[PATCH] D138027: Clean up CFG on JumpThreading

[Pedro Olsen Ferreira via Phabricator via llvm-commits]

arkangath created this revision.
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In some loop situations, when the loop blocks become unreachable, JumpThreading would leave dead blocks behind. It could also generate self-referencing instructions (on a dead cfg).

This patch cleans up those blocks.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D138027

Files:
  llvm/lib/Transforms/Scalar/JumpThreading.cpp
  llvm/test/Transforms/JumpThreading/removed-use.ll
  llvm/test/Transforms/JumpThreading/self-block.ll
  llvm/test/Transforms/JumpThreading/unreachable-loops.ll


Index: llvm/test/Transforms/JumpThreading/unreachable-loops.ll
===================================================================
--- llvm/test/Transforms/JumpThreading/unreachable-loops.ll
+++ llvm/test/Transforms/JumpThreading/unreachable-loops.ll
@@ -100,10 +100,7 @@
 ; CHECK-LABEL: @PR48362(
 ; CHECK-NEXT:  cleanup.cont1500:
 ; CHECK-NEXT:    unreachable
-; CHECK:       if.end1733:
-; CHECK-NEXT:    [[I82:%.*]] = load i32, i32* undef, align 1
-; CHECK-NEXT:    [[TOBOOL1731_NOT:%.*]] = icmp eq i32 [[I82]], 0
-; CHECK-NEXT:    br label [[IF_END1733:%.*]]
+; CHECK-NOT:       if.end1733:
 ;
 cleanup1491:                                      ; preds = %for.body1140
   switch i32 0, label %cleanup2343.loopexit4 [
Index: llvm/test/Transforms/JumpThreading/self-block.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/JumpThreading/self-block.ll
@@ -0,0 +1,25 @@
+; RUN: opt -S -jump-threading %s -o - | FileCheck %s
+
+; Verifies that a self-referencing BasicBlock gets culled.
+; Note: SimplifyCFG would also clean this up but the "add"
+; instruction would become self-referencing when the phi
+; is simplified.
+define void @test() {
+; CHECK-LABEL: @test
+; CHECK: bb397:
+; CHECK-NEXT: ret void
+; CHECK-NOT: bb243:
+entry:
+  br i1 true, label %bb397, label %bb215
+
+bb215:                                            ; preds = %entry
+  br label %bb243
+
+bb243:                                            ; preds = %bb243, %bb215
+  %_4792_ = phi i32 [ %_3125_, %bb243 ], [ 1, %bb215 ]
+  %_3125_ = add i32 %_4792_, 1
+  br label %bb243
+
+bb397:                                            ; preds = %entry
+  ret void
+}
Index: llvm/test/Transforms/JumpThreading/removed-use.ll
===================================================================
--- llvm/test/Transforms/JumpThreading/removed-use.ll
+++ llvm/test/Transforms/JumpThreading/removed-use.ll
@@ -2,8 +2,7 @@
 ; CHECK-LABEL: @foo
 ; CHECK: bb6:
 ; CHECK-NEXT: ret void
-; CHECK: bb3:
-; CHECK: br label %bb3
+; CHECK-NOT: bb3:
 define void @foo() {
 entry:
   br i1 true, label %bb6, label %bb3
Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -449,6 +449,21 @@
                           << '\n');
         LoopHeaders.erase(&BB);
         LVI->eraseBlock(&BB);
+        // If the removed block is a predecessor of a loop block
+        // (referencing itself), then there might be PHIs that will
+        // be simplifiable.  Preserve those to prevent invalid IR.
+        DeleteDeadBlock(&BB, DTU, true);
+        Changed = true;
+        continue;
+      }
+
+      if (pred_size(&BB) == 1 && *pred_begin(&BB) == &BB) {
+        LoopHeaders.erase(&BB);
+        LVI->eraseBlock(&BB);
+        // Manually remove the terminator, otherwise the block
+        // deletion will see a pending reference to the Value being
+        // destroyed, which will trigger assertions.
+        BB.getTerminator()->eraseFromParent();
         DeleteDeadBlock(&BB, DTU);
         Changed = true;
         continue;


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