[llvm] [indvars] Allow widenWithVariantUse to succeed without extend users (PR #71557)

[Nikita Popov via llvm-commits]

================
@@ -365,16 +364,15 @@ define void @zext_postinc_offset_constant_one(ptr %A, i32 %start) {
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ [[TMP0]], [[FOR_BODY_PREHEADER]] ], [ [[INDVARS_IV_NEXT:%.*]], [[FOR_BODY]] ]
-; CHECK-NEXT:    [[J_016_US:%.*]] = phi i32 [ [[INC_US:%.*]], [[FOR_BODY]] ], [ [[START]], [[FOR_BODY_PREHEADER]] ]
 ; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 [[INDVARS_IV]] to i32
 ; CHECK-NEXT:    [[ADD_US:%.*]] = add i32 [[TMP1]], 1
 ; CHECK-NEXT:    [[IDXPROM_US:%.*]] = zext i32 [[ADD_US]] to i64
 ; CHECK-NEXT:    [[ARRAYIDX_US:%.*]] = getelementptr inbounds i32, ptr [[A:%.*]], i64 [[IDXPROM_US]]
 ; CHECK-NEXT:    tail call void @use_ptr(ptr [[ARRAYIDX_US]])
-; CHECK-NEXT:    [[INC_US]] = add nsw i32 [[J_016_US]], -1
-; CHECK-NEXT:    [[CMP2_US:%.*]] = icmp ugt i32 [[INC_US]], 6
+; CHECK-NEXT:    [[TMP2:%.*]] = add nsw i64 [[INDVARS_IV]], 4294967295
----------------
nikic wrote:

I don't really get why this transform is correct. It looks like indvars.iv here is a normal {%start,+,-1} addrec, but then the exit condition here becomes `{%start+4294967295,+,-1} ugt 6`? That's not the same as `{%start-1,+,-1} ugt 6` in a smaller bit width.

Am I missing something here?

https://github.com/llvm/llvm-project/pull/71557