[llvm] [CodeMoverUtils] Enhance CodeMoverUtils to sink an entire BB (PR #87857)

[via llvm-commits]

https://github.com/CongzheUalberta created https://github.com/llvm/llvm-project/pull/87857

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 751be2a064f119af74c7b9b1e52bc904d8aa114d that enhanced hoisting an entire BB, and this patch enhances sinking an entire BB.

>From 73e58d3389bf6431d91316ca2c49fb18dcdfc37d Mon Sep 17 00:00:00 2001
From: Congzhe Cao <c00548033 at china.huawei.com>
Date: Fri, 5 Apr 2024 22:39:09 -0400
Subject: [PATCH]  [CodeMoverUtils] Enhance CodeMoverUtils to sink entire BB

When moving an entire basic block BB after InsertPoint, currently
we check for all instructions 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 751be2a064f119af74c7b9b1e52bc904d8aa114d
that enhanced hoisting an entire BB, and this patch enhances sinking an
entire BB.
---
 llvm/lib/Transforms/Utils/CodeMoverUtils.cpp         | 12 ++++++++++--
 .../Transforms/Utils/CodeMoverUtilsTest.cpp          |  8 ++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

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));
+
       });
 }