| Issue |
164792
|
| Summary |
bpf __builtin_memcpy from large aligned global spills unnecessarily
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
dramforever
|
```c
int meow[512 / sizeof(int)];
void cpy(char *dest)
{
__builtin_memcpy(dest, meow, 512);
}
```
https://godbolt.org/z/9czYacMfT
Compiles with LLVM 19, Fails with this error on >= 20:
```
<source>:3:6: error: Looks like the BPF stack limit is exceeded. Please move large on stack variables into BPF per-cpu array map. For non-kernel uses, the stack can be increased using -mllvm -bpf-stack-size.
3 | void cpy(char *dest)
| ^
<source>:3:6: note: could not determine the original source location for example.c:0:0
<source>:3:6: note: could not determine the original source location for example.c:0:0
```
Reducing the memcpy size to 40 or similar reveals that the code now spills to stack, for seemingly no reason:
```
cpy:
r3 = meow ll
[...]
w2 = *(u32 *)(r3 + 8)
*(u64 *)(r10 - 24) = r2
*(u8 *)(r1 + 8) = w2
w2 = *(u32 *)(r3 + 4)
*(u64 *)(r10 - 16) = r2
*(u8 *)(r1 + 4) = w2
w3 = *(u32 *)(r3 + 0)
*(u64 *)(r10 - 8) = r3
*(u8 *)(r1 + 0) = w3
[...]
```
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