[BasicBlockUtils] Remove broken support for eh pads in SplitEdge()

[nikic]

d81d9e8 changed SplitEdge() to make use of ehAwareSplitEdge() for critical edges where the target is an eh pad. However, the implementation is incorrect at least for landing pads. What is currently produced for the code in the modified unit test is something like this:

continue:
  invoke void @sink()
          to label %normal unwind label %new_bb

new_bb:
  %cp = cleanuppad within %exception []
  cleanupret from %cp unwind label %exception

exception:
  %cleanup = landingpad i8 cleanup
  br label %trivial-eh-handler

This mixes different exception handling mechanisms in a nonsensical way, and is not well-formed IR. To actually "split" the landingpad edge (for a rather loose definition of "split"), I think we'd have to generate something along these lines:

  exception.split:
    %cleanup.split = landingpad i8 cleanup
    br label %exception.cont

  exception:
    %cleanup.orig = landingpad i8 cleanup
    br label %exception.cont
  
  exception.cont:
    %cleanup = phi i8 [ %cleanup.split, %exception_split ], [ %cleanup.orig, %exception ]

I didn't bother actually implementing that, seeing as how nobody noticed the existing codegen being broken in the last four years, so clearly nobody actually needs this function to work with EH edges. Just return nullptr instead.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

d81d9e8 changed SplitEdge() to make use of ehAwareSplitEdge() for critical edges where the target is an eh pad. However, the implementation is incorrect at least for landing pads. What is currently produced for the code in the modified unit test is something like this:

continue:
  invoke void @<!-- -->sink()
          to label %normal unwind label %new_bb

new_bb:
  %cp = cleanuppad within %exception []
  cleanupret from %cp unwind label %exception

exception:
  %cleanup = landingpad i8 cleanup
  br label %trivial-eh-handler

This mixes different exception handling mechanisms in a nonsensical way, and is not well-formed IR. To actually "split" the landingpad edge (for a rather loose definition of "split"), I think we'd have to generate something along these lines:

  exception.split:
    %cleanup.split = landingpad i8 cleanup
    br label %exception.cont

  exception:
    %cleanup.orig = landingpad i8 cleanup
    br label %exception.cont
  
  exception.cont:
    %cleanup = phi i8 [ %cleanup.split, %exception_split ], [ %cleanup.orig, %exception ]

I didn't bother actually implementing that, seeing as how nobody noticed the existing codegen being broken in the last four years, so clearly nobody actually needs this function to work with EH edges. Just return nullptr instead.

---

Full diff: https://github.com/llvm/llvm-project/pull/137816.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Utils/BasicBlockUtils.cpp (-5)
• (modified) llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp (+2-16)

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 429037ab9da47..dce10c0ecd56c 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -768,11 +768,6 @@ BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, DominatorTree *DT,
       CriticalEdgeSplittingOptions(DT, LI, MSSAU).setPreserveLCSSA();
 
   if ((isCriticalEdge(LatchTerm, SuccNum, Options.MergeIdenticalEdges))) {
-    // If it is a critical edge, and the succesor is an exception block, handle
-    // the split edge logic in this specific function
-    if (Succ->isEHPad())
-      return ehAwareSplitEdge(BB, Succ, nullptr, nullptr, Options, BBName);
-
     // If this is a critical edge, let SplitKnownCriticalEdge do it.
     return SplitKnownCriticalEdge(LatchTerm, SuccNum, Options, BBName);
   }
diff --git a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
index c9a6a32851775..e58daed887855 100644
--- a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
+++ b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
@@ -285,22 +285,8 @@ declare void @sink_alt() cold
   EXPECT_TRUE(Ehpad);
 
   BasicBlock *NewBB = SplitEdge(SrcBlock, DestBlock, &DT, &LI, &MSSAU, "");
-
-  MSSA.verifyMemorySSA();
-  EXPECT_TRUE(DT.verify());
-  EXPECT_NE(NewBB, nullptr);
-  EXPECT_EQ(NewBB->getSinglePredecessor(), SrcBlock);
-  EXPECT_EQ(NewBB, SrcBlock->getTerminator()->getSuccessor(SuccNum));
-  EXPECT_EQ(NewBB->getParent(), F);
-
-  bool BBFlag = false;
-  for (BasicBlock &BB : *F) {
-    if (BB.getName() == NewBB->getName()) {
-      BBFlag = true;
-      break;
-    }
-  }
-  EXPECT_TRUE(BBFlag);
+  // SplitEdge cannot split an eh pad edge.
+  EXPECT_EQ(NewBB, nullptr);
 }
 
 TEST(BasicBlockUtils, splitBasicBlockBefore_ex1) {

[efriedma-quic]

LGTM