[PATCH] D126502: [SCEV] Apply conditions involving constants first in applyLoopGuards.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
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In some cases the order conditions are applied in can pessimize results.
In general, applying conditions with constant operands should lead to
better results. I think ideally we would apply conditions with operands
that are not involved in any other condition first, but processing
conditions with constants should be a good start.

Alternatively we could re-write all collected expressions after
collecting them. But that's likely more expensive.

Depends on D126460 <https://reviews.llvm.org/D126460>.

Fixes #55645.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D126502

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp
  llvm/test/Analysis/ScalarEvolution/trip-multiple-guard-info.ll


Index: llvm/test/Analysis/ScalarEvolution/trip-multiple-guard-info.ll
===================================================================
--- llvm/test/Analysis/ScalarEvolution/trip-multiple-guard-info.ll
+++ llvm/test/Analysis/ScalarEvolution/trip-multiple-guard-info.ll
@@ -34,7 +34,7 @@
 ; CHECK:       Loop %for.body: backedge-taken count is (-1 + %num)
 ; CHECK-NEXT:  Loop %for.body: max backedge-taken count is -2
 ; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is (-1 + %num)
-; CHECK:       Loop %for.body: Trip multiple is 2
+; CHECK:       Loop %for.body: Trip multiple is 4
 ;
 entry:
   %u = urem i32 %num, 4
@@ -59,7 +59,7 @@
 ; CHECK:       Loop %for.body: backedge-taken count is (-1 + %num)
 ; CHECK-NEXT:  Loop %for.body: max backedge-taken count is 2147483646
 ; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is (-1 + %num)
-; CHECK:       Loop %for.body: Trip multiple is 2
+; CHECK:       Loop %for.body: Trip multiple is 4
 ;
 entry:
   %u = urem i32 %num, 4
@@ -84,7 +84,7 @@
 ; CHECK:       Loop %for.body: backedge-taken count is (-1 + %num)
 ; CHECK-NEXT:  Loop %for.body: max backedge-taken count is -2
 ; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is (-1 + %num)
-; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK:       Loop %for.body: Trip multiple is 4
 ;
 entry:
   %u = urem i32 %num, 4
@@ -109,7 +109,7 @@
 ; CHECK:       Loop %for.body: backedge-taken count is (-1 + %num)
 ; CHECK-NEXT:  Loop %for.body: max backedge-taken count is 2147483646
 ; CHECK-NEXT:  Loop %for.body: Predicated backedge-taken count is (-1 + %num)
-; CHECK:       Loop %for.body: Trip multiple is 1
+; CHECK:       Loop %for.body: Trip multiple is 4
 ;
 entry:
   %u = urem i32 %num, 4
Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -14548,6 +14548,23 @@
                        LoopEntryPredicate->getSuccessor(0) == Pair.second);
   }
 
+  // Sort conditions so that conditions with constant operands come last. This
+  // allows processing them first in the loop below, which can lead to more
+  // effective expression rewriting.
+  stable_sort(Terms, [this](const std::pair<Value *, bool> &A,
+                            const std::pair<Value *, bool> &B) {
+    auto *CmpA = dyn_cast<ICmpInst>(A.first);
+    auto *CmpB = dyn_cast<ICmpInst>(B.first);
+    if (!CmpA || !CmpB)
+      return true;
+
+    bool ConstA = isa<SCEVConstant>(getSCEV(CmpA->getOperand(0))) ||
+                  isa<SCEVConstant>(getSCEV(CmpA->getOperand(1)));
+    bool ConstB = isa<SCEVConstant>(getSCEV(CmpB->getOperand(0))) ||
+                  isa<SCEVConstant>(getSCEV(CmpB->getOperand(1)));
+    return ConstA != ConstB;
+  });
+
   // Now apply the information from the collected conditions to RewriteMap.
   // Conditions are processed in reverse order, so the earliest conditions is
   // processed first. This ensures the SCEVs with the shortest dependency chains


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