[PATCH dwarves v6 4/5] btf_encoder: Emit true function signatures

[Yonghong Song <yonghong.song@linux.dev>]

When true_signature is enabled, consume the per-parameter analysis the
DWARF loader recorded while saving a function's BTF:
 - Drop parameters that were optimized out of a signature-changed function
   (ftype->signature_changed && param->optimized), adjusting the saved
   parameter count accordingly.
 - When a parameter was reduced to a single aggregate member
   (param->true_sig_member_name is set), emit it under the synthesized name
   "<parameter>__<member>" so the BTF reflects the value actually passed in
   the register.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 btf_encoder.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 633bc61..c8e8437 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -1297,14 +1297,36 @@ static int32_t btf_encoder__save_func(struct btf_encoder *encoder, struct functi
 	state->reordered_parm = ftype->reordered_parm;
 	ftype__for_each_parameter(ftype, param) {
 		const char *name;
+		char *final_name = NULL;
 
 		/* No location info/optimized + reordered means optimized out. */
 		if (ftype->reordered_parm && (!param->has_loc || param->optimized)) {
 			state->nr_parms--;
 			continue;
 		}
-		name = parameter__name(param) ?: "";
+		if (encoder->true_signature && ftype->signature_changed && param->optimized) {
+			state->nr_parms--;
+			continue;
+		}
+
+		name = parameter__name(param);
+		if (!name) {
+			name = "";
+		} else if (param->true_sig_member_name) {
+			/* Non-null param->true_sig_member_name indicates that the parameter
+			 * name is <parameter_name>__<field_name>.
+			 */
+			if (asprintf(&final_name, "%s__%s", name, param->true_sig_member_name) == -1) {
+				err = -ENOMEM;
+				goto out;
+			}
+			name = final_name;
+		}
+
 		str_off = btf__add_str(btf, name);
+		if (final_name)
+			free(final_name);
+
 		if (str_off < 0) {
 			err = str_off;
 			goto out;
-- 
2.53.0-Meta