[PATCH] Recognize n/1 in the SCEV divide function

[Brendon Cahoon]

n/1 generates a quotient equal to n and a remainder of 0.

If this case is not recognized, then the SCEV divide() function can return a remainder that is greater than or equal to the denominator, which means the delinearized subscripts for the test case will be incorrect. The bug will manifest in the dependence analysis pass when delinearizing references. In the test case, the innermost dimension will appear to be the same when they are different by 1 iteration.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9003

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/Delinearization/divide_by_one.ll

Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -726,6 +726,13 @@
       return;
     }
 
+    // A simple case when N/1. The quotient is N.
+    if (Denominator->isOne()) {
+      *Quotient = Numerator;
+      *Remainder = D.Zero;
+      return;
+    }
+
     // Split the Denominator when it is a product.
     if (const SCEVMulExpr *T = dyn_cast<const SCEVMulExpr>(Denominator)) {
       const SCEV *Q, *R;
Index: test/Analysis/Delinearization/divide_by_one.ll
===================================================================
--- /dev/null
+++ test/Analysis/Delinearization/divide_by_one.ll
@@ -0,0 +1,70 @@
+; RUN: opt < %s -analyze -delinearize | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i1:32-i64:64-a:0-n32"
+
+; Check that division by 1 can be delinearized.
+;
+; void test1(unsigned char *dst, int stride, int bs) {
+;   for (int r = bs; r >= 0; --r)
+;     for (int c = 0; c < bs; ++c)
+;       dst[r * stride + c] = dst[(r + 1) * stride + c - 1];
+; }
+
+; AddRec: {{(-1 + ((1 + %bs) * %stride)),+,(-1 * %stride)}<%for.cond1.preheader>,+,1}<nw><%for.body3>
+; CHECK: Inst:  %0 = load i8, i8* %arrayidx, align 1
+; CHECK: Base offset: %dst
+; CHECK: ArrayDecl[UnknownSize][%stride] with elements of 1 bytes.
+; CHECK: ArrayRef[{(1 + %bs),+,-1}<nw><%for.cond1.preheader>][{-1,+,1}<nw><%for.body3>]
+
+; AddRec: {{(%stride * %bs),+,(-1 * %stride)}<%for.cond1.preheader>,+,1}<nw><%for.body3>
+; CHECK: Inst:  store i8 %0, i8* %arrayidx7, align 1
+; CHECK: Base offset: %dst
+; CHECK: ArrayDecl[UnknownSize][%stride] with elements of 1 bytes.
+; CHECK: ArrayRef[{%bs,+,-1}<nsw><%for.cond1.preheader>][{0,+,1}<nuw><nsw><%for.body3>]
+
+define void @test(i8* nocapture %dst, i32 %stride, i32 %bs) {
+entry:
+  %cmp20 = icmp sgt i32 %bs, -1
+  br i1 %cmp20, label %for.cond1.preheader.lr.ph, label %for.end9
+
+for.cond1.preheader.lr.ph:
+  %cmp218 = icmp slt i32 0, %bs
+  br label %for.cond1.preheader
+
+for.cond1.preheader:
+  %r.021 = phi i32 [ %bs, %for.cond1.preheader.lr.ph ], [ %dec, %for.inc8 ]
+  br i1 %cmp218, label %for.body3.lr.ph, label %for.inc8
+
+for.body3.lr.ph:
+  %add = add nsw i32 %r.021, 1
+  %mul = mul nsw i32 %add, %stride
+  %add4 = add i32 %mul, -1
+  %mul5 = mul nsw i32 %r.021, %stride
+  br label %for.body3
+
+for.body3:
+  %c.019 = phi i32 [ 0, %for.body3.lr.ph ], [ %inc, %for.body3 ]
+  %sub = add i32 %add4, %c.019
+  %arrayidx = getelementptr inbounds i8, i8* %dst, i32 %sub
+  %0 = load i8, i8* %arrayidx, align 1
+  %add6 = add nsw i32 %c.019, %mul5
+  %arrayidx7 = getelementptr inbounds i8, i8* %dst, i32 %add6
+  store i8 %0, i8* %arrayidx7, align 1
+  %inc = add nsw i32 %c.019, 1
+  %cmp2 = icmp slt i32 %inc, %bs
+  br i1 %cmp2, label %for.body3, label %for.cond1.for.inc8_crit_edge
+
+for.cond1.for.inc8_crit_edge:
+  br label %for.inc8
+
+for.inc8:
+  %dec = add nsw i32 %r.021, -1
+  %cmp = icmp sgt i32 %dec, -1
+  br i1 %cmp, label %for.cond1.preheader, label %for.cond.for.end9_crit_edge
+
+for.cond.for.end9_crit_edge:
+  br label %for.end9
+
+for.end9:
+  ret void
+}

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