[PATCH dwarves v3 8/9] btf_encoder: Handle optimized parameter properly

Yonghong Song
2026-03-20T19:10:06Z

Ensure to skip optimized parameter so btf can generate
proper true signatures.

In the first patch of the patch set, with DW_CC_nocall filtering, 875 functions
have signature changed. With a series of improvement, eventually only 17 functions
remain and unfortunately these functions cannot be converted to true signatures
due to locations. For example,

0x0242f1f7: DW_TAG_subprogram
DW_AT_name ("memblock_find_in_range")
DW_AT_calling_convention (DW_CC_nocall)
DW_AT_type (0x0242decc "phys_addr_t")
...

0x0242f22e: DW_TAG_formal_parameter
DW_AT_location (indexed (0x14a) loclist = 0x005595bc:
[0xffffffff87a000f9, 0xffffffff87a00178): DW_OP_reg5 RDI
[0xffffffff87a00178, 0xffffffff87a001be): DW_OP_reg14 R14
[0xffffffff87a001be, 0xffffffff87a001c7): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_reg14 R14)
DW_AT_name ("start")
DW_AT_type (0x0242decc "phys_addr_t")
...

0x0242f239: DW_TAG_formal_parameter
DW_AT_location (indexed (0x14b) loclist = 0x005595e6:
[0xffffffff87a000f9, 0xffffffff87a00175): DW_OP_reg4 RSI
[0xffffffff87a00175, 0xffffffff87a001b8): DW_OP_reg3 RBX
[0xffffffff87a001b8, 0xffffffff87a001c7): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
[0xffffffff87a001c7, 0xffffffff87a00214): DW_OP_reg3 RBX)
DW_AT_name ("end")
DW_AT_type (0x0242decc "phys_addr_t")
...

0x0242f245: DW_TAG_formal_parameter
DW_AT_location (indexed (0x14c) loclist = 0x00559610:
[0xffffffff87a001e3, 0xffffffff87a001ef): DW_OP_breg4 RSI+0)
DW_AT_name ("size")
DW_AT_type (0x0242decc "phys_addr_t")
...

0x0242f250: DW_TAG_formal_parameter
DW_AT_const_value (4096)
DW_AT_name ("align")
DW_AT_type (0x0242decc "phys_addr_t")
...

The third parameter 'size' is not from RDX. Hence, true signature is not possible for this function.

I also did some experiments on arm64. The number of signature-changed funcitons
is 863 and finally there are 79 functions cannot be converted to true signatures.
Through dwarf comparison of x86_64 vs. arm64, llvm arm64 backend looks like having
more relaxation to compute parameter values for those signature-changed functions.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
btf_encoder.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index d36984a..8e991b1 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -1257,15 +1257,21 @@ static int32_t btf_encoder__save_func(struct btf_encoder *encoder, struct functi
struct btf *btf = encoder->btf;
struct llvm_annotation *annot;
struct parameter *param;
- uint8_t param_idx = 0;
+ uint8_t param_idx = 0, skip_idx = 0;
int str_off, err = 0;

if (!state)
return -ENOMEM;

+ if (encoder->cu->producer_clang) {
+ ftype__for_each_parameter(ftype, param) {
+ if (param->optimized) skip_idx++;
+ }
+ }
+
state->addr = function__addr(fn);
state->elf = func;
- state->nr_parms = ftype->nr_parms + (ftype->unspec_parms ? 1 : 0);
+ state->nr_parms = ftype->nr_parms - skip_idx + (ftype->unspec_parms ? 1 : 0);
state->ret_type_id = ftype->tag.type == 0 ? 0 : encoder->type_id_off + ftype->tag.type;
if (state->nr_parms > 0) {
state->parms = zalloc(state->nr_parms * sizeof(*state->parms));
@@ -1303,6 +1309,9 @@ static int32_t btf_encoder__save_func(struct btf_encoder *encoder, struct functi
state->nr_parms--;
continue;
}
+ if (encoder->cu->producer_clang && param->optimized)
+ continue;
+
name = parameter__name(param) ?: "";
str_off = btf__add_str(btf, name);
if (str_off < 0) {
--
2.52.0