| commit | a0e222f7c7bc7b6de43391fbbdada8d511004b9c | [log] [tgz] |
|---|---|---|
| author | Kunqiu Chen <camsyn@foxmail.com> | Sat Nov 08 20:52:19 2025 +0800 |
| committer | GitHub <noreply@github.com> | Sat Nov 08 20:52:19 2025 +0800 |
| tree | ddc94369659b91efff77a36b4ae4f35b3884231d | |
| parent | ffb5831fceac7dd0af461f0abd0bd66cdc6e77a8 [diff] |
[SimplifyCFG] Simplify uncond br with icmp & select (#165580)
Previously, SimplifyCFG only simplified unconditional branches when they
met a pattern (`swicth` -> `icmp` -> `br` -> `phi`) as follows:
```LLVM
switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ]
DEFAULT:
%tmp = icmp eq i8 %A, 92
br label %end
end:
... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ]
```
This PR supports a new and more generic pattern (`switch` -> `icmp` ->
`select` -> `br` -> `phi` ) to simplify unconditional branches as
follows:
```LLVM
; BEFORE
case1:
switch i32 %x, label %DEFAULT [
i32 0, label %end
i32 1, label %case2
]
case2:
br label %end
DEFAULT:
%tmp = icmp eq i32 %x, 2
%val = select i1 %tmp, i32 V3, i32 V4
br label %end
end:
... = phi i32 [ V1, %case1 ], [ V2, %case2 ], [ %val, %DEFAULT ]
```
We prefer to split the edge to 'end' so that there are TWO entries of
V3/V4 to the PHI, merging the icmp & select into the switch, as follows:
```LLVM
; AFTER
case1:
switch i32 %x, label %DEFAULT [
i32 0, label %end
i32 1, label %case2
i32 2, label %case3
]
case2:
br label %end
case3:
br label %end
DEFAULT:
br label %end
end:
... = phi i32 [ V1, %case1 ], [ V2, %case2 ], [ V3, %case3 ], [ V4, %DEFAULT]
```
Alive2 Proof: https://alive2.llvm.org/ce/z/jYHM4f
Promising Optimization Impact:
https://github.com/dtcxzyw/llvm-opt-benchmark/pull/3006
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