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Author SHA1 Message Date
XChy
fc6bdb8549
[SimplifyCFG] Reland transform for redirecting phis between unmergeable BB and SuccBB (#68473) 
Reland #67275 with #68953 resolved.
 2023-10-28 17:10:20 +08:00
Muhammad Omair Javaid
431969ede1 Revert "[SimplifyCFG] Transform for redirecting phis between unmergeable BB and SuccBB (#67275)" 
This reverts commit fc86d031fec5e47c6811efd3a871742ad244afdd.

This change breaks LLVM buildbot clang-aarch64-sve-vls-2stage
https://lab.llvm.org/buildbot/#/builders/176/builds/5474
I am going to revert this patch as the bot has been failing for more than a day without a fix.
 2023-09-26 15:47:16 +05:00
XChy
fc86d031fe
[SimplifyCFG] Transform for redirecting phis between unmergeable BB and SuccBB (#67275) 
This patch extends function TryToSimplifyUncondBranchFromEmptyBlock to
handle the similar cases below.

```llvm
define i8 @src(i8 noundef %arg) {
start:
  switch i8 %arg, label %unreachable [
    i8 0, label %case012
    i8 1, label %case1
    i8 2, label %case2
    i8 3, label %end
  ]

unreachable:
  unreachable

case1:
  br label %case012

case2:
  br label %case012

case012:
  %phi1 = phi i8 [ 3, %case2 ], [ 2, %case1 ], [ 1, %start ]
  br label %end

end:
  %phi2 = phi i8 [ %phi1, %case012 ], [ 4, %start ]
  ret i8 %phi2
}
```
The phis here should be merged into one phi, so that we can better
optimize it:

```llvm
define i8 @tgt(i8 noundef %arg) {
start:
  switch i8 %arg, label %unreachable [
    i8 0, label %end
    i8 1, label %case1
    i8 2, label %case2
    i8 3, label %case3
  ]

unreachable:
  unreachable

case1:
  br label %end

case2:
  br label %end

case3:
  br label %end

end:
  %phi = phi i8 [ 4, %case3 ], [ 3, %case2 ], [ 2, %case1 ], [ 1, %start ]
  ret i8 %phi
}
```
Proof:
[normal](https://alive2.llvm.org/ce/z/vAWi88)
[multiple stages](https://alive2.llvm.org/ce/z/DDBQqp)
[multiple stages 2](https://alive2.llvm.org/ce/z/nGkeqN)
[multiple phi combinations](https://alive2.llvm.org/ce/z/VQeEdp)

And lookup table optimization should convert it into add %arg 1.
This patch just match similar CFG structure and merge the phis in
different cases.

Maybe such transform can be applied to other situations besides switch,
but I'm not sure whether it's better than not merging. Therefore, I only
try it in switch,

Related issue:
#63876

[Migrated](https://reviews.llvm.org/D155940)
 2023-09-25 10:13:45 +08:00
Matt Arsenault
07e34d2de5 JumpThreading: Convert tests to -passes 2022-11-27 11:19:28 -05:00
Matt Arsenault
83d5052768 JumpThreading: Convert tests to opaque pointers 
phi-known.ll:test2 required deleting one manual check for a bitcast

Also strip trailing whitespace while we're touching everything.
 2022-11-27 11:19:28 -05:00
Nikita Popov
6f0ca6fd23 [JumpThreading] Insert freeze when unfolding select 
JumpThreading may convert selects into branch instructions,
in which case the condition needs to be frozen (as branch on
poison is immediate undefined behavior, unlike select on poison).

The necessary code for this is already in place, this just enables
the option.

Differential Revision: https://reviews.llvm.org/D125869
 2022-05-21 11:24:27 +02:00
Sam Parker
2663a25fad [JumpThreading] Half the duplicate threshold at Oz 
Duplicating instructions can lead to code size increases but using
a threshold of 3 is good for reducing code size.

Differential Revision: https://reviews.llvm.org/D72916
 2020-02-03 08:40:20 +00:00