[llvm] [SCEV] Discard samesign when analyzing loop invariant exits (PR #181171)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

If the predicate has samesign set, we could either perform the checks with the unsigned predicate and return and unsigned invariant predicate, or we could perform them with the signed predicate and return a signed invariant predicate. The current implementation can end up mixing both, using a signed predicate for one check and an unsigned one for the other.

Avoid this by dropping the samesign flag.

Fixes https://github.com/llvm/llvm-project/issues/180870.

---
Full diff: https://github.com/llvm/llvm-project/pull/181171.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+5) 
- (modified) llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll (+44) 


``````````diff
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 45f1b543b8fc7..1a8f36968adde 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11496,6 +11496,11 @@ ScalarEvolution::getLoopInvariantExitCondDuringFirstIterationsImpl(
   if (!AR || AR->getLoop() != L)
     return std::nullopt;
 
+  // Even if both are valid, we need to consistently chose the unsigned or the
+  // signed predicate below, not mixtures of both. For now, prefer the unsigned
+  // predicate.
+  Pred = Pred.dropSameSign();
+
   // The predicate must be relational (i.e. <, <=, >=, >).
   if (!ICmpInst::isRelational(Pred))
     return std::nullopt;
diff --git a/llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll b/llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll
index d3a5d4c443cb9..c013c91900fee 100644
--- a/llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll
+++ b/llvm/test/Transforms/IndVarSimplify/turn-to-invariant.ll
@@ -1059,3 +1059,47 @@ exit:
 failed:
   ret i32 -1
 }
+
+; PR180870: %range_check1 here cannot be made invariant.
+define i32 @not_invariant_samesign(i32 %unknown_limit, ptr %length_ptr) {
+; CHECK-LABEL: @not_invariant_samesign(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[HEADER:%.*]]
+; CHECK:       header:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LATCH:%.*]] ]
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw i32 [[IV]], 1
+; CHECK-NEXT:    [[RANGE_CHECK1_FIRST_ITER:%.*]] = icmp samesign ult i32 [[IV_NEXT]], [[UNKNOWN_LIMIT:%.*]]
+; CHECK-NEXT:    [[IS_POSITIVE:%.*]] = icmp sgt i32 [[UNKNOWN_LIMIT]], 0
+; CHECK-NEXT:    [[OR_COND:%.*]] = and i1 [[RANGE_CHECK1_FIRST_ITER]], [[IS_POSITIVE]]
+; CHECK-NEXT:    br i1 [[OR_COND]], label [[LATCH]], label [[RET:%.*]]
+; CHECK:       latch:
+; CHECK-NEXT:    [[IV_PREV:%.*]] = add nsw i32 [[IV]], -1
+; CHECK-NEXT:    [[LENGTH:%.*]] = load i32, ptr [[LENGTH_PTR:%.*]], align 4, !range [[RNG0:![0-9]+]]
+; CHECK-NEXT:    [[RANGE_CHECK2:%.*]] = icmp samesign ult i32 [[IV_PREV]], [[LENGTH]]
+; CHECK-NEXT:    br i1 [[RANGE_CHECK2]], label [[HEADER]], label [[RET]]
+; CHECK:       ret:
+; CHECK-NEXT:    [[IV_LCSSA:%.*]] = phi i32 [ [[IV]], [[LATCH]] ], [ [[IV]], [[HEADER]] ]
+; CHECK-NEXT:    ret i32 [[IV_LCSSA]]
+;
+entry:
+  br label %header
+
+header:                                           ; preds = %latch, %entry
+  %iv = phi i32 [ 1, %entry ], [ %iv.next, %latch ]
+  %iv.next = add i32 %iv, 1
+  %range_check1 = icmp samesign ult i32 %iv.next, %unknown_limit
+  %is_positive = icmp sgt i32 %unknown_limit, 0
+  %or.cond = and i1 %range_check1, %is_positive
+  br i1 %or.cond, label %latch, label %ret
+
+latch:                                            ; preds = %header
+  %iv.prev = add i32 %iv, -1
+  %length = load i32, ptr %length_ptr, align 4, !range !0
+  %range_check2 = icmp ult i32 %iv.prev, %length
+  br i1 %range_check2, label %header, label %ret
+
+ret:                                              ; preds = %latch, %header
+  ret i32 %iv
+}
+
+!0 = !{i32 0, i32 2147483647}

``````````

</details>


https://github.com/llvm/llvm-project/pull/181171