[llvm] [Loads] Support dereferenceable assumption with variable size. (PR #128436)

[Florian Hahn via llvm-commits]

================
@@ -185,15 +185,32 @@ define void @deref_assumption_in_preheader_too_small_non_constant_trip_count_acc
 ; CHECK-NEXT:    [[N_VEC:%.*]] = sub i64 [[N]], [[N_MOD_VF]]
 ; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
 ; CHECK:       [[VECTOR_BODY]]:
-; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_LOAD_CONTINUE2:.*]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[INDEX]], 0
-; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr i32, ptr [[A]], i64 [[TMP0]]
 ; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr inbounds i32, ptr [[B]], i64 [[TMP0]]
 ; CHECK-NEXT:    [[TMP3:%.*]] = getelementptr inbounds i32, ptr [[TMP2]], i32 0
 ; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <2 x i32>, ptr [[TMP3]], align 1
 ; CHECK-NEXT:    [[TMP4:%.*]] = icmp sge <2 x i32> [[WIDE_LOAD]], zeroinitializer
-; CHECK-NEXT:    [[TMP5:%.*]] = getelementptr i32, ptr [[TMP1]], i32 0
-; CHECK-NEXT:    [[WIDE_LOAD1:%.*]] = load <2 x i32>, ptr [[TMP5]], align 1
+; CHECK-NEXT:    [[TMP15:%.*]] = xor <2 x i1> [[TMP4]], splat (i1 true)
+; CHECK-NEXT:    [[TMP5:%.*]] = extractelement <2 x i1> [[TMP15]], i32 0
+; CHECK-NEXT:    br i1 [[TMP5]], label %[[PRED_LOAD_IF:.*]], label %[[PRED_LOAD_CONTINUE:.*]]
+; CHECK:       [[PRED_LOAD_IF]]:
+; CHECK-NEXT:    [[TMP16:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP0]]
+; CHECK-NEXT:    [[TMP17:%.*]] = load i32, ptr [[TMP16]], align 1
----------------
fhahn wrote:

Yep, the current code handles the assumption with variable sizes incorrectly due to `getStartAndEndForAccess` wrapping; in this case we pass the unsigned max as trip count (-1) and the end wraps around to 0 (fixed by https://github.com/llvm/llvm-project/pull/128061/)

https://github.com/llvm/llvm-project/pull/128436