[llvm] ddffb74 - [LV] Strip unreachable SCEV-check blocks (#130079)

[via llvm-commits]

Author: Ramkumar Ramachandra
Date: 2025-03-06T19:30:25Z
New Revision: ddffb74afd870d284ba07f1cf6c67117a8ab8b33

URL: https://github.com/llvm/llvm-project/commit/ddffb74afd870d284ba07f1cf6c67117a8ab8b33
DIFF: https://github.com/llvm/llvm-project/commit/ddffb74afd870d284ba07f1cf6c67117a8ab8b33.diff

LOG: [LV] Strip unreachable SCEV-check blocks (#130079)

emitSCEVChecks checks if SCEVCheckCond matches zero, and returns
nullptr. However, it sets SCEVCheckCond as used before it does this,
which prevents it from being removed during cleanup, resulting in
unreachable blocks being emitted. Fix this.

Added: 
    

Modified: 
    llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
    llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll
    llvm/test/Transforms/LoopVectorize/X86/pr36524.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 3890ae2fa3b89..74ddf906ff9fd 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2094,18 +2094,11 @@ class GeneratedRTChecks {
   /// depending on the generated condition.
   BasicBlock *emitSCEVChecks(BasicBlock *Bypass,
                              BasicBlock *LoopVectorPreHeader) {
-    if (!SCEVCheckCond)
+    using namespace llvm::PatternMatch;
+    if (!SCEVCheckCond || match(SCEVCheckCond, m_ZeroInt()))
       return nullptr;
 
-    Value *Cond = SCEVCheckCond;
-    // Mark the check as used, to prevent it from being removed during cleanup.
-    SCEVCheckCond = nullptr;
-    if (auto *C = dyn_cast<ConstantInt>(Cond))
-      if (C->isZero())
-        return nullptr;
-
     auto *Pred = LoopVectorPreHeader->getSinglePredecessor();
-
     BranchInst::Create(LoopVectorPreHeader, SCEVCheckBlock);
     // Create new preheader for vector loop.
     if (OuterLoop)
@@ -2119,10 +2112,14 @@ class GeneratedRTChecks {
     DT->addNewBlock(SCEVCheckBlock, Pred);
     DT->changeImmediateDominator(LoopVectorPreHeader, SCEVCheckBlock);
 
-    BranchInst &BI = *BranchInst::Create(Bypass, LoopVectorPreHeader, Cond);
+    BranchInst &BI =
+        *BranchInst::Create(Bypass, LoopVectorPreHeader, SCEVCheckCond);
     if (AddBranchWeights)
       setBranchWeights(BI, SCEVCheckBypassWeights, /*IsExpected=*/false);
     ReplaceInstWithInst(SCEVCheckBlock->getTerminator(), &BI);
+
+    // Mark the check as used, to prevent it from being removed during cleanup.
+    SCEVCheckCond = nullptr;
     return SCEVCheckBlock;
   }
 

diff  --git a/llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll b/llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll
index cf4fc143fe8c3..ce24d3cfded22 100644
--- a/llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll
@@ -793,8 +793,6 @@ define void @multiple_exit_conditions(ptr %src, ptr noalias %dst) #1 {
 ; DEFAULT-NEXT:    [[BC_RESUME_VAL:%.*]] = phi ptr [ [[IND_END]], [[MIDDLE_BLOCK]] ], [ [[DST]], [[ENTRY:%.*]] ]
 ; DEFAULT-NEXT:    [[BC_RESUME_VAL1:%.*]] = phi i64 [ 512, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY]] ]
 ; DEFAULT-NEXT:    br label [[LOOP:%.*]]
-; DEFAULT:       vector.scevcheck:
-; DEFAULT-NEXT:    unreachable
 ; DEFAULT:       loop:
 ; DEFAULT-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[PTR_IV_NEXT:%.*]], [[LOOP]] ]
 ; DEFAULT-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL1]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
@@ -857,8 +855,6 @@ define void @multiple_exit_conditions(ptr %src, ptr noalias %dst) #1 {
 ; PRED-NEXT:    [[BC_RESUME_VAL:%.*]] = phi ptr [ [[IND_END]], [[MIDDLE_BLOCK]] ], [ [[DST]], [[ENTRY:%.*]] ]
 ; PRED-NEXT:    [[BC_RESUME_VAL2:%.*]] = phi i64 [ [[IND_END1]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY]] ]
 ; PRED-NEXT:    br label [[LOOP:%.*]]
-; PRED:       vector.scevcheck:
-; PRED-NEXT:    unreachable
 ; PRED:       loop:
 ; PRED-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[PTR_IV_NEXT:%.*]], [[LOOP]] ]
 ; PRED-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL2]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]

diff  --git a/llvm/test/Transforms/LoopVectorize/X86/pr36524.ll b/llvm/test/Transforms/LoopVectorize/X86/pr36524.ll
index ee8374f952c7a..90e322e217603 100644
--- a/llvm/test/Transforms/LoopVectorize/X86/pr36524.ll
+++ b/llvm/test/Transforms/LoopVectorize/X86/pr36524.ll
@@ -40,8 +40,6 @@ define void @foo(ptr %ptr, ptr %ptr.2) {
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 80, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ], [ 0, [[VECTOR_MEMCHECK]] ]
 ; CHECK-NEXT:    [[BC_RESUME_VAL2:%.*]] = phi i64 [ 82, [[MIDDLE_BLOCK]] ], [ 2, [[ENTRY]] ], [ 2, [[VECTOR_MEMCHECK]] ]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
-; CHECK:       vector.scevcheck:
-; CHECK-NEXT:    unreachable
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[CAN_IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[CAN_IV_NEXT:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[TMP9:%.*]] = phi i64 [ [[BC_RESUME_VAL2]], [[SCALAR_PH]] ], [ [[TMP12:%.*]], [[LOOP]] ]