[DebugInfo][RemoveDIs] Use splice in Outliner rather than moveBefore

[jmorse]

This patch replaces a utility in the outliner that moves the contents of one basic block into another basic block, with a call to splice instead. I think it's NFC, however I'd like a second pair of eyes to look at it just in case.

The reason for doing this is an edge case in the handling of DPValue objects, the replacement for dbg.values. If there's a variable assignment "dangling" at the end of a block (which happens when we delete the terminator), inserting instructions at end() doesn't shift the DPValue up into the block. We could probably fix this; but it's much easier to use splice at the only call site that does this.

Patch adds --try-experimental-debuginfo-iterators to a test to exercise this code path.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Jeremy Morse (jmorse)

Changes

This patch replaces a utility in the outliner that moves the contents of one basic block into another basic block, with a call to splice instead. I think it's NFC, however I'd like a second pair of eyes to look at it just in case.

The reason for doing this is an edge case in the handling of DPValue objects, the replacement for dbg.values. If there's a variable assignment "dangling" at the end of a block (which happens when we delete the terminator), inserting instructions at end() doesn't shift the DPValue up into the block. We could probably fix this; but it's much easier to use splice at the only call site that does this.

Patch adds --try-experimental-debuginfo-iterators to a test to exercise this code path.

---

Full diff: https://github.com/llvm/llvm-project/pull/79124.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/IPO/IROutliner.cpp (+1-2)
• (modified) llvm/test/Transforms/IROutliner/legal-debug.ll (+1)

diff --git a/llvm/lib/Transforms/IPO/IROutliner.cpp b/llvm/lib/Transforms/IPO/IROutliner.cpp
index a6e19df7c5f1ffc..8e6d0e814372d63 100644
--- a/llvm/lib/Transforms/IPO/IROutliner.cpp
+++ b/llvm/lib/Transforms/IPO/IROutliner.cpp
@@ -154,8 +154,7 @@ struct OutlinableGroup {
 /// \param SourceBB - the BasicBlock to pull Instructions from.
 /// \param TargetBB - the BasicBlock to put Instruction into.
 static void moveBBContents(BasicBlock &SourceBB, BasicBlock &TargetBB) {
-  for (Instruction &I : llvm::make_early_inc_range(SourceBB))
-    I.moveBeforePreserving(TargetBB, TargetBB.end());
+  TargetBB.splice(TargetBB.end(), &SourceBB);
 }
 
 /// A function to sort the keys of \p Map, which must be a mapping of constant
diff --git a/llvm/test/Transforms/IROutliner/legal-debug.ll b/llvm/test/Transforms/IROutliner/legal-debug.ll
index b7b472fa20b3e64..be1182b38fa2d82 100644
--- a/llvm/test/Transforms/IROutliner/legal-debug.ll
+++ b/llvm/test/Transforms/IROutliner/legal-debug.ll
@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --include-generated-funcs
 ; RUN: opt -S -passes=verify,iroutliner -ir-outlining-no-cost < %s | FileCheck %s
+; RUN: opt -S -passes=verify,iroutliner -ir-outlining-no-cost < %s --try-experimental-debuginfo-iterators | FileCheck %s
 
 ; This test checks that debug info is recognized as able to be extracted along
 ; with the other instructions, but is not included in the consolidated function.

[OCHyams]

Makes sense and LGTM