[llvm] 18eaff1 - [ScalarEvolution] Fold %x umin_seq %y if %x cannot be zero

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2022-05-09T15:11:05+02:00
New Revision: 18eaff1510525a2226e1fd0c31c6b176f07ae0a7

URL: https://github.com/llvm/llvm-project/commit/18eaff1510525a2226e1fd0c31c6b176f07ae0a7
DIFF: https://github.com/llvm/llvm-project/commit/18eaff1510525a2226e1fd0c31c6b176f07ae0a7.diff

LOG: [ScalarEvolution] Fold %x umin_seq %y if %x cannot be zero

Fold %x umin_seq %y to %x umin %y if %x cannot be zero. They only
differ in semantics for %x==0.

More generally %x *_seq %y folds to %x * %y if %x cannot be the
saturation fold (though currently we only have umin_seq).

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Analysis/ScalarEvolution/exit-count-select-safe.ll

Removed: 
    


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diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index bbd4e0e26b651..b43526da98a30 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -4129,10 +4129,22 @@ ScalarEvolution::getSequentialMinMaxExpr(SCEVTypes Kind,
       return getSequentialMinMaxExpr(Kind, Ops);
   }
 
-  // In %x umin_seq %y, if %y being poison implies %x is also poison, we can
-  // use a non-sequential umin instead.
+  const SCEV *SaturationPoint;
+  switch (Kind) {
+  case scSequentialUMinExpr:
+    SaturationPoint = getZero(Ops[0]->getType());
+    break;
+  default:
+    llvm_unreachable("Not a sequential min/max type.");
+  }
+
+  // We can replace %x umin_seq %y with %x umin %y if either:
+  //  * %y being poison implies %x is also poison.
+  //  * %x cannot be the saturating value (e.g. zero for umin).
   for (unsigned i = 1, e = Ops.size(); i != e; ++i) {
-    if (::impliesPoison(Ops[i], Ops[i - 1])) {
+    if (::impliesPoison(Ops[i], Ops[i - 1]) ||
+        isKnownViaNonRecursiveReasoning(ICmpInst::ICMP_NE, Ops[i - 1],
+                                        SaturationPoint)) {
       SmallVector<const SCEV *> SeqOps = {Ops[i - 1], Ops[i]};
       Ops[i - 1] = getMinMaxExpr(
           SCEVSequentialMinMaxExpr::getEquivalentNonSequentialSCEVType(Kind),

diff  --git a/llvm/test/Analysis/ScalarEvolution/exit-count-select-safe.ll b/llvm/test/Analysis/ScalarEvolution/exit-count-select-safe.ll
index 9441c8333bb4a..65ea5e09b20bc 100644
--- a/llvm/test/Analysis/ScalarEvolution/exit-count-select-safe.ll
+++ b/llvm/test/Analysis/ScalarEvolution/exit-count-select-safe.ll
@@ -954,15 +954,15 @@ define i32 @logical_and_not_zero(i16 %n, i32 %m) {
 ; CHECK-NEXT:    %n1 = add i32 %n.ext, 1
 ; CHECK-NEXT:    --> (1 + (zext i16 %n to i32))<nuw><nsw> U: [1,65537) S: [1,65537)
 ; CHECK-NEXT:    %i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
-; CHECK-NEXT:    --> {0,+,1}<%loop> U: [0,65537) S: [0,65537) Exits: ((1 + (zext i16 %n to i32))<nuw><nsw> umin_seq %m) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {0,+,1}<%loop> U: [0,65537) S: [0,65537) Exits: ((1 + (zext i16 %n to i32))<nuw><nsw> umin %m) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %i.next = add i32 %i, 1
-; CHECK-NEXT:    --> {1,+,1}<%loop> U: [1,65538) S: [1,65538) Exits: (1 + ((1 + (zext i16 %n to i32))<nuw><nsw> umin_seq %m))<nuw><nsw> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {1,+,1}<%loop> U: [1,65538) S: [1,65538) Exits: (1 + ((1 + (zext i16 %n to i32))<nuw><nsw> umin %m))<nuw><nsw> LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %cond = select i1 %cond_p0, i1 %cond_p1, i1 false
 ; CHECK-NEXT:    --> (%cond_p0 umin_seq %cond_p1) U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %loop: Variant }
 ; CHECK-NEXT:  Determining loop execution counts for: @logical_and_not_zero
-; CHECK-NEXT:  Loop %loop: backedge-taken count is ((1 + (zext i16 %n to i32))<nuw><nsw> umin_seq %m)
+; CHECK-NEXT:  Loop %loop: backedge-taken count is ((1 + (zext i16 %n to i32))<nuw><nsw> umin %m)
 ; CHECK-NEXT:  Loop %loop: max backedge-taken count is 65536
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((1 + (zext i16 %n to i32))<nuw><nsw> umin_seq %m)
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is ((1 + (zext i16 %n to i32))<nuw><nsw> umin %m)
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 ;