[PATCH] D62990: [SCEV][WIP] Try fix PR42175

[Z. Zheng via Phabricator via llvm-commits]

zzheng created this revision.
zzheng added reviewers: efriedma, sanjoy.
Herald added a subscriber: javed.absar.
Herald added a project: LLVM.

https://bugs.llvm.org/show_bug.cgi?id=42175

$ cat loop-small-runtime-upperbound.ll 
target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"

@global = dso_local local_unnamed_addr global i32 0, align 4
@global.1 = dso_local local_unnamed_addr global i8* null, align 4

define dso_local void @hoge(i8 %arg) {
entry:

  %x = load i32, i32* @global, align 4
  %0 = icmp ult i32 %x, 17
  br i1 %0, label %loop, label %exit

loop:

  %iv = phi i32 [ %x, %entry ], [ %iv.next, %loop ]
  %iv.next = add nuw i32 %iv, 8
  %1 = load i8*, i8** @global.1, align 4
  %2 = getelementptr inbounds i8, i8* %1, i32 1
  store i8* %2, i8** @global.1, align 4
  store i8 %arg, i8* %1, align 1
  %3 = icmp ult i32 %iv.next, 17
  br i1 %3, label %loop, label %exit

exit:                                             ; preds = %bb12, %bb

  ret void

}

$ opt loop-small-runtime-upperbound.ll -analyze -scalar-evolution
...

The loop runs a max of 3 iters, but SCEV computes max BE-taken count as 3.
The same issue is also found in test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll, where max BE-taken count is 333 instead of 334.

In computeMaxBECountForLT(), when Start is a (C + %x), where C is a constant and %x is an unknown, getUnsignedRangeMin(Start) returns full-set because of %x. 
But loop entry is guarded by:

  %0 = icmp ult i32 %x, 17

so x is known in [0, 17), thus MinStart shall be C rather than 0.


Repository:
  rL LLVM

https://reviews.llvm.org/D62990

Files:
  include/llvm/Analysis/ScalarEvolution.h
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll


Index: test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
===================================================================
--- test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
+++ test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
@@ -1,7 +1,7 @@
 ; RUN: opt < %s -analyze -scalar-evolution 2>&1 | FileCheck %s
 
 ; CHECK: Loop %bb: backedge-taken count is ((999 + (-1 * %x)) /u 3)
-; CHECK: Loop %bb: max backedge-taken count is 334
+; CHECK: Loop %bb: max backedge-taken count is 333
 
 
 ; This is a tricky testcase for unsigned wrap detection which ScalarEvolution
Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -10434,8 +10434,8 @@
                                                     const SCEV *Stride,
                                                     const SCEV *End,
                                                     unsigned BitWidth,
-                                                    bool IsSigned) {
-
+                                                    bool IsSigned,
+                                                    const Loop *L) {
   assert(!isKnownNonPositive(Stride) &&
          "Stride is expected strictly positive!");
   // Calculate the maximum backedge count based on the range of values
@@ -10444,6 +10444,12 @@
   APInt MinStart =
       IsSigned ? getSignedRangeMin(Start) : getUnsignedRangeMin(Start);
 
+  // If loop entry is guarded by an ULT, we know (Start-Stride) is in [0, End).
+  if (!IsSigned && !isa<SCEVConstant>(Start) && isLoopInvariant(End, L) &&
+      isLoopEntryGuardedByCond(L, ICmpInst::ICMP_ULT,
+          getMinusSCEV(Start, Stride), End))
+    MinStart += getUnsignedRangeMin(Stride);
+
   APInt StrideForMaxBECount =
       IsSigned ? getSignedRangeMin(Stride) : getUnsignedRangeMin(Stride);
 
@@ -10567,7 +10573,7 @@
   // checked above).
   if (!isLoopInvariant(RHS, L)) {
     const SCEV *MaxBECount = computeMaxBECountForLT(
-        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
+        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned, L);
     return ExitLimit(getCouldNotCompute() /* ExactNotTaken */, MaxBECount,
                      false /*MaxOrZero*/, Predicates);
   }
@@ -10604,7 +10610,7 @@
     MaxOrZero = true;
   } else {
     MaxBECount = computeMaxBECountForLT(
-        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
+        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned, L);
   }
 
   if (isa<SCEVCouldNotCompute>(MaxBECount) &&
Index: include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- include/llvm/Analysis/ScalarEvolution.h
+++ include/llvm/Analysis/ScalarEvolution.h
@@ -1838,7 +1838,7 @@
   /// assert these preconditions so please be careful.
   const SCEV *computeMaxBECountForLT(const SCEV *Start, const SCEV *Stride,
                                      const SCEV *End, unsigned BitWidth,
-                                     bool IsSigned);
+                                     bool IsSigned, const Loop *L);
 
   /// Verify if an linear IV with positive stride can overflow when in a
   /// less-than comparison, knowing the invariant term of the comparison,


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