[llvm] 8f79270 - [ScalarEvolution] Fix pointer/int confusion in howManyLessThans.

[Eli Friedman via llvm-commits]

Author: Eli Friedman
Date: 2021-09-09T12:38:33-07:00
New Revision: 8f792707c4e5208bba08d837504c93de28b1f209

URL: https://github.com/llvm/llvm-project/commit/8f792707c4e5208bba08d837504c93de28b1f209
DIFF: https://github.com/llvm/llvm-project/commit/8f792707c4e5208bba08d837504c93de28b1f209.diff

LOG: [ScalarEvolution] Fix pointer/int confusion in howManyLessThans.

In general, howManyLessThans doesn't really want to work with pointers
at all; the result is an integer, and the operands of the icmp are
effectively integers.  However, isLoopEntryGuardedByCond doesn't like
extra ptrtoint casts, so the arguments to isLoopEntryGuardedByCond need
to be computed without those casts.

Somehow, the values got mixed up with the recent howManyLessThans
improvements; fix the confused values, and add a better comment to
explain what's happening.

Differential Revision: https://reviews.llvm.org/D109465

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Analysis/ScalarEvolution/no-wrap-symbolic-becount.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 9d93fbe034d9e..37b2d962d8207 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11778,7 +11778,9 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
   const SCEV *Start = IV->getStart();
 
   // Preserve pointer-typed Start/RHS to pass to isLoopEntryGuardedByCond.
-  // Use integer-typed versions for actual computation.
+  // If we convert to integers, isLoopEntryGuardedByCond will miss some cases.
+  // Use integer-typed versions for actual computation; we can't subtract
+  // pointers in general.
   const SCEV *OrigStart = Start;
   const SCEV *OrigRHS = RHS;
   if (Start->getType()->isPointerTy()) {
@@ -11809,10 +11811,10 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
   // is End and so the result is as above, and if not max(End,Start) is Start
   // so we get a backedge count of zero.
   const SCEV *BECount = nullptr;
-  auto *StartMinusStride = getMinusSCEV(OrigStart, Stride);
+  auto *OrigStartMinusStride = getMinusSCEV(OrigStart, Stride);
   // Can we prove (max(RHS,Start) > Start - Stride?
-  if (isLoopEntryGuardedByCond(L, Cond, StartMinusStride, Start) &&
-      isLoopEntryGuardedByCond(L, Cond, StartMinusStride, RHS)) {
+  if (isLoopEntryGuardedByCond(L, Cond, OrigStartMinusStride, OrigStart) &&
+      isLoopEntryGuardedByCond(L, Cond, OrigStartMinusStride, OrigRHS)) {
     // In this case, we can use a refined formula for computing backedge taken
     // count.  The general formula remains:
     //   "End-Start /uceiling Stride" where "End = max(RHS,Start)"
@@ -11833,10 +11835,8 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
     //   Our preconditions trivially imply no overflow in that form.
     const SCEV *MinusOne = getMinusOne(Stride->getType());
     const SCEV *Numerator =
-        getMinusSCEV(getAddExpr(RHS, MinusOne), StartMinusStride);
-    if (!isa<SCEVCouldNotCompute>(Numerator)) {
-      BECount = getUDivExpr(Numerator, Stride);
-    }
+        getMinusSCEV(getAddExpr(RHS, MinusOne), getMinusSCEV(Start, Stride));
+    BECount = getUDivExpr(Numerator, Stride);
   }
 
   const SCEV *BECountIfBackedgeTaken = nullptr;

diff  --git a/llvm/test/Analysis/ScalarEvolution/no-wrap-symbolic-becount.ll b/llvm/test/Analysis/ScalarEvolution/no-wrap-symbolic-becount.ll
index e04cb652a0c7e..7a6226e9f0125 100644
--- a/llvm/test/Analysis/ScalarEvolution/no-wrap-symbolic-becount.ll
+++ b/llvm/test/Analysis/ScalarEvolution/no-wrap-symbolic-becount.ll
@@ -127,13 +127,13 @@ define void @pointer_iv_nowrap_guard(i32* %startptr, i32* %endptr) local_unnamed
 ; CHECK-NEXT:    %init = getelementptr inbounds i32, i32* %startptr, i64 2000
 ; CHECK-NEXT:    --> (8000 + %startptr)<nuw> U: [8000,0) S: [8000,0)
 ; CHECK-NEXT:    %iv = phi i32* [ %init, %entry ], [ %iv.next, %loop ]
-; CHECK-NEXT:    --> {(8000 + %startptr)<nuw>,+,4}<nuw><%loop> U: [8000,0) S: [8000,0) Exits: (8000 + (4 * ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + ((8004 + (ptrtoint i32* %startptr to i64))<nuw> umax (ptrtoint i32* %endptr to i64))) /u 4))<nuw> + %startptr) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(8000 + %startptr)<nuw>,+,4}<nuw><%loop> U: [8000,0) S: [8000,0) Exits: (8000 + (4 * ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + (ptrtoint i32* %endptr to i64)) /u 4))<nuw> + %startptr) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = getelementptr inbounds i32, i32* %iv, i64 1
-; CHECK-NEXT:    --> {(8004 + %startptr)<nuw>,+,4}<nuw><%loop> U: [8004,0) S: [8004,0) Exits: (8004 + (4 * ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + ((8004 + (ptrtoint i32* %startptr to i64))<nuw> umax (ptrtoint i32* %endptr to i64))) /u 4))<nuw> + %startptr) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {(8004 + %startptr)<nuw>,+,4}<nuw><%loop> U: [8004,0) S: [8004,0) Exits: (8004 + (4 * ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + (ptrtoint i32* %endptr to i64)) /u 4))<nuw> + %startptr) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @pointer_iv_nowrap_guard
-; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + ((8004 + (ptrtoint i32* %startptr to i64))<nuw> umax (ptrtoint i32* %endptr to i64))) /u 4)
+; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + (ptrtoint i32* %endptr to i64)) /u 4)
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is 4611686018427385902
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + ((8004 + (ptrtoint i32* %startptr to i64))<nuw> umax (ptrtoint i32* %endptr to i64))) /u 4)
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-8001 + (-1 * (ptrtoint i32* %startptr to i64)) + (ptrtoint i32* %endptr to i64)) /u 4)
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 ;