| Issue |
180137
|
| Summary |
Unnecessary register spill chains on x86-64-v4
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
ImpleLee
|
I found this pattern from the machine code compiled for x86-64-v4. I don't know llvm ir, so sorry I can't give a smaller replication code. Godbolt link: https://godbolt.org/z/1TzerzGqc (contains the llvm ir passed to llc and the assembly).
Notice this part of the assembly at the top of `.LBB1_11` and this corresponding part before `.LBB1_15`:
```asm
.LBB1_11: # =>This Loop Header: Depth=1
# ...
vmovdqu64 %ymm23, -96(%rsp) # 32-byte Spill
vmovdqa64 %ymm22, %ymm23
vmovdqa64 %ymm14, %ymm22
vmovdqa64 %ymm28, %ymm11
vmovdqa64 %ymm19, %ymm28
vmovdqa64 %ymm17, %ymm19
vmovdqa64 %ymm31, %ymm17
vmovdqa64 %ymm30, %ymm31
vmovdqa64 %ymm29, %ymm30
vmovdqa64 %ymm27, %ymm29
vmovdqa64 %ymm26, %ymm27
vmovdqa64 %ymm24, %ymm26
vmovdqa64 %ymm25, %ymm24
# ...
.LBB1_13:
# ...
jae .LBB1_13 # the only place that jumps to .LBB1_13
# ...
vmovdqa64 %ymm24, %ymm25
vmovdqa64 %ymm26, %ymm24
vmovdqa64 %ymm27, %ymm26
vmovdqa64 %ymm29, %ymm27
vmovdqa64 %ymm30, %ymm29
vmovdqa64 %ymm31, %ymm30
vmovdqa64 %ymm17, %ymm31
vmovdqa64 %ymm19, %ymm17
vmovdqa64 %ymm28, %ymm19
vmovdqa64 %ymm11, %ymm28
vmovdqa64 %ymm22, %ymm14
vmovdqa64 %ymm23, %ymm22
vmovdqu64 -96(%rsp), %ymm23 # 32-byte Reload
# ...
```
They correspond to (probably) basic blocks 301, 310 and 327 in the LLVM IR, corresponding to the part before `.LBB1_13`, `.LBB1_13` and the part after `.LBB1_13` respectively.
These reload part can only be reached after the spill part.
In the spill part, the compiler has the following register spill chains (each pair of `->` indicates the latter place is used to store the value of the former):
```
14 -> 22 -> 23 -> memory
25 -> 24 -> 26 -> 27 -> 29 -> 30 -> 31 -> 17 -> 19 -> 28 -> 11
```
In the restore part, the compiler has the following register restore chains:
```
11 -> 28 -> 19 -> 17 -> 31 -> 30 -> 29 -> 27 -> 26 -> 24 -> 25
memory -> 23 -> 22 -> 14
```
However, these register spill chains are unnecessary, because their whole purpose is to get two free registers `%ymm14` and `%ymm25`, but they can simply use the free registers `%ymm23` (after saving it to the memory) and `%ymm11` in the first place.
![]()
_______________________________________________
llvm-bugs mailing list
[email protected]
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
