llvmbot
commented
3 months ago @llvm/issue-subscribers-clang-codegen
Author: Serhii Pievniev (spevnev)
When compiling a C (I haven't tested other languages) function with more than 6 parameters on x86_64 (I haven't tested other architectures), its call_site will have at most 6 parameters, while the rest, which is stored on the stack (their DW_AT_location is DW_OP_fbreg), can only be seen as formal_parameters in the subprogram DIE.
I believe that this is a bug, since I can't find anything about this limitation in [DWARF5 format](https://dwarfstd.org/doc/DWARF5.pdf).
Minimum example:
```c
__attribute__((noinline)) void test(int a1, int a2, int a3, int a4, int a5, int a6, int a7, int a8) {
printf("%d %d %d %d %d %d %d %d\n", a1, a2, a3, a4, a5, a6, a7, a8);
}
int main(void) {
test(1,2,3,4,5,6,7,8);
return 0;
}
```
```console
$ clang -O2 -g3 example.c
$ llvm-dwarfdump ./a.out
```
DWARF dump of subprogram:
```
0x00000027: DW_TAG_subprogram
DW_AT_low_pc (0x0000000000001140)
DW_AT_high_pc (0x0000000000001176)
DW_AT_frame_base (DW_OP_reg7 RSP)
DW_AT_call_all_calls (true)
DW_AT_name ("test")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_prototyped (true)
DW_AT_external (true)
0x00000032: DW_TAG_formal_parameter
DW_AT_location (indexed (0x0) loclist = 0x0000002c:
[0x0000000000001140, 0x0000000000001164): DW_OP_reg5 RDI
[0x0000000000001164, 0x0000000000001170): DW_OP_reg4 RSI
[0x0000000000001170, 0x0000000000001176): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value)
DW_AT_name ("a1")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
0x0000003b: DW_TAG_formal_parameter
DW_AT_location (indexed (0x1) loclist = 0x0000003f:
[0x0000000000001140, 0x0000000000001150): DW_OP_reg4 RSI
[0x0000000000001150, 0x0000000000001170): DW_OP_reg1 RDX
[0x0000000000001170, 0x0000000000001176): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value)
DW_AT_name ("a2")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
0x00000044: DW_TAG_formal_parameter
DW_AT_location (indexed (0x2) loclist = 0x00000052:
[0x0000000000001140, 0x000000000000114e): DW_OP_reg1 RDX
[0x000000000000114e, 0x0000000000001170): DW_OP_reg2 RCX
[0x0000000000001170, 0x0000000000001176): DW_OP_entry_value(DW_OP_reg1 RDX), DW_OP_stack_value)
DW_AT_name ("a3")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
0x0000004d: DW_TAG_formal_parameter
DW_AT_location (indexed (0x3) loclist = 0x00000065:
[0x0000000000001140, 0x000000000000114c): DW_OP_reg2 RCX
[0x000000000000114c, 0x0000000000001170): DW_OP_reg8 R8
[0x0000000000001170, 0x0000000000001176): DW_OP_entry_value(DW_OP_reg2 RCX), DW_OP_stack_value)
DW_AT_name ("a4")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
0x00000056: DW_TAG_formal_parameter
DW_AT_location (indexed (0x4) loclist = 0x00000078:
[0x0000000000001140, 0x000000000000114a): DW_OP_reg8 R8
[0x000000000000114a, 0x0000000000001170): DW_OP_reg9 R9
[0x0000000000001170, 0x0000000000001176): DW_OP_entry_value(DW_OP_reg8 R8), DW_OP_stack_value)
DW_AT_name ("a5")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
0x0000005f: DW_TAG_formal_parameter
DW_AT_location (indexed (0x5) loclist = 0x0000008b:
[0x0000000000001140, 0x0000000000001147): DW_OP_reg9 R9
[0x0000000000001147, 0x0000000000001170): DW_OP_reg10 R10
[0x0000000000001170, 0x0000000000001176): DW_OP_entry_value(DW_OP_reg9 R9), DW_OP_stack_value)
DW_AT_name ("a6")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
0x00000068: DW_TAG_formal_parameter
DW_AT_location (indexed (0x6) loclist = 0x0000009e:
[0x0000000000001141, 0x000000000000115d): DW_OP_breg7 RSP+16)
DW_AT_name ("a7")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
0x00000071: DW_TAG_formal_parameter
DW_AT_location (indexed (0x7) loclist = 0x000000a5:
[0x0000000000001141, 0x000000000000115d): DW_OP_breg7 RSP+24)
DW_AT_name ("a8")
DW_AT_decl_file ("/app/examples/min.c")
DW_AT_decl_line (3)
DW_AT_type (0x000000b0 "int")
```
Dump of call site:
```
0x0000008a: DW_TAG_call_site
DW_AT_call_origin (0x00000027)
DW_AT_call_return_pc (0x00000000000011aa)
0x00000090: DW_TAG_call_site_parameter
DW_AT_location (DW_OP_reg9 R9)
DW_AT_call_value (DW_OP_lit6)
0x00000095: DW_TAG_call_site_parameter
DW_AT_location (DW_OP_reg8 R8)
DW_AT_call_value (DW_OP_lit5)
0x0000009a: DW_TAG_call_site_parameter
DW_AT_location (DW_OP_reg2 RCX)
DW_AT_call_value (DW_OP_lit4)
0x0000009f: DW_TAG_call_site_parameter
DW_AT_location (DW_OP_reg1 RDX)
DW_AT_call_value (DW_OP_lit3)
0x000000a4: DW_TAG_call_site_parameter
DW_AT_location (DW_OP_reg4 RSI)
DW_AT_call_value (DW_OP_lit2)
0x000000a9: DW_TAG_call_site_parameter
DW_AT_location (DW_OP_reg5 RDI)
DW_AT_call_value (DW_OP_lit1)
```
When compiling a C (I haven't tested other languages) function with more than 6 parameters on x86_64 (I haven't tested other architectures), its call_site will have at most 6 parameters, while the rest, which is stored on the stack (their DW_AT_location is DW_OP_fbreg), can only be seen as formal_parameters in the subprogram DIE.
I believe that this is a bug, since I can't find anything about this limitation in DWARF5 format.
Minimum example:
DWARF dump of subprogram:
Dump of call site: