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

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon created https://github.com/llvm/llvm-project/pull/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.

>From 7b96f8de475540f7a4a61011cb158de4c4eeffdf Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <ramkumar.ramachandra at codasip.com>
Date: Thu, 6 Mar 2025 11:18:22 +0000
Subject: [PATCH] [LV] Strip unreachable SCEV-check blocks

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.
---
 llvm/lib/Transforms/Vectorize/LoopVectorize.cpp             | 6 ++----
 .../LoopVectorize/AArch64/conditional-branches-cost.ll      | 4 ----
 llvm/test/Transforms/LoopVectorize/X86/pr36524.ll           | 2 --
 3 files changed, 2 insertions(+), 10 deletions(-)

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]] ]