[llvm] 3483c28 - [SCEV] ] If RHS >= Start, simplify (Start smax RHS) to RHS for trip counts.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2020-08-11T13:20:24+01:00
New Revision: 3483c28c5bc16d37d6c0f4e760357e4934f83b97

URL: https://github.com/llvm/llvm-project/commit/3483c28c5bc16d37d6c0f4e760357e4934f83b97
DIFF: https://github.com/llvm/llvm-project/commit/3483c28c5bc16d37d6c0f4e760357e4934f83b97.diff

LOG: [SCEV] ] If RHS >= Start, simplify (Start smax RHS) to RHS for trip counts.

This is the max version of D85046.

This change causes binary changes in 44 out of 237 benchmarks (out of
MultiSource/SPEC2000/SPEC2006)

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D85189

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Analysis/ScalarEvolution/smin-smax-folds.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 23ad77ff6054..75d2d39e7114 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10561,7 +10561,13 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
   if (isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS))
     BECount = BECountIfBackedgeTaken;
   else {
-    End = IsSigned ? getSMaxExpr(RHS, Start) : getUMaxExpr(RHS, Start);
+    // If we know that RHS >= Start in the context of loop, then we know that
+    // max(RHS, Start) = RHS at this point.
+    if (isLoopEntryGuardedByCond(
+            L, IsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE, RHS, Start))
+      End = RHS;
+    else
+      End = IsSigned ? getSMaxExpr(RHS, Start) : getUMaxExpr(RHS, Start);
     BECount = computeBECount(getMinusSCEV(End, Start), Stride, false);
   }
 

diff  --git a/llvm/test/Analysis/ScalarEvolution/smin-smax-folds.ll b/llvm/test/Analysis/ScalarEvolution/smin-smax-folds.ll
index 15ec72317d9a..98475926872d 100644
--- a/llvm/test/Analysis/ScalarEvolution/smin-smax-folds.ll
+++ b/llvm/test/Analysis/ScalarEvolution/smin-smax-folds.ll
@@ -98,13 +98,13 @@ define void @smax_simplify_with_guard(i32 %start, i32 %n) {
 ; CHECK-LABEL:  'smax_simplify_with_guard'
 ; CHECK-NEXT:  Classifying expressions for: @smax_simplify_with_guard
 ; CHECK-NEXT:    %k.0.i26 = phi i32 [ %start, %loop.ph ], [ %inc.i, %loop ]
-; CHECK-NEXT:    -->  {%start,+,1}<nsw><%loop> U: full-set S: full-set      Exits: (%start smax %n)     LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    -->  {%start,+,1}<nsw><%loop> U: full-set S: full-set      Exits: %n     LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %inc.i = add nsw i32 %k.0.i26, 1
-; CHECK-NEXT:    -->  {(1 + %start),+,1}<nw><%loop> U: full-set S: full-set     Exits: (1 + (%start smax %n))       LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    -->  {(1 + %start),+,1}<nw><%loop> U: full-set S: full-set     Exits: (1 + %n)       LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @smax_simplify_with_guard
-; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-1 * %start) + (%start smax %n))
+; CHECK-NEXT:  Loop %loop: backedge-taken count is ((-1 * %start) + %n)
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is -1
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-1 * %start) + (%start smax %n))
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((-1 * %start) +  %n)
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 entry: