| Issue |
104853
|
| Summary |
[RISC-V] Unlinked obj file branches contain a fake infinite-loop destination.
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
patrick-rivos
|
Using the same input assembly file from:
```c
int x = 0;
int min(int y)
{
while (x != 192) {
x += 1;
}
return y;
}
int main() {
min(1);
}
```
gnu-as assembled program looks normal:
```
map-binutils.o: file format elf64-littleriscv
Disassembly of section .text:
0000000000000000 <min>:
0: 000005b7 lui a1, 0x0
4: 0005a683 lw a3, 0x0(a1)
8: 0c000613 li a2, 0xc0
c: 00c68463 beq a3, a2, 0x14 <.LBB0_2>
10: 00c5a023 sw a2, 0x0(a1)
0000000000000014 <.LBB0_2>:
14: 8082 ret
0000000000000016 <main>:
16: 00000537 lui a0, 0x0
1a: 00052603 lw a2, 0x0(a0)
1e: 0c000593 li a1, 0xc0
22: 00b60463 beq a2, a1, 0x2a <.LBB1_2>
26: 00b52023 sw a1, 0x0(a0)
000000000000002a <.LBB1_2>:
2a: 4501 li a0, 0x0
2c: 8082 ret
2e: 0000 unimp
```
But LLVM's obj file looks like: (each branch is an infinite loop)
```
map.o: file format elf64-littleriscv
Disassembly of section .text:
0000000000000000 <min>:
0: 000005b7 lui a1, 0x0
4: 0005a683 lw a3, 0x0(a1)
8: 0c000613 li a2, 0xc0
c: 00c68063 beq a3, a2, 0xc <min+0xc>
10: 00c5a023 sw a2, 0x0(a1)
14: 8082 ret
0000000000000016 <main>:
16: 00000537 lui a0, 0x0
1a: 00052603 lw a2, 0x0(a0)
1e: 0c000593 li a1, 0xc0
22: 00b60063 beq a2, a1, 0x22 <main+0xc>
26: 00b52023 sw a1, 0x0(a0)
2a: 4501 li a0, 0x0
2c: 8082 ret
```
gnu-objdump and llvm-objdump agree on this output so it's probably not on the objdump side of things.
When actually linked the offsets are updated to the correct addresses (with both lld/ld):
```
000000000001129e <min>:
1129e: 000155b7 lui a1, 0x15
112a2: 2f05a683 lw a3, 0x2f0(a1)
112a6: 0c000613 li a2, 0xc0
112aa: 00c68463 beq a3, a2, 0x112b2 <min+0x14>
112ae: 2ec5a823 sw a2, 0x2f0(a1)
112b2: 8082 ret
```
I think the actual dests in the LLVM obj file are encoded using R_RISCV_BRANCH.
This behavior makes inspecting unlinked object files unintuitive at first glance.
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