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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Ramkumar Ramachandra (artagnon)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/130079.diff


3 Files Affected:

- (modified) llvm/lib/Transforms/Vectorize/LoopVectorize.cpp (+2-4) 
- (modified) llvm/test/Transforms/LoopVectorize/AArch64/conditional-branches-cost.ll (-4) 
- (modified) llvm/test/Transforms/LoopVectorize/X86/pr36524.ll (-2) 


``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index cb860a472d8f7..5f29f1a918244 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -2095,15 +2095,13 @@ 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();
 
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]] ]

``````````

</details>


https://github.com/llvm/llvm-project/pull/130079