[PATCH] D149786: [SCEV] Try smaller ZExts when using loop guard info.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: nikic, reames, mkazantsev.
Herald added subscribers: StephenFan, hiraditya.
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fhahn requested review of this revision.
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If we didn't find the extact ZExt expr in the rewrite map, check if
there's an entry for a smaller ZExt we can use instead.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D149786

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll


Index: llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll
===================================================================
--- llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll
+++ llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll
@@ -9,14 +9,14 @@
 ; CHECK-NEXT:    %conv8 = zext i32 %and to i64
 ; CHECK-NEXT:    --> (zext i1 (trunc i32 %conv to i1) to i64) U: [0,2) S: [0,2)
 ; CHECK-NEXT:    %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
-; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,2) S: [0,2) Exits: (zext i1 (trunc i32 %conv to i1) to i64) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {0,+,1}<nuw><nsw><%loop> U: [0,1) S: [0,1) Exits: 0 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = add i64 %iv, 1
-; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%loop> U: [1,3) S: [1,3) Exits: (1 + (zext i1 (trunc i32 %conv to i1) to i64))<nuw><nsw> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {1,+,1}<nuw><nsw><%loop> U: [1,2) S: [1,2) Exits: 1 LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @loop_guard_improves_exact_backedge_taken_count_1
-; CHECK-NEXT:  Loop %loop: backedge-taken count is (zext i1 (trunc i32 %conv to i1) to i64)
-; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is 1
-; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is (zext i1 (trunc i32 %conv to i1) to i64)
-; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (zext i1 (trunc i32 %conv to i1) to i64)
+; CHECK-NEXT:  Loop %loop: backedge-taken count is 0
+; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is 0
+; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is 0
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is 0
 ; CHECK-NEXT:   Predicates:
 ; CHECK:       Loop %loop: Trip multiple is 1
 ;
@@ -54,7 +54,7 @@
 ; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is (zext i1 (trunc i32 %conv to i1) to i64)
 ; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (zext i1 (trunc i32 %conv to i1) to i64)
 ; CHECK-NEXT:   Predicates:
-; CHECK:       Loop %loop: Trip multiple is 1
+; CHECK:       Loop %loop: Trip multiple is 2
 ;
 entry:
   %and = and i32 %conv, 1
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -14924,9 +14924,27 @@
 
   const SCEV *visitZeroExtendExpr(const SCEVZeroExtendExpr *Expr) {
     auto I = Map.find(Expr);
-    if (I == Map.end())
+    if (I == Map.end()) {
+      // If we didn't find the extact ZExt expr in the map, check if there's an
+      // entry for a smaller ZExt we can use instead.
+      Type *Ty = Expr->getType();
+      const SCEV *Op = Expr->getOperand(0);
+      unsigned Bitwidth = Ty->getScalarSizeInBits() / 2;
+      if (Bitwidth % 8 == 0) {
+        while (Bitwidth >= 8 &&
+               Bitwidth > Op->getType()->getScalarSizeInBits()) {
+          Type *NarrowTy = IntegerType::get(SE.getContext(), Bitwidth);
+          auto *NarrowExt = SE.getZeroExtendExpr(Op, NarrowTy);
+          auto I = Map.find(NarrowExt);
+          if (I != Map.end())
+            return SE.getZeroExtendExpr(I->second, Ty);
+          Bitwidth = Bitwidth / 2;
+        }
+      }
+
       return SCEVRewriteVisitor<SCEVLoopGuardRewriter>::visitZeroExtendExpr(
           Expr);
+    }
     return I->second;
   }
 


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