[PATCH dwarves v2 8/9] btf_encoder: Handle optimized parameter properly

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

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 aa7cd1c..0cd4124 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) {
-- 
2.47.3