[CodeMoverUtils] Enhance CodeMoverUtils to sink an entire BB

[CongzheUalberta]

When moving an entire basic block after InsertPoint, currently we check each instruction whether their users are dominated by InsertPoint, however, this can be improved such that even a user is not dominated InsertPoint, as long as it appears as a subsequent instruction in the same BB, it is safe to move.

This patch is similar to commit 751be2a that enhanced hoisting an entire BB, and this patch enhances sinking an entire BB. Please refer to the added functionality in test case llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp that was not supported without this patch.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Congzhe (CongzheUalberta)

Changes

When moving an entire basic block after InsertPoint, currently we check each instruction whether their users are dominated by InsertPoint, however, this can be improved such that even a user is not dominated InsertPoint, as long as it appears as a subsequent instruction in the same BB, it is safe to move.

This patch is similar to commit 751be2a that enhanced hoisting an entire BB, and this patch enhances sinking an entire BB.

---

Full diff: https://github.com/llvm/llvm-project/pull/87857.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Utils/CodeMoverUtils.cpp (+10-2)
• (modified) llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp (+8)

diff --git a/llvm/lib/Transforms/Utils/CodeMoverUtils.cpp b/llvm/lib/Transforms/Utils/CodeMoverUtils.cpp
index 6a2dae5bab68ee..8b9526ceda9a3a 100644
--- a/llvm/lib/Transforms/Utils/CodeMoverUtils.cpp
+++ b/llvm/lib/Transforms/Utils/CodeMoverUtils.cpp
@@ -337,8 +337,16 @@ bool llvm::isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
   if (isReachedBefore(&I, &InsertPoint, &DT, PDT))
     for (const Use &U : I.uses())
       if (auto *UserInst = dyn_cast<Instruction>(U.getUser()))
-        if (UserInst != &InsertPoint && !DT.dominates(&InsertPoint, U))
-          return false;
+        if (UserInst != &InsertPoint && !DT.dominates(&InsertPoint, U)) {
+          // If UserInst is an instruction that appears later in the same BB as
+          // I, then it is okay to move since I will still be available when
+          // UserInst is executed.
+          if (CheckForEntireBlock && I.getParent() == UserInst->getParent() &&
+              DT.dominates(&I, UserInst))
+            continue;
+          if (UserInst != &InsertPoint && !DT.dominates(&InsertPoint, U))
+            return false;
+        }
   if (isReachedBefore(&InsertPoint, &I, &DT, PDT))
     for (const Value *Op : I.operands())
       if (auto *OpInst = dyn_cast<Instruction>(Op)) {
diff --git a/llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp b/llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp
index 8554d1a33cadee..e599450f25e0ee 100644
--- a/llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp
+++ b/llvm/unittests/Transforms/Utils/CodeMoverUtilsTest.cpp
@@ -615,6 +615,7 @@ TEST(CodeMoverUtils, IsSafeToMoveTest3) {
         Instruction *CmpInst = getInstructionByName(F, "cmp");
         BasicBlock *BB0 = getBasicBlockByName(F, "for.body");
         BasicBlock *BB1 = getBasicBlockByName(F, "for.latch");
+        BasicBlock *BB2 = getBasicBlockByName(F, "for.end");
 
         // Can move as the incoming block of %inc for %i (%for.latch) dominated
         // by %cmp.
@@ -625,6 +626,13 @@ TEST(CodeMoverUtils, IsSafeToMoveTest3) {
         // before %add2 although %add does not dominate InsertPoint.
         EXPECT_TRUE(
             isSafeToMoveBefore(*BB1, *BB0->getTerminator(), DT, &PDT, &DI));
+
+        // Can move as the operands of instructions in BB1 either dominate
+        // InsertPoint or appear before that instruction, e.g., %add appears
+        // before %add2 although %add does not dominate InsertPoint.
+        EXPECT_TRUE(
+            isSafeToMoveBefore(*BB1, *BB2->getTerminator(), DT, &PDT, &DI));
+
       });
 }
 

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[CongzheUalberta]

Thank you Whitney for the review! I very much appreciate it.
I've now merged this PR.