| Issue |
135639
|
| Summary |
[GreedyRegAlloc] Multiple spill reloads into same register without intermediate def/overwrite
|
| Labels |
llvm:regalloc
|
| Assignees |
|
| Reporter |
JanekvO
|
llvm-project git hash: `c9eebc7af440dc012c94d25351eaba92e6a57910`
command: `llc --mtriple=aarch64 aarch64_reprod.ll`
Reproducible provided for aarch64, but am seeing it for amdgpu as well (I assume it can happen for any target using greedy).
>From the AArch64 reproducible provided (`aarch64_reprod.s`):
```
...
fmov d16, d19
fmul d7, d6, d7
ldr d6, [sp, #40] // 8-byte Folded Reload
fnmul d13, d1, d18
fmul d24, d3, d24
fmul d11, d30, d22
mul w11, w1, w11
fmadd d26, d9, d22, d26
fmul d17, d8, d17
cmp w8, #3
fmadd d31, d6, d18, d15
fmul d27, d27, d28
ldr d6, [sp, #40] // 8-byte Folded Reload
fmadd d9, d0, d18, d14
fmul d8, d4, d23
...
```
Where offset `#40` is reloaded multiple times from the stack into `d6`. Between the 2 reloads, a use exists as `fmadd d31, d6, d18, d15` but no defs for `d6` that would overwrite the initial reload. Looking into it, it seems to be an unfortunate split+spill combination (albeit, the split in the aarch64 example is particularly confusing. The amdgpu case I've been looking at shows a more direct `split->spill->superfluous stack reload` combination.) where `spillAroundUses` will indiscriminately reload for all uses within an interval regardless of whether a previous reload is (or could be) still live. Within the `aarch64_reprod_regalloc.txt` dumps the virtregs in question are `%170` and `%171`.
I'm personally not too familiar with greedy regalloc details so I'm not sure how easy a fix will be but it seems that regalloc changes are (understandably) under a bit more scrutiny so I'm not convinced that it'll be a trivial fix but I'd like to hear from the people more familiar with the greedy regalloc, if possible. Perhaps there is some low hanging fruit I'm overlooking.
[aarch64_reprod_regalloc.txt](https://github.com/user-attachments/files/19739025/aarch64_reprod_regalloc.txt)
[aarch64_reprod.ll.txt](https://github.com/user-attachments/files/19739078/aarch64_reprod.ll.txt)
[aarch64_reprod.s.txt](https://github.com/user-attachments/files/19739079/aarch64_reprod.s.txt)
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