *tag)
@@ -827,7 +829,8 @@ 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)
--
1.8.3.1
