From 1d2c27b23eceec7b6a79e3f0b21d24068fbd50f3 Mon Sep 17 00:00:00 2001
From: Wojciech Matyjewicz <wmatyjewicz@fastmail.fm>
Date: Tue, 12 Feb 2008 15:09:36 +0000
Subject: [PATCH] Fix PR2002. Suppose n is the initial value for the induction
 variable (with step 1) and m is its final value. Then, the correct trip count
 is SMAX(m,n)-n. Previously, we used SMAX(0,m-n), but m-n may overflow and
 can't in general be interpreted as signed.

Patch by Nick Lewycky.

llvm-svn: 47007
---
 llvm/lib/Analysis/ScalarEvolution.cpp            | 10 ++++------
 .../ScalarEvolution/2008-02-12-SMAXTripCount.ll  | 16 ++++++++++++++++
 2 files changed, 20 insertions(+), 6 deletions(-)
 create mode 100644 llvm/test/Analysis/ScalarEvolution/2008-02-12-SMAXTripCount.ll

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 122dba3e6ac0..65cee82a9649 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2527,19 +2527,17 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L, bool isSigned) {
   if (AddRec->isAffine()) {
     // The number of iterations for "{n,+,1} < m", is m-n.  However, we don't
     // know that m is >= n on input to the loop.  If it is, the condition
-    // returns true zero times.  To handle both cases, we return SMAX(0, m-n).
+    // returns true zero times.  To handle both cases, we return SMAX(m, n)-n.
 
     // FORNOW: We only support unit strides.
     SCEVHandle One = SE.getIntegerSCEV(1, RHS->getType());
     if (AddRec->getOperand(1) != One)
       return UnknownValue;
 
-    SCEVHandle Iters = SE.getMinusSCEV(RHS, AddRec->getOperand(0));
+    SCEVHandle Start = AddRec->getOperand(0);
+    SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS, Start) : (SCEVHandle)RHS;
 
-    if (isSigned)
-      return SE.getSMaxExpr(SE.getIntegerSCEV(0, RHS->getType()), Iters);
-    else
-      return Iters;
+    return SE.getMinusSCEV(End, Start);
   }
 
   return UnknownValue;
diff --git a/llvm/test/Analysis/ScalarEvolution/2008-02-12-SMAXTripCount.ll b/llvm/test/Analysis/ScalarEvolution/2008-02-12-SMAXTripCount.ll
new file mode 100644
index 000000000000..292ea99b6b5d
--- /dev/null
+++ b/llvm/test/Analysis/ScalarEvolution/2008-02-12-SMAXTripCount.ll
@@ -0,0 +1,16 @@
+; RUN: llvm-as < %s | opt -scalar-evolution -analyze | grep {Loop loop: ( 100 + ( -100 smax  %n)) iterations!}
+; PR2002
+
+define void @foo(i8 %n) {
+entry:
+	br label %loop
+loop:
+	%i = phi i8 [ -100, %entry ], [ %i.inc, %next ]
+	%cond = icmp slt i8 %i, %n
+	br i1 %cond, label %next, label %return
+next:
+        %i.inc = add i8 %i, 1
+	br label %loop
+return:
+	ret void
+}