[all-commits] [llvm/llvm-project] 4997af: [SimplifyCFG] Simplify nested branches (#97067)

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 4997af98a008e71a3df61707559710d1c2769839
      https://github.com/llvm/llvm-project/commit/4997af98a008e71a3df61707559710d1c2769839
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-07-01 (Mon, 01 Jul 2024)

  Changed paths:
    M llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    M llvm/test/CodeGen/ARM/and-cmp0-sink.ll
    A llvm/test/Transforms/SimplifyCFG/branch-nested.ll

  Log Message:
  -----------
  [SimplifyCFG] Simplify nested branches (#97067)

This patch folds the following pattern (I don't know what to call this):
```
bb0:
  br i1 %cond1, label %bb1, label %bb2
bb1:
  br i1 %cond2, label %bb3, label %bb4
bb2:
  br i1 %cond2, label %bb4, label %bb3
bb3:
  ...
bb4:
  ...
```
into
```
bb0:
  %cond = xor i1 %cond1, %cond2
  br i1 %cond, label %bb4, label %bb3
bb3:
  ...
bb4:
  ...
```

Alive2: https://alive2.llvm.org/ce/z/5iOJEL
Closes https://github.com/llvm/llvm-project/issues/97022.
Closes https://github.com/llvm/llvm-project/issues/83417.

I found this pattern in some verilator-generated code, which is widely
used in RTL simulation. This fold will reduces branches and improves the
performance of CPU frontend. To my surprise, this pattern is also common
in C/C++ code base.
Affected libraries/applications:
cmake/cvc5/freetype/git/gromacs/jq/linux/openblas/openmpi/openssl/php/postgres/ruby/sqlite/wireshark/z3/...



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