[LAA] Fix picking context instr in evaluatePtrAddRec for multiple preds.

A loop may have more than one predecessor out of the loop. In that case,
just pick the first non-phi instruction in the loop header.

Author: fhahn

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -234,9 +234,14 @@ static bool evaluatePtrAddRecAtMaxBTCWillNotWrap(
 
   // Check if we have a suitable dereferencable assumption we can use.
   if (!StartPtrV->canBeFreed()) {
+    Instruction *CtxI = &*L->getHeader()->getFirstNonPHIIt();
+    if (BasicBlock *LoopPred = L->getLoopPredecessor()) {
+      if (isa<BranchInst>(LoopPred->getTerminator()))
+        CtxI = LoopPred->getTerminator();
+    }
+
     RetainedKnowledge DerefRK = getKnowledgeValidInContext(
-        StartPtrV, {Attribute::Dereferenceable}, *AC,
-        L->getLoopPredecessor()->getTerminator(), DT);
+        StartPtrV, {Attribute::Dereferenceable}, *AC, CtxI, DT);
     if (DerefRK) {
       DerefBytesSCEV =
           SE.getUMaxExpr(DerefBytesSCEV, SE.getSCEV(DerefRK.IRArgValue));

llvm/test/Analysis/LoopAccessAnalysis/early-exit-runtime-checks.ll

@@ -565,6 +565,138 @@ e.2:
   ret void
 }
 
+define void @all_exits_dominate_latch_countable_exits_at_most_500_iterations_known_deref_via_assumption_multiple_predecessors(ptr %A, ptr %B, i1 %c) nosync nofree {
+; CHECK-LABEL: 'all_exits_dominate_latch_countable_exits_at_most_500_iterations_known_deref_via_assumption_multiple_predecessors'
+; CHECK-NEXT:    loop.header:
+; CHECK-NEXT:      Memory dependences are safe with run-time checks
+; CHECK-NEXT:      Dependences:
+; CHECK-NEXT:      Run-time memory checks:
+; CHECK-NEXT:      Check 0:
+; CHECK-NEXT:        Comparing group GRP0:
+; CHECK-NEXT:          %gep.B = getelementptr inbounds i32, ptr %B, i64 %iv
+; CHECK-NEXT:        Against group GRP1:
+; CHECK-NEXT:          %gep.A = getelementptr inbounds i32, ptr %A, i64 %iv
+; CHECK-NEXT:      Grouped accesses:
+; CHECK-NEXT:        Group GRP0:
+; CHECK-NEXT:          (Low: %B High: (2000 + %B))
+; CHECK-NEXT:            Member: {%B,+,4}<nuw><%loop.header>
+; CHECK-NEXT:        Group GRP1:
+; CHECK-NEXT:          (Low: %A High: (2000 + %A))
+; CHECK-NEXT:            Member: {%A,+,4}<nuw><%loop.header>
+; CHECK-EMPTY:
+; CHECK-NEXT:      Non vectorizable stores to invariant address were not found in loop.
+; CHECK-NEXT:      SCEV assumptions:
+; CHECK-EMPTY:
+; CHECK-NEXT:      Expressions re-written:
+;
+entry:
+  call void @llvm.assume(i1 true) [ "dereferenceable"(ptr %A, i64 2000) ]
+  call void @llvm.assume(i1 true) [ "dereferenceable"(ptr %B, i64 2000) ]
+  br i1 %c, label %then, label %else
+
+then:
+  br label %loop.header
+
+else:
+  br label %loop.header
+
+loop.header:
+  %iv = phi i64 [ 0, %then ], [ 0, %else ], [ %iv.next, %latch ]
+  %gep.A = getelementptr inbounds i32, ptr %A, i64 %iv
+  %gep.B = getelementptr inbounds i32, ptr %B, i64 %iv
+  %l = load i32, ptr %gep.A, align 4
+  store i32 0, ptr %gep.B, align 4
+  %cntable.c.1 = icmp ult i64 %iv, 1000
+  %iv.next = add nuw nsw i64 %iv, 1
+  br i1 %cntable.c.1, label %b2, label %e.1
+
+b2:
+  %uncntable.c.0 = icmp eq i32 %l, 0
+  br i1 %uncntable.c.0, label %e.2, label %b3
+
+b3:
+  %cntable.c.2 = icmp eq i64 %iv.next, 500
+  br i1 %cntable.c.2, label %cleanup4, label %latch
+
+latch:
+  br label %loop.header
+
+cleanup4:
+  ret void
+
+e.1:
+  ret void
+
+e.2:
+  ret void
+}
+
+define void @all_exits_dominate_latch_countable_exits_at_most_500_iterations_known_deref_via_assumption_multiple_predecessors_no_valid(ptr %A, ptr %B, i1 %c) nosync nofree {
+; CHECK-LABEL: 'all_exits_dominate_latch_countable_exits_at_most_500_iterations_known_deref_via_assumption_multiple_predecessors_no_valid'
+; CHECK-NEXT:    loop.header:
+; CHECK-NEXT:      Memory dependences are safe with run-time checks
+; CHECK-NEXT:      Dependences:
+; CHECK-NEXT:      Run-time memory checks:
+; CHECK-NEXT:      Check 0:
+; CHECK-NEXT:        Comparing group GRP0:
+; CHECK-NEXT:          %gep.B = getelementptr inbounds i32, ptr %B, i64 %iv
+; CHECK-NEXT:        Against group GRP1:
+; CHECK-NEXT:          %gep.A = getelementptr inbounds i32, ptr %A, i64 %iv
+; CHECK-NEXT:      Grouped accesses:
+; CHECK-NEXT:        Group GRP0:
+; CHECK-NEXT:          (Low: %B High: inttoptr (i64 -1 to ptr))
+; CHECK-NEXT:            Member: {%B,+,4}<nuw><%loop.header>
+; CHECK-NEXT:        Group GRP1:
+; CHECK-NEXT:          (Low: %A High: (2000 + %A))
+; CHECK-NEXT:            Member: {%A,+,4}<nuw><%loop.header>
+; CHECK-EMPTY:
+; CHECK-NEXT:      Non vectorizable stores to invariant address were not found in loop.
+; CHECK-NEXT:      SCEV assumptions:
+; CHECK-EMPTY:
+; CHECK-NEXT:      Expressions re-written:
+;
+entry:
+  call void @llvm.assume(i1 true) [ "dereferenceable"(ptr %A, i64 2000) ]
+  br i1 %c, label %then, label %else
+
+then:
+  call void @llvm.assume(i1 true) [ "dereferenceable"(ptr %B, i64 2000) ]
+  br label %loop.header
+
+else:
+  br label %loop.header
+
+loop.header:
+  %iv = phi i64 [ 0, %then ], [ 0, %else ], [ %iv.next, %latch ]
+  %gep.A = getelementptr inbounds i32, ptr %A, i64 %iv
+  %gep.B = getelementptr inbounds i32, ptr %B, i64 %iv
+  %l = load i32, ptr %gep.A, align 4
+  store i32 0, ptr %gep.B, align 4
+  %cntable.c.1 = icmp ult i64 %iv, 1000
+  %iv.next = add nuw nsw i64 %iv, 1
+  br i1 %cntable.c.1, label %b2, label %e.1
+
+b2:
+  %uncntable.c.0 = icmp eq i32 %l, 0
+  br i1 %uncntable.c.0, label %e.2, label %b3
+
+b3:
+  %cntable.c.2 = icmp eq i64 %iv.next, 500
+  br i1 %cntable.c.2, label %cleanup4, label %latch
+
+latch:
+  br label %loop.header
+
+cleanup4:
+  ret void
+
+e.1:
+  ret void
+
+e.2:
+  ret void
+}
+
 define void @all_exits_dominate_latch_countable_exits_at_most_500_iterations_deref_via_assumption_too_small(ptr %A, ptr %B) nosync nofree {
 ; CHECK-LABEL: 'all_exits_dominate_latch_countable_exits_at_most_500_iterations_deref_via_assumption_too_small'
 ; CHECK-NEXT:    loop.header: