[llvm] bb8e023 - llvm-reduce: improve basic-blocks removal pass

[Markus Lavin via llvm-commits]

Author: Markus Lavin
Date: 2022-05-24T09:51:25+02:00
New Revision: bb8e02325ffc96d4a3a21aa9eefabb8bd205b214

URL: https://github.com/llvm/llvm-project/commit/bb8e02325ffc96d4a3a21aa9eefabb8bd205b214
DIFF: https://github.com/llvm/llvm-project/commit/bb8e02325ffc96d4a3a21aa9eefabb8bd205b214.diff

LOG: llvm-reduce: improve basic-blocks removal pass

When the single branch target of a block has been removed try updating
it to target a  block that is kept (by scanning forward in the sequence)
instead of replacing the branch with a return instruction. Doing so
reduces the risk of breaking loop structures meaning that when the loop
is 'interesting' these reductions should have more blocks eliminated.

Differential Revision: https://reviews.llvm.org/D125766

Added: 
    llvm/test/tools/llvm-reduce/remove-bbs-sequence.ll
    llvm/test/tools/llvm-reduce/remove-bbs-sequence.py

Modified: 
    llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/test/tools/llvm-reduce/remove-bbs-sequence.ll b/llvm/test/tools/llvm-reduce/remove-bbs-sequence.ll
new file mode 100644
index 0000000000000..0b2fdb5bd8bde
--- /dev/null
+++ b/llvm/test/tools/llvm-reduce/remove-bbs-sequence.ll
@@ -0,0 +1,30 @@
+; RUN: llvm-reduce --delta-passes=basic-blocks --test %python --test-arg %p/remove-bbs-sequence.py %s -o %t
+; RUN: cat %t | FileCheck %s
+
+; The interestingness test is that the CFG contains a loop. Verify that the
+; unnecessary bb2 and bb3 are removed while still maintaining a loop.
+
+define void @main() {
+  bb0:
+    br label %bb1
+  bb1:
+    br label %bb2
+  bb2:
+    br label %bb3
+  bb3:
+    %phi = phi i32 [ undef, %bb2 ]
+    br label %bb4
+  bb4:
+    br label %bb1
+}
+
+; CHECK:define void @main() {
+; CHECK-NEXT: bb0:
+; CHECK-NEXT:   br label %bb1
+; CHECK-EMPTY:
+; CHECK-NEXT: bb1:
+; CHECK-NEXT:   br label %bb4
+; CHECK-EMPTY:
+; CHECK-NEXT: bb4:
+; CHECK-NEXT:   br label %bb1
+; CHECK-NEXT:}

diff  --git a/llvm/test/tools/llvm-reduce/remove-bbs-sequence.py b/llvm/test/tools/llvm-reduce/remove-bbs-sequence.py
new file mode 100755
index 0000000000000..f38d501d13700
--- /dev/null
+++ b/llvm/test/tools/llvm-reduce/remove-bbs-sequence.py
@@ -0,0 +1,15 @@
+import subprocess
+import sys
+
+opt = subprocess.run( [ 'opt', '-passes=print<loops>','-disable-output', sys.argv[1]], stdout=subprocess.PIPE, stderr=subprocess.PIPE )
+
+stdout = opt.stdout.decode()
+
+pattern = 'Loop at depth 1 containing'
+
+if (pattern in opt.stderr.decode()):
+  print('This is interesting!')
+  sys.exit(0)
+else:
+  print('This is NOT interesting!')
+  sys.exit(1)

diff  --git a/llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp b/llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp
index a9b9db2d09ee7..2b319efb3868a 100644
--- a/llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp
+++ b/llvm/tools/llvm-reduce/deltas/ReduceBasicBlocks.cpp
@@ -47,6 +47,19 @@ static void replaceBranchTerminator(BasicBlock &BB,
   Term->eraseFromParent();
 
   if (ChunkSuccessors.empty()) {
+    // Scan forward in BB list to try find a block that is kept.
+    Function &F = *BB.getParent();
+    Function::iterator FI = BB.getIterator();
+    FI++;
+    while (FI != F.end()) {
+      auto &FIB = *FI;
+      if (BBsToKeep.count(&FIB) && !isa<PHINode>(FIB.begin())) {
+        BranchInst::Create(&FIB, &BB);
+        return;
+      }
+      FI++;
+    }
+    // If that fails then resort to replacing with a ret.
     auto *FnRetTy = BB.getParent()->getReturnType();
     ReturnInst::Create(BB.getContext(),
                        FnRetTy->isVoidTy() ? nullptr : UndefValue::get(FnRetTy),