From 3c59cf5aa79b8c17f59663a0ea4caede6828565f Mon Sep 17 00:00:00 2001
From: Nikita Popov <nikita.ppv@gmail.com>
Date: Mon, 6 Sep 2021 22:18:11 +0200
Subject: [PATCH] [SCEV] Fix applyLoopGuards() with range check idiom (PR51760)

Due to a typo, this replaced %x with umax(C1, umin(C2, %x + C3))
rather than umax(C1, umin(C2, %x)). This didn't make a difference
for the existing tests, because the result is only used for range
calculation, and %x will usually have an unknown starting range,
and the additional offset keeps it unknown. However, if %x already
has a known range, we may compute a result range that is too
small.

(cherry picked from commit 8d54c8a0c3d7d4a50186ae7087780c6082e5bb46)
---
 lib/Analysis/ScalarEvolution.cpp              |  2 +-
 .../max-backedge-taken-count-guard-info.ll    | 40 +++++++++++++++++++
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index f22d834b5e5..2d980e6935b 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -13969,7 +13969,7 @@ const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
       if (ExactRegion.isWrappedSet() || ExactRegion.isFullSet())
         return false;
       auto I = RewriteMap.find(LHSUnknown->getValue());
-      const SCEV *RewrittenLHS = I != RewriteMap.end() ? I->second : LHS;
+      const SCEV *RewrittenLHS = I != RewriteMap.end() ? I->second : LHSUnknown;
       RewriteMap[LHSUnknown->getValue()] = getUMaxExpr(
           getConstant(ExactRegion.getUnsignedMin()),
           getUMinExpr(RewrittenLHS, getConstant(ExactRegion.getUnsignedMax())));
diff --git a/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll b/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
index 9e7957a3736..7dd21827d0b 100644
--- a/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
+++ b/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
@@ -1327,6 +1327,46 @@ exit:
   ret void
 }
 
+; Same as @optimized_range_check_unsigned, but %N already has a range limited
+; to [2,4) beforehand.
+define void @optimized_range_check_unsigned3(i16* %pred, i1 %c) {
+; CHECK-LABEL: 'optimized_range_check_unsigned3'
+; CHECK-NEXT:  Classifying expressions for: @optimized_range_check_unsigned3
+; CHECK-NEXT:    %N = select i1 %c, i32 2, i32 3
+; CHECK-NEXT:    --> %N U: [2,4) S: [2,4)
+; CHECK-NEXT:    %N.off = add i32 %N, -1
+; CHECK-NEXT:    --> (-1 + %N)<nsw> U: [1,3) S: [1,3)
+; CHECK-NEXT:    %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop ]
+; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,3) S: [0,3) Exits: (-1 + %N)<nsw> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    %gep = getelementptr inbounds i16, i16* %pred, i32 %iv
+; CHECK-NEXT:    --> {%pred,+,2}<nuw><%loop> U: full-set S: full-set Exits: ((2 * (zext i32 (-1 + %N)<nsw> to i64))<nuw><nsw> + %pred) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    %iv.next = add nuw nsw i32 %iv, 1
+; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%loop> U: [1,4) S: [1,4) Exits: %N LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:  Determining loop execution counts for: @optimized_range_check_unsigned3
+; CHECK-NEXT:  Loop %loop: backedge-taken count is (-1 + %N)<nsw>
+; CHECK-NEXT:  Loop %loop: max backedge-taken count is 2
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (-1 + %N)<nsw>
+; CHECK-NEXT:   Predicates:
+; CHECK:       Loop %loop: Trip multiple is 1
+;
+entry:
+  %N = select i1 %c, i32 2, i32 3
+  %N.off = add i32 %N, -1
+  %cmp = icmp ult i32 %N.off, 7
+  br i1 %cmp, label %loop, label %exit
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop ]
+  %gep = getelementptr inbounds i16, i16* %pred, i32 %iv
+  store i16 0, i16* %gep, align 2
+  %iv.next = add nuw nsw i32 %iv, 1
+  %ec = icmp eq i32 %iv.next, %N
+  br i1 %ec, label %exit, label %loop
+
+exit:
+  ret void
+}
+
 ; Similar to @optimized_range_check_unsigned, but the initial compare checks
 ; against unsigned max (-1), which breaks the range check idiom.
 define void @not_optimized_range_check_unsigned1(i16* %pred, i32 %N) {